Move thumbnails into their own source file

And clean up identifiers.

7 files changed, 605 insertions, 545 deletions

diff --git a/fastiv.c b/fastiv.c
index b253df1..0a8b426 100644
--- a/fastiv.c
+++ b/fastiv.c
@@ -31,6 +31,7 @@
 #include "fiv-browser.h"
 #include "fiv-io.h"
 #include "fiv-sidebar.h"
+#include "fiv-thumbnail.h"
 #include "fiv-view.h"
 #include "xdg.h"
 
@@ -644,12 +645,12 @@ on_open_location(G_GNUC_UNUSED GtkPlacesSidebar *sidebar, GFile *location,
 static void
 on_toolbar_zoom(G_GNUC_UNUSED GtkButton *button, gpointer user_data)
 {
-	FivIoThumbnailSize size = FIV_IO_THUMBNAIL_SIZE_COUNT;
+	FivThumbnailSize size = FIV_THUMBNAIL_SIZE_COUNT;
 	g_object_get(g.browser, "thumbnail-size", &size, NULL);
 
 	size += (gintptr) user_data;
-	g_return_if_fail(size >= FIV_IO_THUMBNAIL_SIZE_MIN &&
-		size <= FIV_IO_THUMBNAIL_SIZE_MAX);
+	g_return_if_fail(size >= FIV_THUMBNAIL_SIZE_MIN &&
+		size <= FIV_THUMBNAIL_SIZE_MAX);
 
 	g_object_set(g.browser, "thumbnail-size", size, NULL);
 }
@@ -658,10 +659,10 @@ static void
 on_notify_thumbnail_size(
 	GObject *object, GParamSpec *param_spec, G_GNUC_UNUSED gpointer user_data)
 {
-	FivIoThumbnailSize size = 0;
+	FivThumbnailSize size = 0;
 	g_object_get(object, g_param_spec_get_name(param_spec), &size, NULL);
-	gtk_widget_set_sensitive(g.plus, size < FIV_IO_THUMBNAIL_SIZE_MAX);
-	gtk_widget_set_sensitive(g.minus, size > FIV_IO_THUMBNAIL_SIZE_MIN);
+	gtk_widget_set_sensitive(g.plus, size < FIV_THUMBNAIL_SIZE_MAX);
+	gtk_widget_set_sensitive(g.minus, size > FIV_THUMBNAIL_SIZE_MIN);
 }
 
 static void
@@ -1236,16 +1237,16 @@ main(int argc, char *argv[])
 		if (!path_arg)
 			exit_fatal("no path given");
 
-		FivIoThumbnailSize size = 0;
-		for (; size < FIV_IO_THUMBNAIL_SIZE_COUNT; size++)
-			if (!strcmp(fiv_io_thumbnail_sizes[size].thumbnail_spec_name,
+		FivThumbnailSize size = 0;
+		for (; size < FIV_THUMBNAIL_SIZE_COUNT; size++)
+			if (!strcmp(fiv_thumbnail_sizes[size].thumbnail_spec_name,
 					thumbnail_size))
 				break;
-		if (size >= FIV_IO_THUMBNAIL_SIZE_COUNT)
+		if (size >= FIV_THUMBNAIL_SIZE_COUNT)
 			exit_fatal("unknown thumbnail size: %s", thumbnail_size);
 
 		GFile *target = g_file_new_for_path(path_arg);
-		if (!fiv_io_produce_thumbnail(target, size, &error))
+		if (!fiv_thumbnail_produce(target, size, &error))
 			exit_fatal("%s", error->message);
 		g_object_unref(target);
 		return 0;
diff --git a/fiv-browser.c b/fiv-browser.c
index 84828d2..0ef212e 100644
--- a/fiv-browser.c
+++ b/fiv-browser.c
@@ -22,6 +22,7 @@
 
 #include "fiv-browser.h"
 #include "fiv-io.h"
+#include "fiv-thumbnail.h"
 #include "fiv-view.h"
 
 // --- Widget ------------------------------------------------------------------
@@ -43,7 +44,7 @@
 struct _FivBrowser {
 	GtkWidget parent_instance;
 
-	FivIoThumbnailSize item_size;       ///< Thumbnail size
+	FivThumbnailSize item_size;         ///< Thumbnail size
 	int item_height;                    ///< Thumbnail height in pixels
 	int item_spacing;                   ///< Space between items in pixels
 
@@ -303,8 +304,8 @@ rescale_thumbnail(cairo_surface_t *thumbnail, double row_height)
 
 	double scale_x = 1;
 	double scale_y = 1;
-	if (width > FIV_IO_WIDE_THUMBNAIL_COEFFICIENT * height) {
-		scale_x = FIV_IO_WIDE_THUMBNAIL_COEFFICIENT * row_height / width;
+	if (width > FIV_THUMBNAIL_WIDE_COEFFICIENT * height) {
+		scale_x = FIV_THUMBNAIL_WIDE_COEFFICIENT * row_height / width;
 		scale_y = round(scale_x * height) / height;
 	} else {
 		scale_y = row_height / height;
@@ -350,7 +351,7 @@ rescale_thumbnail(cairo_surface_t *thumbnail, double row_height)
 	pixman_image_unref(dest);
 
 	cairo_surface_set_user_data(
-		scaled, &fiv_io_key_thumbnail_lq, (void *) (intptr_t) 1, NULL);
+		scaled, &fiv_thumbnail_key_lq, (void *) (intptr_t) 1, NULL);
 	cairo_surface_destroy(thumbnail);
 	cairo_surface_mark_dirty(scaled);
 	return scaled;
@@ -366,8 +367,7 @@ entry_add_thumbnail(gpointer data, gpointer user_data)
 	FivBrowser *browser = FIV_BROWSER(user_data);
 	GFile *file = g_file_new_for_uri(self->uri);
 	self->thumbnail = rescale_thumbnail(
-		fiv_io_lookup_thumbnail(file, browser->item_size),
-		browser->item_height);
+		fiv_thumbnail_lookup(file, browser->item_size), browser->item_height);
 	if (self->thumbnail)
 		goto out;
 
@@ -508,7 +508,7 @@ thumbnailer_next(FivBrowser *self)
 	GError *error = NULL;
 	self->thumbnailer = g_subprocess_new(G_SUBPROCESS_FLAGS_NONE, &error,
 		PROJECT_NAME, "--thumbnail",
-		fiv_io_thumbnail_sizes[self->item_size].thumbnail_spec_name, "--", path,
+		fiv_thumbnail_sizes[self->item_size].thumbnail_spec_name, "--", path,
 		NULL);
 	g_free(path);
 	if (error) {
@@ -542,7 +542,7 @@ thumbnailer_start(FivBrowser *self)
 	thumbnailer_abort(self);
 
