When simultaneously fading two items (one from fully opaque to transparent and the other one vice-versa) you can briefly see the black background through the images leading to an ugly flicker.
By only doing this when the old image could be seen (when the new one has a smaller size) we avoid this in the common case of full-size wallpapers.
Added a large and tiny wallpaper, randomly switched between them, also played around with various scale modes. It no longer flickers everytime the wallpaper is changed.
This obviously breaks (as in you can see the old image which then suddenly disapperas once the animation finishes) images with alpha channel but I don't think we actively support/encourge that? It's just a corner-case anyway imho.
I noticed that because kscreenlocker greeter sets the image size after the wallpaper has loaded causing it to reload (source size changed) and this flickered when the lock screen showed up. This issue needs to be addressed separately but this patch still gives us a more polished look, especially when using a slideshow wallpaper.
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hmm, it shouldn't cause flicker, fading between 2 different sized wallpapers should be just fine
The correct way to fix this is to use a custom shader that does approximately this:
uniform sampler2D texture1;
uniform sampler2D texture2;
uniform float fadeProgress;
varying vec2 texcoord1;
varying vec2 texcoord2;
void main(void) {
vec4 texel1 = texture2D(texture1, texcoord1);
vec4 texel2 = texture2D(texture2, texcoord2);
gl_FragColor = mix(texel1, texel2, fadeProgress);
}As an added bonus you only render each pixel once, and you don't need blending to be enabled, so it's much faster. Especially on tiled renderers.