Andreas,

Thank you very much for you answer, it help me a lot! I just included all
these sanity checks to my code.

But I still don't find the way to get the alpha part of pixel. With this
code:

   alpha := (pixel bitAnd: 16rFF000000) >> 24.

alpha is always 0.  Is It the way to get the alpha part for pixels in 32
bits Forms?

The other 3 bytes hold the RGB triplet and I can access them without
problems.

TIA,

Diego


> Diego,
>
> There are actually various problems in the code you're quoting. A safe
> way to deal with this is along the following lines:
>
> 	| formOop bitsOop bitsPtr |
> 	self var: #bitsPtr type: 'unsigned int*'.
>
> 	"... grab other stuff before..."
> 	bitsOop := interpreterProxy fetchPointer: 0 ofObject: formOop.
>
> 	"first test if bitsOop is really what we're expecting"
> 	((interpreterProxy isInteger: bitsOop) or:[
> 		(interpreterProxy isWords: bitsOop) not])
> 			ifTrue:[^interpreterProxy primitiveFail].
>
> [note: those tests above *MUST* be done as it is quite possible that
> forms contain either integers (surface references) or byte arrays
> (hibernated). Not testing for this will hose your system].
>
> 	"now test if the length of the bitmap is right"
> 	(interpreterProxy slotSizeOf: bitsOop) = expectedSize
> 		ifFalse:[^interpreterProxy primitiveFail]
>
> [again, this test is needed if for some reason the format of the form
> has been screwed up. the expectedSize here is whatever the number of
> words for a form of that size/depth is expected to be; e.g., for a
> 32bpp form just width*height]
>
> 	"now grab the pointer to the bits"
> 	bitsPtr := interpreterProxy firstIndexableField: bitsOop.
>
> [At this point you have a C 'unsigned int *' pointer to those bits. If
> you wanted to work on a byte pointer instead you could just change the
> declaration to be 'unsigned char *' but watch the indexing!]
>
> 	0 to: expectedSize-1 do:[:i| "for all words..."
> 		pixel := bitsPtr at: i.
> 		... etc ...
>
> You should be pretty safe doing it the above way as it will ensure all
> of the invariants are checked.
>
> Cheers,
>  - Andreas
>
>> -----Original Message-----
>> From: squeak-dev-bounces at lists.squeakfoundation.org
>> [mailto:squeak-dev-bounces at lists.squeakfoundation.org] On
>> Behalf Of diegogomezdeck at consultar.com
>> Sent: Monday, April 07, 2003 3:58 PM
>> To: squeak-dev at lists.squeakfoundation.org
>> Subject: Form manipulation from a Plugin
>>
>>
>> Hi list...
>>
>> I'm processing a 32 bits Form in a slang-plugin.
>>
>> The method receive the Form as a parameter:
>>
>>    formOop := interpreterProxy stackObjectValue: 1.
>>
>> Then I ask for widht, height and a pointer to bits:
>>
>>    bits := interpreterProxy fetchPointer: 0 ofObject: formOop.
>>    width := self fetchIntOrFloat: 1 ofObject: formOop.
>>    height := self fetchIntOrFloat: 2 ofObject: formOop.
>>
>> Then I iterate over the bits:
>>
>>    bits
>>       to: bits + (height - 1 * (width * 4))
>>       by: 4
>>       do: [:pos |
>>          pixel := interpreterProxy longAt: pos.
>>          alpha := (pixel bitAnd: 16rFF000000) >> 24.
>>          <some other stuff>
>>       ].
>>
>> The problem I have is the following: The alpha part of pixel
>> is always 0.
>> (I don't need to say that the form has not transparent colors)
>>
>> Any idea of my problem?
>>
>>
>> TIA,
>>
>> Diego Gomez Deck
>>
>>
>>
>>
>>