 	// TODO(p): Leave out all paths containing .cache/thumbnails altogether.
-	gchar *thumbnails_dir = fiv_io_get_thumbnail_root();
+	gchar *thumbnails_dir = fiv_thumbnail_get_root();
 	GFile *thumbnails = g_file_new_for_path(thumbnails_dir);
 	g_free(thumbnails_dir);
 	GFile *current = g_file_new_for_path(self->path);
@@ -558,7 +558,7 @@ thumbnailer_start(FivBrowser *self)
 		if (entry->icon)
 			missing = g_list_prepend(missing, entry);
 		else if (cairo_surface_get_user_data(
-			entry->thumbnail, &fiv_io_key_thumbnail_lq))
+			entry->thumbnail, &fiv_thumbnail_key_lq))
 			lq = g_list_prepend(lq, entry);
 	}
 
@@ -778,14 +778,14 @@ fiv_browser_get_property(
 }
 
 static void
-set_item_size(FivBrowser *self, FivIoThumbnailSize size)
+set_item_size(FivBrowser *self, FivThumbnailSize size)
 {
-	if (size < FIV_IO_THUMBNAIL_SIZE_MIN || size > FIV_IO_THUMBNAIL_SIZE_MAX)
+	if (size < FIV_THUMBNAIL_SIZE_MIN || size > FIV_THUMBNAIL_SIZE_MAX)
 		return;
 
 	if (size != self->item_size) {
 		self->item_size = size;
-		self->item_height = fiv_io_thumbnail_sizes[self->item_size].size;
+		self->item_height = fiv_thumbnail_sizes[self->item_size].size;
 		reload_thumbnails(self);
 
 		g_object_notify_by_pspec(
@@ -822,7 +822,7 @@ fiv_browser_get_preferred_width(GtkWidget *widget, gint *minimum, gint *natural)
 	GtkBorder padding = {};
 	gtk_style_context_get_padding(style, GTK_STATE_FLAG_NORMAL, &padding);
 	*minimum = *natural =
-		FIV_IO_WIDE_THUMBNAIL_COEFFICIENT * self->item_height + padding.left +
+		FIV_THUMBNAIL_WIDE_COEFFICIENT * self->item_height + padding.left +
 		2 * self->item_border_x + padding.right;
 }
 
@@ -1100,7 +1100,7 @@ fiv_browser_class_init(FivBrowserClass *klass)
 
 	browser_properties[PROP_THUMBNAIL_SIZE] = g_param_spec_enum(
 		"thumbnail-size", "Thumbnail size", "The thumbnail height to use",
-		FIV_TYPE_IO_THUMBNAIL_SIZE, FIV_IO_THUMBNAIL_SIZE_NORMAL,
+		FIV_TYPE_THUMBNAIL_SIZE, FIV_THUMBNAIL_SIZE_NORMAL,
 		G_PARAM_READWRITE);
 	g_object_class_install_properties(
 		object_class, N_PROPERTIES, browser_properties);
@@ -1145,7 +1145,7 @@ fiv_browser_init(FivBrowser *self)
 	self->layouted_rows = g_array_new(FALSE, TRUE, sizeof(Row));
 	g_array_set_clear_func(self->layouted_rows, (GDestroyNotify) row_free);
 
-	set_item_size(self, FIV_IO_THUMBNAIL_SIZE_NORMAL);
+	set_item_size(self, FIV_THUMBNAIL_SIZE_NORMAL);
 	self->selected = -1;
 	self->glow = cairo_image_surface_create(CAIRO_FORMAT_A1, 0, 0);
 
diff --git a/fiv-io.c b/fiv-io.c
index 26b6e89..759cb4e 100644
--- a/fiv-io.c
+++ b/fiv-io.c
@@ -20,9 +20,7 @@
 #include <cairo.h>
 #include <errno.h>
 #include <glib.h>
-#include <glib/gstdio.h>
 
-#include <spng.h>
 #include <turbojpeg.h>
 #include <webp/decode.h>
 #include <webp/demux.h>
@@ -78,7 +76,6 @@
 #include "wuffs-mirror-release-c/release/c/wuffs-v0.3.c"
 
 #include "fiv-io.h"
-#include "xdg.h"
 
 #if CAIRO_VERSION >= 11702 && X11_ACTUALLY_SUPPORTS_RGBA128F_OR_WE_USE_OPENGL
 #define FIV_CAIRO_RGBA128F
@@ -135,6 +132,22 @@ fiv_io_all_supported_media_types(void)
 	return (char **) g_ptr_array_free(types, FALSE);
 }
 
+int
+fiv_io_filecmp(GFile *location1, GFile *location2)
+{
+	if (g_file_has_prefix(location1, location2))
+		return +1;
+	if (g_file_has_prefix(location2, location1))
+		return -1;
+
+	gchar *name1 = g_file_get_parse_name(location1);
+	gchar *name2 = g_file_get_parse_name(location2);
+	int result = g_utf8_collate(name1, name2);
+	g_free(name1);
+	g_free(name2);
+	return result;
+}
+
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
 #define FIV_IO_ERROR fiv_io_error_quark()
@@ -421,9 +434,6 @@ fiv_io_profile_finalize(cairo_surface_t *image, FivIoProfile target)
 	return image;
 }
 
-// From libwebp, verified to exactly match [x * a / 255].
-#define PREMULTIPLY8(a, x) (((uint32_t) (x) * (uint32_t) (a) * 32897U) >> 23)
-
 static void
 fiv_io_premultiply_argb32(cairo_surface_t *surface)
 {
@@ -2297,8 +2307,6 @@ cairo_user_data_key_t fiv_io_key_loops;
 cairo_user_data_key_t fiv_io_key_page_next;
 cairo_user_data_key_t fiv_io_key_page_previous;
 
