Puzzled about Form#stencil

> Hello Squeakers!
>
> I am really puzzled about this: I have 2 PNGs, both with transparent
> pixels. Although both are rendered accordingly when I ask for the
> "stencil" (map of transparent vs. opaque pixels) one of them gives me
> the correct image, whereas for the other I just get a black box.
>
> Using the files attached, "triangle-sketch" gives me the correct mask
> and "home-gimp..." gives me the black box.
>
> I evaluate this in a workspace, first by d&d the files into Squeak
> (4.3, all-in-one, Mac), and then d&d into the workspace:
>
> (SketchMorph withForm: sketch305 form stencil) openInHand.
> (SketchMorph withForm: sketch1737 form stencil) openInHand.
>
> I show the situation in the attached screenshot.
>
> Any idea why this is so? Thanks in advance!
>
> Sebastian
> <triangle-sketch.png><home-gimp-100x100.png><SqueakScreen.1.jpg>

>
> sketch305 form bits first hex
> ==> '16r0'
>
> sketch1737 form bits first hex
> ==> '16rFFFFFF'
>
> - Bert -
>
>

> I still don't get it... wouldn't it mean that it should be a full
> white opaque pixel? (I'm no graphics expert)
>
> And still, isn't this contradictory?
>
> self isTransparentAt: 0@0.   ==> false
> self colorAt: 0@0.       ==> Color transparent
>
> Thanks again!
>
> Sebastian
>
>
> 2012/11/14 Bert Freudenberg <[hidden email]>:
>>
>> sketch305 form bits first hex
>> ==> '16r0'
>>
>> sketch1737 form bits first hex
>> ==> '16rFFFFFF'
>>
>> - Bert -
>>
>>

> Thanks again!
>
> Sebastian
>
>
> 2012/11/14 Bert Freudenberg <[hidden email]>:
>>
>> sketch305 form bits first hex
>> ==> '16r0'
>>
>> sketch1737 form bits first hex
>> ==> '16rFFFFFF'
>>
>> - Bert -

> On 14.11.2012, at 17:57, Sebastian Nozzi <[hidden email]> wrote:
>
>> I still don't get it... wouldn't it mean that it should be a full
>> white opaque pixel? (I'm no graphics expert)
>
> No, it's a white pixel with alpha (opaqueness) of 0. Opaque white would be 16rFFFFFFFF, not 16r00FFFFFF.
>
>> And still, isn't this contradictory?
>>
>> self isTransparentAt: 0@0.   ==> false
>> self colorAt: 0@0.       ==> Color transparent
>
> For historical reasons, only a pixel value of 0 is treated as transparent by BitBlt.
>
> - Bert -
>
>> Thanks again!
>>
>> Sebastian
>>
>>
>> 2012/11/14 Bert Freudenberg <[hidden email]>:
>>>
>>> sketch305 form bits first hex
>>> ==> '16r0'
>>>
>>> sketch1737 form bits first hex
>>> ==> '16rFFFFFF'
>>>
>>> - Bert -
>