https://bugs.kde.org/show_bug.cgi?id=468203
--- Comment #21 from Fushan Wen <qydwhotm...@gmail.com> ---
For anyone interested, I think this is still a bug in GTK3(Wayland only) and
GTK4. See `gtk_css_image_surface_draw` for example, image_width and
image_height are hardcoded values (For GTK3 it's 18 on Wayland, 50 on X11),
which means when the size of a background image exceeds 18x18 on Wayland, it
will be blurry.
```
static void
gtk_css_image_surface_draw (GtkCssImage *image,
cairo_t *cr,
double width,
double height)
{
GtkCssImageSurface *surface = GTK_CSS_IMAGE_SURFACE (image);
int image_width, image_height;
image_width = cairo_image_surface_get_width (surface->surface);
image_height = cairo_image_surface_get_height (surface->surface);
if (image_width == 0 || image_height == 0 || width <= 0 || height <= 0)
return;
/* Update cache image if size is different */
if (surface->cache == NULL ||
ABS (width - surface->width) > 0.001 ||
ABS (height - surface->height) > 0.001)
{
double xscale, yscale;
cairo_t *cache;
/* We need the device scale (HiDPI mode) to calculate the proper size in
* pixels for the image surface and set the cache device scale
*/
cairo_surface_get_device_scale (cairo_get_target (cr), &xscale, &yscale);
/* Save original size to preserve precision */
surface->width = width;
surface->height = height;
/* Destroy old cache if any */
g_clear_pointer (&surface->cache, cairo_surface_destroy);
/* Image big enough to contain scaled image with subpixel precision */
surface->cache = cairo_surface_create_similar_image (surface->surface,
CAIRO_FORMAT_ARGB32,
ceil (width*xscale),
ceil
(height*yscale));
cairo_surface_set_device_scale (surface->cache, xscale, yscale);
cache = cairo_create (surface->cache);
cairo_rectangle (cache, 0, 0, width, height);
cairo_scale (cache, width / image_width, height / image_height);
cairo_set_source_surface (cache, surface->surface, 0, 0);
cairo_fill (cache);
cairo_destroy (cache);
}
cairo_rectangle (cr, 0, 0, width, height);
cairo_set_source_surface (cr, surface->cache ? surface->cache :
surface->surface, 0, 0);
cairo_fill (cr);
}
```
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