-cairo_user_data_key_t fiv_io_key_thumbnail_lq;
-
 cairo_surface_t *
 fiv_io_open(
 	const gchar *path, FivIoProfile profile, gboolean enhance, GError **error)
@@ -2754,465 +2762,3 @@ fiv_io_save_metadata(cairo_surface_t *page, const gchar *path, GError **error)
 	}
 	return TRUE;
 }
-
-// --- Thumbnails --------------------------------------------------------------
-
-#ifndef __linux__
-#define st_mtim st_mtimespec
-#endif  // ! __linux__
-
-GType
-fiv_io_thumbnail_size_get_type(void)
-{
-	static gsize guard;
-	if (g_once_init_enter(&guard)) {
-#define XX(name, value, dir) {FIV_IO_THUMBNAIL_SIZE_ ## name, \
-	"FIV_IO_THUMBNAIL_SIZE_" #name, #name},
-		static const GEnumValue values[] = {FIV_IO_THUMBNAIL_SIZES(XX) {}};
-#undef XX
-		GType type = g_enum_register_static(
-			g_intern_static_string("FivIoThumbnailSize"), values);
-		g_once_init_leave(&guard, type);
-	}
-	return guard;
-}
-
-#define XX(name, value, dir) {value, dir},
-FivIoThumbnailSizeInfo
-	fiv_io_thumbnail_sizes[FIV_IO_THUMBNAIL_SIZE_COUNT] = {
-		FIV_IO_THUMBNAIL_SIZES(XX)};
-#undef XX
-
-static void
-mark_thumbnail_lq(cairo_surface_t *surface)
-{
-	cairo_surface_set_user_data(
-		surface, &fiv_io_key_thumbnail_lq, (void *) (intptr_t) 1, NULL);
-}
-
-gchar *
-fiv_io_get_thumbnail_root(void)
-{
-	gchar *cache_dir = get_xdg_home_dir("XDG_CACHE_HOME", ".cache");
-	gchar *thumbnails_dir = g_build_filename(cache_dir, "thumbnails", NULL);
-	g_free(cache_dir);
-	return thumbnails_dir;
-}
-
-// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-
-// In principle similar to rescale_thumbnail() from fiv-browser.c.
-static cairo_surface_t *
-rescale_thumbnail(cairo_surface_t *thumbnail, double row_height)
-{
-	cairo_format_t format = cairo_image_surface_get_format(thumbnail);
-	int width = cairo_image_surface_get_width(thumbnail);
-	int height = cairo_image_surface_get_height(thumbnail);
-
-	double scale_x = 1;
-	double scale_y = 1;
-	if (width > FIV_IO_WIDE_THUMBNAIL_COEFFICIENT * height) {
-		scale_x = FIV_IO_WIDE_THUMBNAIL_COEFFICIENT * row_height / width;
-		scale_y = round(scale_x * height) / height;
-	} else {
-		scale_y = row_height / height;
-		scale_x = round(scale_y * width) / width;
-	}
-	if (scale_x == 1 && scale_y == 1)
-		return cairo_surface_reference(thumbnail);
-
-	int projected_width = round(scale_x * width);
-	int projected_height = round(scale_y * height);
-	cairo_surface_t *scaled = cairo_image_surface_create(
-		(format == CAIRO_FORMAT_RGB24 || format == CAIRO_FORMAT_RGB30)
-			? CAIRO_FORMAT_RGB24
-			: CAIRO_FORMAT_ARGB32,
-		projected_width, projected_height);
-
-	cairo_t *cr = cairo_create(scaled);
-	cairo_scale(cr, scale_x, scale_y);
-
-	cairo_set_source_surface(cr, thumbnail, 0, 0);
-	cairo_pattern_t *pattern = cairo_get_source(cr);
-	cairo_pattern_set_filter(pattern, CAIRO_FILTER_BEST);
-	cairo_pattern_set_extend(pattern, CAIRO_EXTEND_PAD);
-
-	cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);
-	cairo_paint(cr);
-	cairo_destroy(cr);
-	mark_thumbnail_lq(scaled);
-	return scaled;
-}
-
-static WebPData
-encode_thumbnail(cairo_surface_t *surface)
-{
-	WebPData bitstream = {};
-	WebPConfig config = {};
-	if (!WebPConfigInit(&config) || !WebPConfigLosslessPreset(&config, 6))
-		return bitstream;
-
-	config.near_lossless = 95;
-	config.thread_level = true;
-	if (!WebPValidateConfig(&config))
-		return bitstream;
-
-	bitstream.bytes = fiv_io_encode_webp(surface, &config, &bitstream.size);
-	return bitstream;
-}
-
-static void
-save_thumbnail(cairo_surface_t *thumbnail, const char *path, GString *thum)
-{
-	WebPMux *mux = WebPMuxNew();
-	WebPData bitstream = encode_thumbnail(thumbnail);
-	gboolean ok = WebPMuxSetImage(mux, &bitstream, true) == WEBP_MUX_OK;
-	WebPDataClear(&bitstream);
-
-	WebPData data = {.bytes = (const uint8_t *) thum->str, .size = thum->len};
-	ok = ok && WebPMuxSetChunk(mux, "THUM", &data, false) == WEBP_MUX_OK;
-
-	WebPData assembled = {};
-	WebPDataInit(&assembled);
-	ok = ok && WebPMuxAssemble(mux, &assembled) == WEBP_MUX_OK;
-	WebPMuxDelete(mux);
-	if (!ok) {
-		g_warning("thumbnail encoding failed");
-		return;
-	}
-
-	GError *e = NULL;
-	while (!g_file_set_contents(
-		path, (const gchar *) assembled.bytes, assembled.size, &e)) {
-		bool missing_parents =
-			e->domain == G_FILE_ERROR && e->code == G_FILE_ERROR_NOENT;
-		g_debug("%s: %s", path, e->message);
-		g_clear_error(&e);
-		if (!missing_parents)
-			break;
-
-		gchar *dirname = g_path_get_dirname(path);
-		int err = g_mkdir_with_parents(dirname, 0755);
-		if (err)
-			g_debug("%s: %s", dirname, g_strerror(errno));
-
-		g_free(dirname);
-		if (err)
-			break;
-	}
-
-	// It would be possible to create square thumbnails as well,
-	// but it seems like wasted effort.
