[9fans] libdraw α differences between plan 9 and p9p

[9fans] libdraw α differences between plan 9 and p9p

[erik quanstrom]

i've noticed a difference in how the α channel looks in p9p vs.
plan 9. for example when porting mahjongg to plan9port,
i need to make this change to make the selection highlighting
apparent:

/sys/src/games/mahjongg//mahjongg.c:75,81 - ./mahjongg.c:75,81
  {
  	Rectangle one = Rect(0, 0, 1, 1);
  	
- 	selected = eallocimage(one, 1, RGBA32, setalpha(DPalebluegreen, 0x5f));
+ 	selected = eallocimage(one, 1, RGBA32, setalpha(DPalebluegreen, 0x9f));

is a mapping needed to convert libdraw α values to X?

- erik

p.s. p9p mahjongg is on sources but it's missing the tile sets and lib configuration.

Re: [9fans] libdraw α differences between plan 9 and p9p

[Russ Cox]

> i've noticed a difference in how the α channel looks in p9p vs.
> plan 9. for example when porting mahjongg to plan9port,
> i need to make this change to make the selection highlighting
> apparent:
> 
> - 	selected = eallocimage(one, 1, RGBA32, setalpha(DPalebluegreen, 0x5f));
> + 	selected = eallocimage(one, 1, RGBA32, setalpha(DPalebluegreen, 0x9f));
> 
> is a mapping needed to convert libdraw α values to X?

This is the wrong question.  The right question is why you 
changed this line:

  	if(level.board[d][x][y].clicked)
-   		draw(img, r, selected, nil, ZP);
+   		draw(img, r, selected, selected, ZP);

Since you're drawing selected through itself, you're effectively
squaring the alpha, and (0x9F^2)/255 is approximately 0x5F, so
this change necessitates the first one.

I see why you changed the draw call, though, since on X, it
just ignored the alpha channel in selected and just draws
a big solid-color rectangle over the tile.  This was due to a 
bug in the X wrappers around libdraw and is now fixed.

Russ

P.S. Upas/marshal encodes subjects "correctly" now too.