[maemo-developers] SDL, pixel format and little endian issue

[From: Michael Stepanov]

On Mon, Sep 1, 2008 at 4:16 PM, Frantisek Dufka <dufkaf at seznam.cz> wrote:

> Michael Stepanov wrote:
>
>> Hi,
>>
>> I have a SDL application where I have to replace color of specified
>> pixels. So, I have a color in RGB representation and convert it to the pixel
>> color using that approach:
>>
>> Uint32 PixelSrc = (0 << PF->Rshift) | (128 << PF->Gshift) | (192 <<
>> PF->Bshift);
>>
>> But as result I get color 0x10c0 instead of 0x0080C0. Why it's happens?
>>
>
> You did not mention pixel format, if you use device native 16 bit format
> (RGB565?) you cannot use values like 128 or 192 since it will overflow 5 or
> 6 bits. Also you should do proper masking.
>
>  Moreover, I found following values for Xshift for little endian:
>>
>> const int rshift = 0, gshift = 8, bshift = 16, ashift = 24;
>> Uini32 PixelDest = (0 << rshift) | (128 << gshift) | (192 << bshift);
>>
>> But the byte order is different. I get 0xc08000 but it should be 0x0080c0.
>>
>
> 00c08000 seems fine to me for 0<<0 | 0x80<<8 |0xC0<<16.
>
> I think you should add a bit more of your example code and also tell us
> what you want to do (and maybe even why).


Ok, there is an SDL application which was ported from x86 to Nokia. The
issue with color is following. The application finds some icons on the
picture (they are all pink - 255, 102, 255) and replace them by proper color
- yellow, red, gray etc. The replacement colors are 32bit but Nokia uses
16bit as you already mentioned. So, I found the way how to compare pink
color with actual color of specified pixel:

SDL_GetRGB((Uint32)Pixel, m_pScreenImage->format, &red, &green, &blue);

where Pixel is retrieved from the surface in the loop.

But now I have to fill pink pixel by specified color. And as I understand I
should convert that color from 32bit to 16bit? How can I do it?

Maybe that's why Xshift works wrongly because I try to use 32bit color
definition with 16bit surface?


>
>
> Frantisek
>



-- 
Cheers,
Michael
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