-	WebPDataClear(&assembled);
-}
-
-gboolean
-fiv_io_produce_thumbnail(GFile *target, FivIoThumbnailSize size, GError **error)
-{
-	g_return_val_if_fail(size >= FIV_IO_THUMBNAIL_SIZE_MIN &&
-		size <= FIV_IO_THUMBNAIL_SIZE_MAX, FALSE);
-
-	// Local files only, at least for now.
-	gchar *path = g_file_get_path(target);
-	if (!path)
-		return FALSE;
-
-	GMappedFile *mf = g_mapped_file_new(path, FALSE, error);
-	if (!mf) {
-		g_free(path);
-		return FALSE;
-	}
-
-	GStatBuf st = {};
-	if (g_stat(path, &st)) {
-		set_error(error, g_strerror(errno));
-		g_free(path);
-		return FALSE;
-	}
-
-	// TODO(p): Add a flag to avoid loading all pages and frames.
-	FivIoProfile sRGB = fiv_io_profile_new_sRGB();
-	gsize filesize = g_mapped_file_get_length(mf);
-	cairo_surface_t *surface = fiv_io_open_from_data(
-		g_mapped_file_get_contents(mf), filesize, path, sRGB, FALSE, error);
-
-	g_free(path);
-	g_mapped_file_unref(mf);
-	if (sRGB)
-		fiv_io_profile_free(sRGB);
-	if (!surface)
-		return FALSE;
-
-	// Boilerplate copied from fiv_io_lookup_thumbnail().
-	gchar *uri = g_file_get_uri(target);
-	gchar *sum = g_compute_checksum_for_string(G_CHECKSUM_MD5, uri, -1);
-	gchar *thumbnails_dir = fiv_io_get_thumbnail_root();
-
-	GString *thum = g_string_new("");
-	g_string_append_printf(
-		thum, "%s%c%s%c", "Thumb::URI", 0, uri, 0);
-	g_string_append_printf(
-		thum, "%s%c%ld%c", "Thumb::Mtime", 0, (long) st.st_mtim.tv_sec, 0);
-	g_string_append_printf(
-		thum, "%s%c%ld%c", "Thumb::Size", 0, (long) filesize, 0);
-	g_string_append_printf(thum, "%s%c%d%c", "Thumb::Image::Width", 0,
-		cairo_image_surface_get_width(surface), 0);
-	g_string_append_printf(thum, "%s%c%d%c", "Thumb::Image::Height", 0,
-		cairo_image_surface_get_height(surface), 0);
-
-	// Without a CMM, no conversion is attempted.
-	if (sRGB) {
-		g_string_append_printf(
-			thum, "%s%c%s%c", "Thumb::ColorSpace", 0, "sRGB", 0);
-	}
-
-	for (int use = size; use >= FIV_IO_THUMBNAIL_SIZE_MIN; use--) {
-		cairo_surface_t *scaled =
-			rescale_thumbnail(surface, fiv_io_thumbnail_sizes[use].size);
-		gchar *path = g_strdup_printf("%s/wide-%s/%s.webp", thumbnails_dir,
-			fiv_io_thumbnail_sizes[use].thumbnail_spec_name, sum);
-		save_thumbnail(scaled, path, thum);
-		cairo_surface_destroy(scaled);
-		g_free(path);
-	}
-
-	g_string_free(thum, TRUE);
-
-	g_free(thumbnails_dir);
-	g_free(sum);
-	g_free(uri);
-	cairo_surface_destroy(surface);
-	return TRUE;
-}
-
-static cairo_surface_t *
-read_wide_thumbnail(
-	const gchar *path, const gchar *uri, time_t mtime, GError **error)
-{
-	// TODO(p): Validate fiv_io_key_thum.
-	(void) uri;
-	(void) mtime;
-	return fiv_io_open(path, NULL, FALSE, error);
-}
-
-// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-
-static int  // tri-state
-check_spng_thumbnail_texts(struct spng_text *texts, uint32_t texts_len,
-	const gchar *target, time_t mtime)
-{
-	// May contain Thumb::Image::Width Thumb::Image::Height,
-	// but those aren't interesting currently (would be for fast previews).
-	bool need_uri = true, need_mtime = true;
-	for (uint32_t i = 0; i < texts_len; i++) {
-		struct spng_text *text = texts + i;
-		if (!strcmp(text->keyword, "Thumb::URI")) {
-			need_uri = false;
-			if (strcmp(target, text->text))
-				return false;
-		}
-		if (!strcmp(text->keyword, "Thumb::MTime")) {
-			need_mtime = false;
-			if (atol(text->text) != mtime)
-				return false;
-		}
-	}
-	return need_uri || need_mtime ? -1 : true;
-}
-
-static int  // tri-state
-check_spng_thumbnail(spng_ctx *ctx, const gchar *target, time_t mtime, int *err)
-{
-	uint32_t texts_len = 0;
-	if ((*err = spng_get_text(ctx, NULL, &texts_len)))
-		return false;
-
-	int result = false;
-	struct spng_text *texts = g_malloc0_n(texts_len, sizeof *texts);
-	if (!(*err = spng_get_text(ctx, texts, &texts_len)))
-		result = check_spng_thumbnail_texts(texts, texts_len, target, mtime);
-	g_free(texts);
-	return result;
-}
-
-static cairo_surface_t *
-read_spng_thumbnail(
-	const gchar *path, const gchar *uri, time_t mtime, GError **error)
-{
-	FILE *fp;
-	cairo_surface_t *result = NULL;
-	if (!(fp = fopen(path, "rb"))) {
-		set_error(error, g_strerror(errno));
-		return NULL;
-	}
-
-	errno = 0;
-	spng_ctx *ctx = spng_ctx_new(0);
-	if (!ctx) {
-		set_error(error, g_strerror(errno));
-		goto fail_init;
-	}
-
-	int err;
-	size_t size = 0;
-	if ((err = spng_set_png_file(ctx, fp)) ||
-		(err = spng_set_image_limits(ctx, INT16_MAX, INT16_MAX)) ||
-		(err = spng_decoded_image_size(ctx, SPNG_FMT_RGBA8, &size))) {
-		set_error(error, spng_strerror(err));
-		goto fail;
-	}
-	if (check_spng_thumbnail(ctx, uri, mtime, &err) == false) {
-		set_error(error, err ? spng_strerror(err) : "mismatch");
-		goto fail;
-	}
-
-	struct spng_ihdr ihdr = {};
