Cursor translucency problem?

[lenrik1589]

Description

Inconsistent blending of cursor image when using translucency in the image

Expected Behavior

Using cursor(img, x, y) and exporting the image and using it with X should produce same visual result

Current Behavior

System: P2D: (colored sections are 100/255 alpha) (colored sections are 10/255 alpha)

Steps to Reproduce

1. create image with translucency
2. apply using cursor(img, x, y)
3. observe how translucent colors interact with the backdrop
4. export the image.save(file)
5. create left_ptr using xcursorgen
6. import as a cursor into the system and apply
7. observe as cursor now blends differently with the background

(might include code a little later, when come home)

Your Environment

• Processing version: 4.3 (imported through gradle as implementation("com.github.micycle1:processing-core-4:4.+"))
• Operating System and OS version: Linux (gemeric arch, kernel 6.6.7)
• Other information:
  • Wayland Plasma 6.0 Beta 1
  • kwin 5.90.0
  • Xwayland 23.2.3-1
  • mesa 23.3.1
  • Java: 21

Note: the discrepancy existed even when i was using a native x11 session with stable Plasma 5 and processing 3.5 in the processing editor

IIRC there might be a workaround, of using a different renderer, but i can not use it as i found no way to consistently resize a window after initialization or programmatically close other windows (produced by the same program but different PApplet) with anything other than P2D or P3D as i currently use reflection to gain access internal resize method and ((PSurfaceJOGL)surface).finish() ((GLWindow)surface.native).dispose() to close windows

(the older version of description particularly, i am running into issue that using P2D (xwayland + opengl) as the renderer for all my sketch windows and other graphics, my custom cursor (using cursor(img, hotspotX, hotspotY)) seems to render incorrectly compared to cursor theme made by exporting the same images as png files, and assembling into a xcursor image to be used with the system )

Possible Causes / Solutions

It might be a discrepancy between how OpenGL and KWin handle cursors

[lenrik1589]

Asked in KWin matrix room, and got this:

maybe rgb values are not premultiplied with alpha?

so i added a lerp(color(0, 0, 0, 0), fullColor, alpha(fullColor)) and it really helped

well, that's expected iirc we expect rgb values to be premultiplied with alpha channel

(quotes are from matrix user @zzag:kde.org) So it really seems that on certain renderers + platforms image given to cursor(img) and cursor(img, x, y) needs to be alpha-premultiplied?