[Vm-dev] Bug in BitBlt. Need help.

[Juan Vuletich]

Hi Folks,

A recent issue with AA StrikeFonts reported in squeak-dev made me do 
some tests with BitBlt. I found that a couple of operations work ok on 
the simulator but give incorrect results on the VM. I guess there might 
be some 32 bit overflow somewhere, or some value that should be unsigned 
and is int. The following code:

" 20    rgbAdd:with:
 21    rgbSub:with:
 27    rgbMax:with
 28    rgbMin:with:
 29    rgbMinInvert:with:
 37    rgbMul:with: "
rules _ #(20 21 27 28 29 37).
results _ rules collect: [ :r |
   f1 := Form extent: 8 at 8 depth: 32.
   f1 fillColor: (Color r: 0.2 g: 0.2 b: 0.3 alpha: 0.2).
   f2 := Form extent: 8 at 8 depth: 32.
   f2 fillColor: (Color r: 0.1 g: 1.0 b: 0.1 alpha: 0.1).
   bb :=(BitBlt toForm: f1) sourceForm: f2; combinationRule: r; 
copyBitsSimulated.
   c := f1 colorAt: 2 at 2.
   (c red roundTo: 0.001) @(c alpha roundTo: 0.001) ].

Answers the following:
 #(0.297 at 0.298 0.102 at 0.102 0.199 at 0.2 0.098 at 0.098 0.199 at 0.2 0.02 at 0.02)
Each point is for one of the tested rules. The x is the value for red, 
and the y is the value for alpha. I this case, the values are the same 
(or at least very close).

But if I change #copyBitsSimulated for #copyBits I get:
 #(0.297 at 1.0 0.102 at 0.102 0.199 at 0.2 0.098 at 0.098 0.199 at 0.902 0.02 at 0.02)

Note that rgbAdd and rgbMinInvert have completely wrong values. I guess 
this has to do with alpha being the MSB in each word, and perhaps there 
is some overflow, but looking at the code, and running some tests in the 
simulator I could not find it. I didn't go as far as using a C debugger, 
and perhaps somebody can help.

Thanks,
Juan Vuletich