-	struct spng_trns trns = {};
-	spng_get_ihdr(ctx, &ihdr);
-	bool may_be_translucent = !spng_get_trns(ctx, &trns) ||
-		ihdr.color_type == SPNG_COLOR_TYPE_GRAYSCALE_ALPHA ||
-		ihdr.color_type == SPNG_COLOR_TYPE_TRUECOLOR_ALPHA;
-
-	cairo_surface_t *surface = cairo_image_surface_create(
-		may_be_translucent ? CAIRO_FORMAT_ARGB32 : CAIRO_FORMAT_RGB24,
-		ihdr.width, ihdr.height);
-
-	cairo_status_t surface_status = cairo_surface_status(surface);
-	if (surface_status != CAIRO_STATUS_SUCCESS) {
-		set_error(error, cairo_status_to_string(surface_status));
-		goto fail_data;
-	}
-
-	uint32_t *data = (uint32_t *) cairo_image_surface_get_data(surface);
-	g_assert((size_t) cairo_image_surface_get_stride(surface) *
-		cairo_image_surface_get_height(surface) == size);
-
-	cairo_surface_flush(surface);
-	if ((err = spng_decode_image(ctx, data, size, SPNG_FMT_RGBA8,
-		SPNG_DECODE_TRNS | SPNG_DECODE_GAMMA))) {
-		set_error(error, spng_strerror(err));
-		goto fail_data;
-	}
-
-	// The specification does not say where the required metadata should be,
-	// it could very well be broken up into two parts.
-	if (check_spng_thumbnail(ctx, uri, mtime, &err) != true) {
-		set_error(
-			error, err ? spng_strerror(err) : "mismatch or not a thumbnail");
-		goto fail_data;
-	}
-
-	// pixman can be mildly abused to do this operation, but it won't be faster.
-	if (may_be_translucent) {
-		for (size_t i = size / sizeof *data; i--; ) {
-			const uint8_t *unit = (const uint8_t *) &data[i];
-			uint32_t a = unit[3],
-				b = PREMULTIPLY8(a, unit[2]),
-				g = PREMULTIPLY8(a, unit[1]),
-				r = PREMULTIPLY8(a, unit[0]);
-			data[i] = a << 24 | r << 16 | g << 8 | b;
-		}
-	} else {
-		for (size_t i = size / sizeof *data; i--; ) {
-			uint32_t rgba = g_ntohl(data[i]);
-			data[i] = rgba << 24 | rgba >> 8;
-		}
-	}
-
-	cairo_surface_mark_dirty((result = surface));
-
-fail_data:
-	if (!result)
-		cairo_surface_destroy(surface);
-fail:
-	spng_ctx_free(ctx);
-fail_init:
-	fclose(fp);
-	return result;
-}
-
-cairo_surface_t *
-fiv_io_lookup_thumbnail(GFile *target, FivIoThumbnailSize size)
-{
-	g_return_val_if_fail(size >= FIV_IO_THUMBNAIL_SIZE_MIN &&
-		size <= FIV_IO_THUMBNAIL_SIZE_MAX, NULL);
-
-	// Local files only, at least for now.
-	gchar *path = g_file_get_path(target);
-	if (!path)
-		return NULL;
-
-	GStatBuf st = {};
-	int err = g_stat(path, &st);
-	g_free(path);
-	if (err)
-		return NULL;
-
-	gchar *uri = g_file_get_uri(target);
-	gchar *sum = g_compute_checksum_for_string(G_CHECKSUM_MD5, uri, -1);
-	gchar *thumbnails_dir = fiv_io_get_thumbnail_root();
-
-	// The lookup sequence is: nominal..max, then mirroring back to ..min.
-	cairo_surface_t *result = NULL;
-	GError *error = NULL;
-	for (int i = 0; i < FIV_IO_THUMBNAIL_SIZE_COUNT; i++) {
-		FivIoThumbnailSize use = size + i;
-		if (use > FIV_IO_THUMBNAIL_SIZE_MAX)
-			use = FIV_IO_THUMBNAIL_SIZE_MAX - i;
-
-		const char *name = fiv_io_thumbnail_sizes[use].thumbnail_spec_name;
-		gchar *wide =
-			g_strdup_printf("%s/wide-%s/%s.webp", thumbnails_dir, name, sum);
-		result = read_wide_thumbnail(wide, uri, st.st_mtim.tv_sec, &error);
-		if (error) {
-			g_debug("%s: %s", wide, error->message);
-			g_clear_error(&error);
-		}
-		g_free(wide);
-		if (result) {
-			// Higher up we can't distinguish images smaller than the thumbnail.
-			// Also, try not to rescale the already rescaled.
-			if (use != size)
-				mark_thumbnail_lq(result);
-			break;
-		}
-
-		gchar *path =
-			g_strdup_printf("%s/%s/%s.png", thumbnails_dir, name, sum);
-		result = read_spng_thumbnail(path, uri, st.st_mtim.tv_sec, &error);
-		if (error) {
-			g_debug("%s: %s", path, error->message);
-			g_clear_error(&error);
-		}
-		g_free(path);
-		if (result) {
-			// Whatever produced it, we may be able to outclass it.
-			mark_thumbnail_lq(result);
-			break;
-		}
-	}
-
-	// TODO(p): We can definitely extract embedded thumbnails, but it should be
-	// done as a separate stage--the file may be stored on a slow device.
-
-	g_free(thumbnails_dir);
-	g_free(sum);
-	g_free(uri);
-	return result;
-}
-
-int
-fiv_io_filecmp(GFile *location1, GFile *location2)
-{
-	if (g_file_has_prefix(location1, location2))
-		return +1;
-	if (g_file_has_prefix(location2, location1))
-		return -1;
-
-	gchar *name1 = g_file_get_parse_name(location1);
-	gchar *name2 = g_file_get_parse_name(location2);
-	int result = g_utf8_collate(name1, name2);
-	g_free(name1);
-	g_free(name2);
-	return result;
-}
diff --git a/fiv-io.h b/fiv-io.h
index 52fd691..54e2727 100644
--- a/fiv-io.h
+++ b/fiv-io.h
@@ -31,6 +31,9 @@ FivIoProfile fiv_io_profile_new(const void *data, size_t len);
 FivIoProfile fiv_io_profile_new_sRGB(void);
 void fiv_io_profile_free(FivIoProfile self);
 
