Image Format

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URL: https://www.opengl.org/wiki/Image_Format

         Image Format

An Image Format describes the way that the images in Textures and renderbuffers store their data. They define the meaning of the image's data.
There are three basic kinds of image formats: color, depth, and depth/stencil. Unless otherwise specified, all formats can be used for textures and renderbuffers equally. Also, unless otherwise specified, all formats can be multisampled equally.

Color formats

Colors in OpenGL are stored in RGBA format. That is, each color has a Red, Green, Blue, and Alpha component. The Alpha value does not have an intrinsic meaning; it only does what the shader that uses it wants to. Usually, Alpha is used as a translucency value, but do not make the mistake of confining your thinking to just that. Alpha means whatever you want it to.
Note: Technically, any of the 4 color values can take on whatever meaning you give them in a shader. Shaders are arbitrary programs; they can consider a color value to represent a texture coordinate, a Fresnel index, a normal, or anything else they so desire. They're just numbers; it's how you use them that defines their meaning.
Color formats can be stored in one of 3 ways: normalized integers, floating-point, or integral. Both normalized integer and floating-point formats will resolve, in the shader, to a vector of floating-point values, whereas integral formats will resolve to a vector of integers.
Normalized integer formats themselves are broken down into 2 kinds: unsigned normalized and signed normalized. Unsigned normalized integers store floating-point values on the range [0, 1], while signed normalized integers store values on the range [-1, 1].
Integral formats are also divided into signed and unsigned integers. Signed integers are 2's complement integer values.
Image formats do not have to store each component. When the shader samples such a texture, it will still resolve to a 4-value RGBA vector. The components not stored by the image format are filled in automatically. Zeros are used if R, G, or B is missing, while a missing Alpha always resolves to 1.

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___ How To Get the Image Format with Third Library FreeImage.h
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FREE_IMAGE_FORMAT fif = FIF_UNKNOWN;
FIBITMAP* dib(0);
///\ check the file signature and deduce its format..
const char * is_Path = "data\textures\golddiag.jpg" ;
fif = FreeImage_GetFileType(is_Path, 0)
if (FIF_UNKNOWN == fif) {
    // try to guess the file format from the file extension..
    fif = FreeImage_GetFIFFromFilename(is_Path);
}
if (FIF_UNKNOWN == fif) {
    return;
}

/** I/O image format identifiers. --From FreeImage.h
*/
FI_ENUM(FREE_IMAGE_FORMAT) {
FIF_UNKNOWN = -1,
FIF_BMP = 0,
FIF_ICO = 1,
FIF_JPEG = 2,
FIF_JNG = 3,
FIF_KOALA = 4,
FIF_LBM = 5,
FIF_IFF = FIF_LBM,
FIF_MNG = 6,
FIF_PBM = 7,
FIF_PBMRAW = 8,
FIF_PCD = 9,
FIF_PCX = 10,
FIF_PGM = 11,
FIF_PGMRAW = 12,
FIF_PNG = 13,
FIF_PPM = 14,
FIF_PPMRAW = 15,
FIF_RAS = 16,
FIF_TARGA = 17,
FIF_TIFF = 18,
FIF_WBMP = 19,
FIF_PSD = 20,
FIF_CUT = 21,
FIF_XBM = 22,
FIF_XPM = 23,
FIF_DDS = 24,
FIF_GIF = 25,
FIF_HDR = 26,
FIF_FAXG3 = 27,
FIF_SGI = 28,
FIF_EXR = 29,
FIF_J2K = 30,
FIF_JP2 = 31,
FIF_PFM = 32,
FIF_PICT = 33,
FIF_RAW = 34,
FIF_WEBP = 35,
FIF_JXR = 36
};