[SDL] Pixel byte manipulation question.

[matt hull]

On Oct 3, 2007, at 4:07 PM, Paul Duffy wrote:

> On Wed, 2007-10-03 at 13:49 -0500, matt hull wrote:
>>>
>>>> Anyway, it is possible to have a pixel laid out with the RGB masks
>>>> you
>>>> listed. Not common, but possible.
>>>
>>> Gack. I'll have to see if the screen blitting functions would accept
>>> such a format, I'd rather avoid having to check the dest/source  
>>> masks
>>> every time. If the format is reasonably predictable then the worst I
>>> have to do is use some ifdefs to use the appropriate formatting per
>>> platform rather than having to use runtime detection. I'm mainly
>>> trying
>>> to avoid doing anything unnecessary that would slow things down.
>>>
>>
>> ifdef's should be avoided.  those are only for compile time.
>
> Yep. and when I use them they will be for compile time and sorting the
> code appropriate for the architecture.
>
> Anyway, found the answer in that << will actually push in bits from  
> the
> left to the right on a little endian system and right to left on a big
> endian system (I make it sound like the hardware abstraction in C is a
> bad thing) so:
>
> a Uint32 is given the values 15 << 16 | 235 << 8 | 124
>
> If passed to a 4 byte array as *(Uint32 *)arr = i, the values in the
> array will be 124, 235, 15, 0 in a little-endian system whereas in a
> big-endian system they will be 0, 15, 235, 124.
>

that is an incorrect assumption.  i tried that myself and had to fix  
my application.  you need to use the mask.  the system endianness  
doesnt tell all.

> So, assuming the RGBmask and colorkey are of the appropriate values  
> (and
> they should be) all that's required to pass the complete value in this
> way on a big-endian system is << 8 whereas a little endian system will
> require some byte-swapping too.
>
> Of course, I could just use bit-shifting to pass the appropriate  
> values
> one at a time but that would require a lot of checking (at least  
> with 16
> bit values) of the RGB masks which I'm trying to avoid.

how often are you changing your pixels ?   is it possible you could  
set up your palette or surface during an initialization?

matt