+// From libwebp, verified to exactly match [x * a / 255].
+#define PREMULTIPLY8(a, x) (((uint32_t) (x) * (uint32_t) (a) * 32897U) >> 23)
+
 // --- Loading -----------------------------------------------------------------
 
 extern const char *fiv_io_supported_media_types[];
@@ -69,9 +72,6 @@ extern cairo_user_data_key_t fiv_io_key_page_next;
 /// There is no wrap-around. This is a weak pointer.
 extern cairo_user_data_key_t fiv_io_key_page_previous;
 
-/// If non-NULL, indicates a thumbnail of insufficient quality.
-extern cairo_user_data_key_t fiv_io_key_thumbnail_lq;
-
 cairo_surface_t *fiv_io_open(
 	const gchar *path, FivIoProfile profile, gboolean enhance, GError **error);
 cairo_surface_t *fiv_io_open_from_data(const char *data, size_t len,
@@ -113,48 +113,3 @@ FivIoOrientation fiv_io_exif_orientation(const guint8 *exif, gsize len);
 /// Save metadata attached by this module in Exiv2 format.
 gboolean fiv_io_save_metadata(
 	cairo_surface_t *page, const gchar *path, GError **error);
-
-// --- Thumbnails --------------------------------------------------------------
-
-// And this is how you avoid glib-mkenums.
-typedef enum _FivIoThumbnailSize {
-#define FIV_IO_THUMBNAIL_SIZES(XX) \
-	XX(SMALL,  128, "normal") \
-	XX(NORMAL, 256, "large") \
-	XX(LARGE,  512, "x-large") \
-	XX(HUGE,  1024, "xx-large")
-#define XX(name, value, dir) FIV_IO_THUMBNAIL_SIZE_ ## name,
-	FIV_IO_THUMBNAIL_SIZES(XX)
-#undef XX
-	FIV_IO_THUMBNAIL_SIZE_COUNT,
-
-	FIV_IO_THUMBNAIL_SIZE_MIN = 0,
-	FIV_IO_THUMBNAIL_SIZE_MAX = FIV_IO_THUMBNAIL_SIZE_COUNT - 1
-} FivIoThumbnailSize;
-
-GType fiv_io_thumbnail_size_get_type(void) G_GNUC_CONST;
-#define FIV_TYPE_IO_THUMBNAIL_SIZE (fiv_io_thumbnail_size_get_type())
-
-typedef struct _FivIoThumbnailSizeInfo {
-	int size;                           ///< Nominal size in pixels
-	const char *thumbnail_spec_name;    ///< thumbnail-spec directory name
-} FivIoThumbnailSizeInfo;
-
-extern FivIoThumbnailSizeInfo
-	fiv_io_thumbnail_sizes[FIV_IO_THUMBNAIL_SIZE_COUNT];
-
-enum {
-	FIV_IO_WIDE_THUMBNAIL_COEFFICIENT = 2
-};
-
-/// Returns this user's root thumbnail directory.
-gchar *fiv_io_get_thumbnail_root(void);
-
-/// Generates wide thumbnails of up to the specified size, saves them in cache.
-gboolean fiv_io_produce_thumbnail(
-	GFile *target, FivIoThumbnailSize size, GError **error);
-
-/// Retrieves a thumbnail of the most appropriate quality and resolution
-/// for the target file.
-cairo_surface_t *fiv_io_lookup_thumbnail(
-	GFile *target, FivIoThumbnailSize size);
diff --git a/fiv-thumbnail.c b/fiv-thumbnail.c
new file mode 100644
index 0000000..1384213
--- /dev/null
+++ b/fiv-thumbnail.c
@@ -0,0 +1,494 @@
+//
+// fiv-thumbnail.c: thumbnail management
+//
+// Copyright (c) 2021, Přemysl Eric Janouch <p@janouch.name>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+//
+
+#include <spng.h>
+#include <webp/encode.h>
+#include <webp/mux.h>
+#include <glib/gstdio.h>
+
+#include <math.h>
+#include <stdbool.h>
+
+#include "fiv-io.h"
+#include "fiv-thumbnail.h"
+#include "xdg.h"
+
+#ifndef __linux__
+#define st_mtim st_mtimespec
+#endif  // ! __linux__
+
+// TODO(p): Consider merging back with fiv-io.
+#define FIV_THUMBNAIL_ERROR fiv_thumbnail_error_quark()
+
+G_DEFINE_QUARK(fiv-thumbnail-error-quark, fiv_thumbnail_error)
+
+enum FivThumbnailError {
+	FIV_THUMBNAIL_ERROR_IO
+};
+
+static void
+set_error(GError **error, const char *message)
+{
+	g_set_error_literal(
+		error, FIV_THUMBNAIL_ERROR, FIV_THUMBNAIL_ERROR_IO, message);
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+GType
+fiv_thumbnail_size_get_type(void)
+{
+	static gsize guard;
+	if (g_once_init_enter(&guard)) {
+#define XX(name, value, dir) {FIV_THUMBNAIL_SIZE_ ## name, \
+	"FIV_THUMBNAIL_SIZE_" #name, #name},
+		static const GEnumValue values[] = {FIV_THUMBNAIL_SIZES(XX) {}};
+#undef XX
+		GType type = g_enum_register_static(
+			g_intern_static_string("FivThumbnailSize"), values);
+		g_once_init_leave(&guard, type);
+	}
+	return guard;
+}
+
+#define XX(name, value, dir) {value, dir},
+FivThumbnailSizeInfo
+	fiv_thumbnail_sizes[FIV_THUMBNAIL_SIZE_COUNT] = {
+		FIV_THUMBNAIL_SIZES(XX)};
+#undef XX
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+cairo_user_data_key_t fiv_thumbnail_key_lq;
+
+static void
+mark_thumbnail_lq(cairo_surface_t *surface)
+{
+	cairo_surface_set_user_data(
+		surface, &fiv_thumbnail_key_lq, (void *) (intptr_t) 1, NULL);
+}
+
+gchar *
+fiv_thumbnail_get_root(void)
+{
+	gchar *cache_dir = get_xdg_home_dir("XDG_CACHE_HOME", ".cache");
+	gchar *thumbnails_dir = g_build_filename(cache_dir, "thumbnails", NULL);
+	g_free(cache_dir);
+	return thumbnails_dir;
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+// In principle similar to rescale_thumbnail() from fiv-browser.c.
+static cairo_surface_t *
+rescale_thumbnail(cairo_surface_t *thumbnail, double row_height)
+{
+	cairo_format_t format = cairo_image_surface_get_format(thumbnail);
+	int width = cairo_image_surface_get_width(thumbnail);
+	int height = cairo_image_surface_get_height(thumbnail);
+
+	double scale_x = 1;
+	double scale_y = 1;
+	if (width > FIV_THUMBNAIL_WIDE_COEFFICIENT * height) {
+		scale_x = FIV_THUMBNAIL_WIDE_COEFFICIENT * row_height / width;
+		scale_y = round(scale_x * height) / height;
+	} else {
+		scale_y = row_height / height;
+		scale_x = round(scale_y * width) / width;
+	}
+	if (scale_x == 1 && scale_y == 1)
+		return cairo_surface_reference(thumbnail);
+
+	int projected_width = round(scale_x * width);
+	int projected_height = round(scale_y * height);
+	cairo_surface_t *scaled = cairo_image_surface_create(
+		(format == CAIRO_FORMAT_RGB24 || format == CAIRO_FORMAT_RGB30)
+			? CAIRO_FORMAT_RGB24
+			: CAIRO_FORMAT_ARGB32,
+		projected_width, projected_height);
+
+	cairo_t *cr = cairo_create(scaled);
+	cairo_scale(cr, scale_x, scale_y);
+
+	cairo_set_source_surface(cr, thumbnail, 0, 0);
+	cairo_pattern_t *pattern = cairo_get_source(cr);
+	cairo_pattern_set_filter(pattern, CAIRO_FILTER_BEST);
+	cairo_pattern_set_extend(pattern, CAIRO_EXTEND_PAD);
+
+	cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);
+	cairo_paint(cr);
+	cairo_destroy(cr);
+	mark_thumbnail_lq(scaled);
+	return scaled;
+}
+
+static WebPData
+encode_thumbnail(cairo_surface_t *surface)
+{
+	WebPData bitstream = {};
+	WebPConfig config = {};
