Calculate masks for SDL_CreateRGBSurface?

[paul_nicholls]

Hi all,
I want to be able to simply create a SDL surface using the SDL_CreateRGBSurface() routine, but only if I am given a width, height, and one of a few common bit depths. I need to calculate the 4 masks for that routine that depends on the endiness of the system.

So given a bit depth like so:

Type
    TBitDepth = (
        eBitDepth_8Bit,
        eBitDepth_15Bit,
        eBitDepth_16Bit,
        eBitDepth_24Bit,
        eBitDepth_32Bit
    );

Is there any 'easy' way of calculating the R, G, B and A masks for that depth and endiness?

Procedure CalculateRGBAMasks(Const ABitDepth               : TBitDepth;
                             Var   RMask,GMask,BMask,AMask : LongWord);
Begin
    // calculate masks required for bit depth?
End;

I am figuring it may be something like so (for little endian)

Procedure CalculateRGBAMasks(Const ABitDepth               : TBitDepth;
                             Var   RMask,GMask,BMask,AMask : LongWord);
Begin
    RMask := 0;
    GMask := 0;
    BMask := 0;
    AMask := 0;
    If SDL_BYTEORDER = SDL_LIL_ENDIAN Then
    Begin
        If ABitDepth In[eBitDepth_15Bit,eBitDepth_16Bit] Then
        Begin
            RMask := 31;         // 5 bits wide
            GMask := 63 Shl 5;   // 6 bits wide
            BMask := 31 Shl 11;  // 5 bits wide
        End
        Else
        If ABitDepth = eBitDepth_24Bit Then
        Begin
            RMask := 8;
            GMask := 8 Shl 8;
            BMask := 8 Shl 16;
        End
        Else
        If ABitDepth = eBitDepth_32Bit Then
        Begin
            RMask := 8;
            GMask := 8 Shl 8;
            BMask := 8 Shl 16;
            AMask := 8 Shl 24;
        End;
    End
    Else
    // bit endian format
    Begin
    End;
End;

If the little endian part is correct, I am not sure what the big endian version should be...

Thanks for any help you can give me :-)
cheers,
Paul

[paul_nicholls]

Ok, after testing on my Windows PC, the small endian version seems correct, but I am not sure what the big endian code would look like, and I can't test it anyway...

if someone could workout some code and maybe test it on their big endian machine (if possible) that would be great

Below is the routine that would use the calculated masks.

Function CreateSurface(Const AWidth,AHeight : Word;
                       Const ABitDepth      : TBitDepth) : PSDL_Surface;
Var
    bpp        : Byte;
    RMask      : LongWord;
    GMask      : LongWord;
    BMask      : LongWord;
    AMask      : LongWord;
Begin
    Case ABitDepth Of
        eBitDepth_8Bit  : bpp := 8;
        eBitDepth_15Bit : bpp := 15;
        eBitDepth_16Bit : bpp := 16;
        eBitDepth_24Bit : bpp := 24;
        eBitDepth_32Bit : bpp := 32;
    Else
        bpp := 16;
    End;
    CalculateRGBAMasks(ABitDepth,RMask,GMask,BMask,AMask);
    Result := SDL_CreateRGBSurface(SDL_SWSURFACE,
                                   AWidth,
                                   AHeight,
                                   bpp,
                                   RMask,
                                   GMask,
                                   BMask,
                                   AMask);
    FillChar(PByte(Result^.pixels)^,AWidth * AHeight * Result^.format^.BytesPerPixel,0);
End;

cheers,
Paul