Details Contained within an SDL_PixelFormat Struct

An SDL_PixelFormat structure holds all the necessary details for SDL to interpret the raw pixel data stored within an SDL_Surface. It essentially defines the "rules" for translating the bytes in memory into actual colors.

Here are the key members of the SDL_PixelFormat struct:

• format: An enumeration value (like SDL_PIXELFORMAT_RGB888) that uniquely identifies the format. This is often the most convenient way to check a format type.
• palette: A pointer to an SDL_Palette structure. This is only used for "palettized" or "indexed" color modes, where pixel values are indices into a color lookup table. For direct color modes like RGB888, this is usually nullptr.
• BitsPerPixel: The total number of bits used to store a single pixel's data (e.g., 8, 16, 24, 32).
• BytesPerPixel: The total number of bytes used to store a single pixel's data. This is typically BitsPerPixel / 8.

The following members describe how the Red, Green, Blue, and Alpha (transparency) channels are packed within the bits of a single pixel:

• Rmask, Gmask, Bmask, Amask: These are bitmasks used to isolate the bits corresponding to each color channel. Applying a bitwise AND operation between a pixel's integer value and a mask yields the value of that specific channel (shifted to the mask's position). We cover masks in more detail separately.
• Rloss, Gloss, Bloss, Aloss: The number of bits lost for each channel when packing the color. Usually 0 for common formats.
• Rshift, Gshift, Bshift, Ashift: The number of bits to right-shift a masked pixel value to get the channel's value aligned to the least significant bit (LSB).
• refcount: Used internally by SDL for reference counting. You generally don't need to interact with this directly.
• next: Used internally by SDL to link pixel formats together. You generally don't need to interact with this directly.

Example Interpretation

Let's consider a common 32-bit format like SDL_PIXELFORMAT_RGBA8888. The SDL_PixelFormat for this might contain:

• BitsPerPixel: 32
• BytesPerPixel: 4
• Rmask: 0xFF000000 (Red channel in the most significant byte)
• Gmask: 0x00FF0000 (Green channel in the next byte)
• Bmask: 0x0000FF00 (Blue channel in the next byte)
• Amask: 0x000000FF (Alpha channel in the least significant byte)
• Rshift: 24
• Gshift: 16
• Bshift: 8
• Ashift: 0

This tells SDL that for a 32-bit pixel value, the first 8 bits are Red, the next 8 are Green, the next 8 are Blue, and the last 8 are Alpha. The masks allow SDL functions like SDL_MapRGB and SDL_GetRGB to correctly pack and unpack color components into/from the raw integer pixel representation according to these rules.

You typically get an SDL_PixelFormat* from an existing SDL_Surface (via its format member) rather than creating one manually.