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

[From: Michael Stepanov]

Just fixed issue with wrong color of pixels by creating 16bit pixel instead
of 32bit:

Uint16  PlutoPixelDest = ((ReplacementColor.R() >> PF->Rloss) << PF->Rshift)
+
                             ((ReplacementColor.G() >> PF->Gloss) <<
PF->Gshift) +
                             ((ReplacementColor.B() >> PF->Bloss) <<
PF->Bshift);

where ReplacementColor is structure with 32bit RGB.


On Mon, Sep 1, 2008 at 4:27 PM, Michael Stepanov <michael at stepanoff.org>wrote:

>
>
> 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
>



-- 
Cheers,
Michael
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