+	if (!WebPConfigInit(&config) || !WebPConfigLosslessPreset(&config, 6))
+		return bitstream;
+
+	config.near_lossless = 95;
+	config.thread_level = true;
+	if (!WebPValidateConfig(&config))
+		return bitstream;
+
+	bitstream.bytes = fiv_io_encode_webp(surface, &config, &bitstream.size);
+	return bitstream;
+}
+
+static void
+save_thumbnail(cairo_surface_t *thumbnail, const char *path, GString *thum)
+{
+	WebPMux *mux = WebPMuxNew();
+	WebPData bitstream = encode_thumbnail(thumbnail);
+	gboolean ok = WebPMuxSetImage(mux, &bitstream, true) == WEBP_MUX_OK;
+	WebPDataClear(&bitstream);
+
+	WebPData data = {.bytes = (const uint8_t *) thum->str, .size = thum->len};
+	ok = ok && WebPMuxSetChunk(mux, "THUM", &data, false) == WEBP_MUX_OK;
+
+	WebPData assembled = {};
+	WebPDataInit(&assembled);
+	ok = ok && WebPMuxAssemble(mux, &assembled) == WEBP_MUX_OK;
+	WebPMuxDelete(mux);
+	if (!ok) {
+		g_warning("thumbnail encoding failed");
+		return;
+	}
+
+	GError *e = NULL;
+	while (!g_file_set_contents(
+		path, (const gchar *) assembled.bytes, assembled.size, &e)) {
+		bool missing_parents =
+			e->domain == G_FILE_ERROR && e->code == G_FILE_ERROR_NOENT;
+		g_debug("%s: %s", path, e->message);
+		g_clear_error(&e);
+		if (!missing_parents)
+			break;
+
+		gchar *dirname = g_path_get_dirname(path);
+		int err = g_mkdir_with_parents(dirname, 0755);
+		if (err)
+			g_debug("%s: %s", dirname, g_strerror(errno));
+
+		g_free(dirname);
+		if (err)
+			break;
+	}
+
+	// It would be possible to create square thumbnails as well,
+	// but it seems like wasted effort.
+	WebPDataClear(&assembled);
+}
+
+gboolean
+fiv_thumbnail_produce(GFile *target, FivThumbnailSize max_size, GError **error)
+{
+	g_return_val_if_fail(max_size >= FIV_THUMBNAIL_SIZE_MIN &&
+		max_size <= FIV_THUMBNAIL_SIZE_MAX, FALSE);
+
+	// Local files only, at least for now.
+	gchar *path = g_file_get_path(target);
+	if (!path)
+		return FALSE;
+
+	GMappedFile *mf = g_mapped_file_new(path, FALSE, error);
+	if (!mf) {
+		g_free(path);
+		return FALSE;
+	}
+
+	GStatBuf st = {};
+	if (g_stat(path, &st)) {
+		set_error(error, g_strerror(errno));
+		g_free(path);
+		return FALSE;
+	}
+
+	// TODO(p): Add a flag to avoid loading all pages and frames.
+	FivIoProfile sRGB = fiv_io_profile_new_sRGB();
+	gsize filesize = g_mapped_file_get_length(mf);
+	cairo_surface_t *surface = fiv_io_open_from_data(
+		g_mapped_file_get_contents(mf), filesize, path, sRGB, FALSE, error);
+
+	g_free(path);
+	g_mapped_file_unref(mf);
+	if (sRGB)
+		fiv_io_profile_free(sRGB);
+	if (!surface)
+		return FALSE;
+
+	// Boilerplate copied from fiv_thumbnail_lookup().
+	gchar *uri = g_file_get_uri(target);
+	gchar *sum = g_compute_checksum_for_string(G_CHECKSUM_MD5, uri, -1);
+	gchar *thumbnails_dir = fiv_thumbnail_get_root();
+
+	GString *thum = g_string_new("");
+	g_string_append_printf(
+		thum, "%s%c%s%c", "Thumb::URI", 0, uri, 0);
+	g_string_append_printf(
+		thum, "%s%c%ld%c", "Thumb::Mtime", 0, (long) st.st_mtim.tv_sec, 0);
+	g_string_append_printf(
+		thum, "%s%c%ld%c", "Thumb::Size", 0, (long) filesize, 0);
+	g_string_append_printf(thum, "%s%c%d%c", "Thumb::Image::Width", 0,
+		cairo_image_surface_get_width(surface), 0);
+	g_string_append_printf(thum, "%s%c%d%c", "Thumb::Image::Height", 0,
+		cairo_image_surface_get_height(surface), 0);
+
+	// Without a CMM, no conversion is attempted.
+	if (sRGB) {
+		g_string_append_printf(
+			thum, "%s%c%s%c", "Thumb::ColorSpace", 0, "sRGB", 0);
+	}
+
+	for (int use = max_size; use >= FIV_THUMBNAIL_SIZE_MIN; use--) {
+		cairo_surface_t *scaled =
+			rescale_thumbnail(surface, fiv_thumbnail_sizes[use].size);
+		gchar *path = g_strdup_printf("%s/wide-%s/%s.webp", thumbnails_dir,
+			fiv_thumbnail_sizes[use].thumbnail_spec_name, sum);
+		save_thumbnail(scaled, path, thum);
+		cairo_surface_destroy(scaled);
+		g_free(path);
+	}
+
+	g_string_free(thum, TRUE);
+
+	g_free(thumbnails_dir);
+	g_free(sum);
+	g_free(uri);
+	cairo_surface_destroy(surface);
+	return TRUE;
+}
+
+static cairo_surface_t *
+read_wide_thumbnail(
+	const gchar *path, const gchar *uri, time_t mtime, GError **error)
+{
+	// TODO(p): Validate fiv_io_key_thum.
+	(void) uri;
+	(void) mtime;
+	return fiv_io_open(path, NULL, FALSE, error);
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+static int  // tri-state
+check_spng_thumbnail_texts(struct spng_text *texts, uint32_t texts_len,
+	const gchar *target, time_t mtime)
+{
+	// May contain Thumb::Image::Width Thumb::Image::Height,
+	// but those aren't interesting currently (would be for fast previews).
+	bool need_uri = true, need_mtime = true;
+	for (uint32_t i = 0; i < texts_len; i++) {
+		struct spng_text *text = texts + i;
+		if (!strcmp(text->keyword, "Thumb::URI")) {
+			need_uri = false;
+			if (strcmp(target, text->text))
+				return false;
+		}
+		if (!strcmp(text->keyword, "Thumb::MTime")) {
+			need_mtime = false;
+			if (atol(text->text) != mtime)
+				return false;
+		}
+	}
+	return need_uri || need_mtime ? -1 : true;
+}
+
+static int  // tri-state
+check_spng_thumbnail(spng_ctx *ctx, const gchar *target, time_t mtime, int *err)
+{
+	uint32_t texts_len = 0;
+	if ((*err = spng_get_text(ctx, NULL, &texts_len)))
+		return false;
+
+	int result = false;
+	struct spng_text *texts = g_malloc0_n(texts_len, sizeof *texts);
+	if (!(*err = spng_get_text(ctx, texts, &texts_len)))
+		result = check_spng_thumbnail_texts(texts, texts_len, target, mtime);
+	g_free(texts);
+	return result;
+}
+
+static cairo_surface_t *
+read_spng_thumbnail(
+	const gchar *path, const gchar *uri, time_t mtime, GError **error)
+{
+	FILE *fp;
+	cairo_surface_t *result = NULL;
+	if (!(fp = fopen(path, "rb"))) {
+		set_error(error, g_strerror(errno));
+		return NULL;
+	}
+
+	errno = 0;
+	spng_ctx *ctx = spng_ctx_new(0);
+	if (!ctx) {
+		set_error(error, g_strerror(errno));
+		goto fail_init;
+	}
+
+	int err;
+	size_t size = 0;
+	if ((err = spng_set_png_file(ctx, fp)) ||
+		(err = spng_set_image_limits(ctx, INT16_MAX, INT16_MAX)) ||
+		(err = spng_decoded_image_size(ctx, SPNG_FMT_RGBA8, &size))) {
+		set_error(error, spng_strerror(err));
+		goto fail;
+	}
+	if (check_spng_thumbnail(ctx, uri, mtime, &err) == false) {
+		set_error(error, err ? spng_strerror(err) : "mismatch");
+		goto fail;
+	}
+
+	struct spng_ihdr ihdr = {};
+	struct spng_trns trns = {};
