Hi folks,
Unfortunately, the problem is not in the fonts code. There is a bug in Bitblt, rule 28. Rule 28 is supposed to take the minimum value between source and destination. I use it to render 32 bit per pixel fonts. Unfortunately there is a bug in BitBlt. The result values for r, g and b are computed correctly. But the alpha channel is always set at zero (i.e. transparent). Usually the VM / OS ignores the alpha channel of the display. But with composite window managers on Linux, it is honored, and the bug is exposed.
You can try for yourself. Evaluate:
(Display pixelValueAt: 10@10)printStringBase: 16
You'll get something like this '16rFF455286'. The FF at the first byte is alpha. FF means opaque. Now, run the following code, and with the mouse go and paint the top left area of your display. Then do click. Do not clean the garbage on the screen yet.
f := Form extent: 20 @ 20 depth: 32. f fillWithColor: Color red. aBitBlt := BitBlt destForm: Display sourceForm: f fillColor: nil combinationRule: 28 destOrigin: Sensor cursorPoint sourceOrigin: 0 @ 0 extent: f extent clipRect: Display computeBoundingBox. [Sensor anyButtonPressed] whileFalse: [aBitBlt destOrigin: Sensor cursorPoint - (10@10). aBitBlt copyBits].
Now check (f pixelValueAt: 10@10) printStringBase: 16. You'll see that inside form f, the alpha is FF, i.e. opaque again.
Now print (Display pixelValueAt: 10@10) printStringBase: 16. I get '16r450000'. Adding the missing leading zeros, it is '16r00450000'. You can see that Bitblt correctly left the red unchanged and made zero green and blue. Unfortunately, alpha went to zero.
I believe this is a bug. Rule 28 means "take the minimum between source and destination". It could do the same for alpha, or it could leave alpha unchanged. Both would be acceptable behaviors. But setting alpha to zero for no good reason is a bug.
Perhaps there is some bitblt rule that can be used for setting alpha to 255 after rendering text. If someone knows how to do it, it could be a good workaround.
Another workaround is to do Prederences setPreferencesFrom: #((subPixelRenderFonts false)) This will turn off subpixel rendering, using only gray levels for AA. This uses BitBlt rule 34, that does not have this problem.
Cheers, Juan Vuletich
Mariano Martinez Peck wrote:
WRT composite window managers on Linux, it seems I need to make sure the alpha channel is set to 1.0. I'll do it. I'll need help with testing it, though.Just let me know when and how and I will test it as soon as I have time. I was one of those who has that problem.
cheers,
mariano
pd: juan: si me pedis algo, pedimelo en castellano jajaj ;)
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