Bert Freudenberg wrote:
> Folks, it would be really helpful if you adapt the subject when the 
> discussion strays from the original topic of a thread. Thanks.
>
Indeed. Apologies.

> Comments below.
>
> On 30.03.2009, at 00:44, Juan Vuletich wrote:
>
>> It is a BitBlt bug. Evaluate and print this:
>>
>> f1 _ Form extent: 8 at 8 depth: 32.
>> f1 fillColor: (Color r: 1.0 g: 1.0 b: 0.7 alpha: 0.6).
>> f2 _ Form extent: 8 at 8 depth: 32.
>> f2 fillColor: (Color r: 0.5 g: 1.0 b: 0.7 alpha: 0.7).
>> bb _(BitBlt toForm: f1) sourceForm: f2; combinationRule: 28; copyBits.
>> f1 display.
>> (f1 colorAt: 2 at 2) alpha
>>
>> Rule 28 is rgbMin. The resulting color should be (Color r: 0.5 g: 1.0 
>> b: 0.7 alpha: 0.6). But alpha is set to zero. This is a bug, and it 
>> is what causes wrong alpha values in Display in Cuis.
>
> Not a bug per se, it's called rgbMin and not rgbaMin, right? Besides, 
> even it would take the min of the alpha, then the alpha in the frame 
> buffer would still not be 1 to indicate opaqueness.
>
I believe it is a bug. Reasonable answers would be alpha = 1 (always 
generate opaque colors), 0.6 (previous destination alpha), 0.7 (max 
alpha between source and destination) or 0.65 (mean alpha between source 
and destination). Any of these results would be consistent with the 
result for other bit depths. Setting alpha to zero means that if the 
destination depth is 32, rule 28 will always fill destination with 
transparent, ignoring everything else. I think this can never be the 
desired result.

BTW, the same problem happens with the following rules:
    20    rgbAdd: sourceWord with: destinationWord.  Sum of color components
    21    rgbSub: sourceWord with: destinationWord.  Difference of color 
components
    27    rgbMax: sourceWord with: destinationWord.  Max of each color 
component.
    28    rgbMin: sourceWord with: destinationWord.  Min of each color 
component.
    29    rgbMin: sourceWord bitInvert32 with: destinationWord.  Min 
with (max-source)
    37    rgbMul:with:

All these rules are currently useless if destination is 32 bit deep.

> I can't remember what the rgbMin rule is used for. Changing it might 
> break something, or it might now. Does anyone know?
>
It should break nothing. Nobody can be actually using that transparent 
in a meaningful way.

> OTOH, isn't using min for text rendering a hack at best, which only 
> works for black text?
>
Yes. A hack at its best! See BitBlt >> 
#installStrikeFont:foregroundColor:backgroundColor: (in Cuis). It is the 
best I could do without requiring rule 41, that is not always available. 
For other colors (not black) I turn off subpixel rendering, and use some 
other rule (if possible, 34, alpha blend). See that I chose the rule and 
color map carefully for different text color and destination depth.

> - Bert -
>

Cheers,
Juan Vuletich