+	spng_get_ihdr(ctx, &ihdr);
+	bool may_be_translucent = !spng_get_trns(ctx, &trns) ||
+		ihdr.color_type == SPNG_COLOR_TYPE_GRAYSCALE_ALPHA ||
+		ihdr.color_type == SPNG_COLOR_TYPE_TRUECOLOR_ALPHA;
+
+	cairo_surface_t *surface = cairo_image_surface_create(
+		may_be_translucent ? CAIRO_FORMAT_ARGB32 : CAIRO_FORMAT_RGB24,
+		ihdr.width, ihdr.height);
+
+	cairo_status_t surface_status = cairo_surface_status(surface);
+	if (surface_status != CAIRO_STATUS_SUCCESS) {
+		set_error(error, cairo_status_to_string(surface_status));
+		goto fail_data;
+	}
+
+	uint32_t *data = (uint32_t *) cairo_image_surface_get_data(surface);
+	g_assert((size_t) cairo_image_surface_get_stride(surface) *
+		cairo_image_surface_get_height(surface) == size);
+
+	cairo_surface_flush(surface);
+	if ((err = spng_decode_image(ctx, data, size, SPNG_FMT_RGBA8,
+		SPNG_DECODE_TRNS | SPNG_DECODE_GAMMA))) {
+		set_error(error, spng_strerror(err));
+		goto fail_data;
+	}
+
+	// The specification does not say where the required metadata should be,
+	// it could very well be broken up into two parts.
+	if (check_spng_thumbnail(ctx, uri, mtime, &err) != true) {
+		set_error(
+			error, err ? spng_strerror(err) : "mismatch or not a thumbnail");
+		goto fail_data;
+	}
+
+	// pixman can be mildly abused to do this operation, but it won't be faster.
+	if (may_be_translucent) {
+		for (size_t i = size / sizeof *data; i--; ) {
+			const uint8_t *unit = (const uint8_t *) &data[i];
+			uint32_t a = unit[3],
+				b = PREMULTIPLY8(a, unit[2]),
+				g = PREMULTIPLY8(a, unit[1]),
+				r = PREMULTIPLY8(a, unit[0]);
+			data[i] = a << 24 | r << 16 | g << 8 | b;
+		}
+	} else {
+		for (size_t i = size / sizeof *data; i--; ) {
+			uint32_t rgba = g_ntohl(data[i]);
+			data[i] = rgba << 24 | rgba >> 8;
+		}
+	}
+
+	cairo_surface_mark_dirty((result = surface));
+
+fail_data:
+	if (!result)
+		cairo_surface_destroy(surface);
+fail:
+	spng_ctx_free(ctx);
+fail_init:
+	fclose(fp);
+	return result;
+}
+
+cairo_surface_t *
+fiv_thumbnail_lookup(GFile *target, FivThumbnailSize size)
+{
+	g_return_val_if_fail(size >= FIV_THUMBNAIL_SIZE_MIN &&
+		size <= FIV_THUMBNAIL_SIZE_MAX, NULL);
+
+	// Local files only, at least for now.
+	gchar *path = g_file_get_path(target);
+	if (!path)
+		return NULL;
+
+	GStatBuf st = {};
+	int err = g_stat(path, &st);
+	g_free(path);
+	if (err)
+		return NULL;
+
+	gchar *uri = g_file_get_uri(target);
+	gchar *sum = g_compute_checksum_for_string(G_CHECKSUM_MD5, uri, -1);
+	gchar *thumbnails_dir = fiv_thumbnail_get_root();
+
+	// The lookup sequence is: nominal..max, then mirroring back to ..min.
+	cairo_surface_t *result = NULL;
+	GError *error = NULL;
+	for (int i = 0; i < FIV_THUMBNAIL_SIZE_COUNT; i++) {
+		FivThumbnailSize use = size + i;
+		if (use > FIV_THUMBNAIL_SIZE_MAX)
+			use = FIV_THUMBNAIL_SIZE_MAX - i;
+
+		const char *name = fiv_thumbnail_sizes[use].thumbnail_spec_name;
+		gchar *wide =
+			g_strdup_printf("%s/wide-%s/%s.webp", thumbnails_dir, name, sum);
+		result = read_wide_thumbnail(wide, uri, st.st_mtim.tv_sec, &error);
+		if (error) {
+			g_debug("%s: %s", wide, error->message);
+			g_clear_error(&error);
+		}
+		g_free(wide);
+		if (result) {
+			// Higher up we can't distinguish images smaller than the thumbnail.
+			// Also, try not to rescale the already rescaled.
+			if (use != size)
+				mark_thumbnail_lq(result);
+			break;
+		}
+
+		gchar *path =
+			g_strdup_printf("%s/%s/%s.png", thumbnails_dir, name, sum);
+		result = read_spng_thumbnail(path, uri, st.st_mtim.tv_sec, &error);
+		if (error) {
+			g_debug("%s: %s", path, error->message);
+			g_clear_error(&error);
+		}
+		g_free(path);
+		if (result) {
+			// Whatever produced it, we may be able to outclass it.
+			mark_thumbnail_lq(result);
+			break;
+		}
+	}
+
+	// TODO(p): We can definitely extract embedded thumbnails, but it should be
+	// done as a separate stage--the file may be stored on a slow device.
+
+	g_free(thumbnails_dir);
+	g_free(sum);
+	g_free(uri);
+	return result;
+}
diff --git a/fiv-thumbnail.h b/fiv-thumbnail.h
new file mode 100644
index 0000000..9dcf3f6
--- /dev/null
+++ b/fiv-thumbnail.h
@@ -0,0 +1,64 @@
+//
+// fiv-thumbnail.h: thumbnail management
+//
+// Copyright (c) 2021, Přemysl Eric Janouch <p@janouch.name>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+//
+
+#pragma once
+
+#include <cairo.h>
+#include <gio/gio.h>
+#include <glib.h>
+
+// And this is how you avoid glib-mkenums.
+typedef enum _FivThumbnailSize {
+#define FIV_THUMBNAIL_SIZES(XX) \
+	XX(SMALL,  128, "normal") \
+	XX(NORMAL, 256, "large") \
+	XX(LARGE,  512, "x-large") \
+	XX(HUGE,  1024, "xx-large")
+#define XX(name, value, dir) FIV_THUMBNAIL_SIZE_ ## name,
+	FIV_THUMBNAIL_SIZES(XX)
+#undef XX
+	FIV_THUMBNAIL_SIZE_COUNT,
+
+	FIV_THUMBNAIL_SIZE_MIN = 0,
+	FIV_THUMBNAIL_SIZE_MAX = FIV_THUMBNAIL_SIZE_COUNT - 1
+} FivThumbnailSize;
+
+GType fiv_thumbnail_size_get_type(void) G_GNUC_CONST;
+#define FIV_TYPE_THUMBNAIL_SIZE (fiv_thumbnail_size_get_type())
+
+typedef struct _FivThumbnailSizeInfo {
+	int size;                           ///< Nominal size in pixels
+	const char *thumbnail_spec_name;    ///< thumbnail-spec directory name
+} FivThumbnailSizeInfo;
+
+extern FivThumbnailSizeInfo fiv_thumbnail_sizes[FIV_THUMBNAIL_SIZE_COUNT];
+
+enum { FIV_THUMBNAIL_WIDE_COEFFICIENT = 2 };
+
+/// If non-NULL, indicates a thumbnail of insufficient quality.
+extern cairo_user_data_key_t fiv_thumbnail_key_lq;
+
+/// Returns this user's root thumbnail directory.
+gchar *fiv_thumbnail_get_root(void);
+
+/// Generates wide thumbnails of up to the specified size, saves them in cache.
+gboolean fiv_thumbnail_produce(
+	GFile *target, FivThumbnailSize max_size, GError **error);
+
+/// Retrieves a thumbnail of the most appropriate quality and resolution
+/// for the target file.
+cairo_surface_t *fiv_thumbnail_lookup(GFile *target, FivThumbnailSize size);
diff --git a/meson.build b/meson.build
index ecad76f..50ac6f2 100644
--- a/meson.build
+++ b/meson.build
@@ -71,7 +71,7 @@ resources = gnome.compile_resources('resources',
 )
 
 exe = executable('fastiv', 'fastiv.c', 'fiv-view.c', 'fiv-io.c',
-	'fiv-browser.c', 'fiv-sidebar.c', 'xdg.c', resources,
+	'fiv-browser.c', 'fiv-sidebar.c', 'fiv-thumbnail.c', 'xdg.c', resources,
 	install : true,
 	dependencies : [dependencies])