6.1.1.1 Internal Texture Formats

If you care about the quality of your textures or want to conserve the amount of texture memory your application requires (and often conserving texture memory helps improve performance), you should definitely use appropriate internal formats. Internal texture formats were introduced in OpenGL 1.1. Table 1 lists the available internal texture formats. If your texture is known to be only gray-scale or luminance values, choosing the GL_ LUMINANCE format instead of GL_ RGB typically cuts your texture memory usage by one third. Requesting more efficient internal format sizes can also help. The GL_ RGB8 internal texture format requests 8 bits of red, green, and blue precision for each texel. The more space efficient GL_ RGB4 internal texture format uses only 4 bits per component making it require only half the texture memory of the GL_ RGB8 format. Of course, the GL_ RGB4 format only has 16 distinct values per component instead of 256 values for the GL_ RGB8 format. However, if minimizing texture memory usage (and often improving texturing performance too) is more important than better texture quality, the GL_ RGB4 format is a better choice. In the case where the source image for your texture only has 4 bits of color resolution per component, there is absolutely no reason to request a format with more than 4 bits of color resolution.

Table 1: OpenGL Internal Texture Formats. Each internal texture format has a corresponding base internal format and its desired component resolutions.

Sized	Base	R	G	B	A	L	I
Internal Format	Internal Format	bits	bits	bits	bits	bits	bits
ALPHA4	ALPHA				4
ALPHA8	ALPHA				8
ALPHA12	ALPHA				12
ALPHA16	ALPHA				16
LUMINANCE4	LUMINANCE					4
LUMINANCE8	LUMINANCE					8
LUMINANCE12	LUMINANCE					12
LUMINANCE16	LUMINANCE					16
LUMINANCE4_ALPHA4	LUMINANCE_ALPHA				4	4
LUMINANCE6_ALPHA2	LUMINANCE_ALPHA				2	6
LUMINANCE8_ALPHA8	LUMINANCE_ALPHA				8	8
LUMINANCE12_ALPHA4	LUMINANCE_ALPHA				12	4
LUMINANCE16_ALPHA16	LUMINANCE_ALPHA				16	16
INTENSITY4	INTENSITY						4
INTENSITY8	INTENSITY						8
INTENSITY12	INTENSITY						12
INTENSITY16	INTENSITY						16
R3_G3_B2	RGB	3	3	2
RGB4	RGB	4	4	4
RGB5	RGB	5	5	5
RGB8	RGB	8	8	8
RGB10	RGB	10	10	10
RGB12	RGB	12	12	12
RGB16	RGB	16	16	16
RGBA2	RGBA	2	2	2	2
RGBA4	RGBA	4	4	4	4
RGB5_A1	RGBA	5	5	5	1
RGBA8	RGBA	8	8	8	8
RGB10_A2	RGBA	10	10	10	2
RGBA12	RGBA	12	12	12	12
RGBA16	RGBA	16	16	16	16

Some words of advice about internal texture formats: If you do not request a specific internal resolution for your texture image because you requested a GL_ RGBA internal format instead of a size-specific internal format such as GL_ RGBA8 or GL_ RGBA4, your OpenGL implementation is free to pick the ``most appropriate'' format for the particular implementation. If a smaller texture format has better texturing performance, the implementation is free to choose the smaller format. This means if you care about maintaining a particular level of internal format resolution, selecting a size-specific texture format is strongly recommended.

Some words of warning about internal texture formats: Not all OpenGL implementations are expected to support all the available internal texture formats. This means just because you request a GL_ LUMIANCE12_ALPHA4 format (to pick a format that is likely to be obscure) does not mean that your texture is guaranteed to be stored in this format. The size-specific internal texture formats are merely hints. If the best the OpenGL implementation can provide is GL_ LUMINANCE8_ALPHA8, this will be the format you get, even though is provides less luminance precision and more alpha precision than you requested.