size of compressed texture in memory

Hi! I’d like to know how much memory does an s3tc compressed texture consume in the vgacard’s texture memory? 32bit and 24bit textures also. Can someone help me?

Normally you get a ratio from 1:4 to 1:6 compared to uncompressed textures. The size taken by an uncompressed texture depends on its resolution, but also internal format.

For example, GL_RGBA8 should take 4 bytes per texel. If your texture is 512x512, it’s 1 Mb of memory. If it’s compressed, it’s probably taking around 200 Kb.


And using mipmaps, the used space is 1.3333333 times the level 0 mipmap (for all levels together!) for uncompressed textures.


The compression ratio depends what you’re comparing with what.

Lets take for example a GeForce (1/2/3/4, the whole familly…). Pixels are either encoded in 16bits or 32bits (in uncompressed formats).

So this give:

DXT1/DXT1a (aka S3TC) compared to RGB (16bits) has a compression ratio of 1:4
DXT1/DXT1a compared to RGB (24bits) and RGBA (32bits) has a ratio of 1:8
(compared to 24bits it would be 1:6 but on GeForce 24bits pixels are aligned on 32bits boundaries)

DXT3 and DXT5 have a compression ratio of 1:4 compared to RGB (24bits) and RGBA (32bits), and 1:2 compared to RGB (16bits)
(Note it doesn’t make much sense to use DXT3 or DXT5 when working with RGB and not RGBA)

To put it in another way:

DXT1/DXT1a/S3TC stores 16 pixels in 64 bits.
DXT3/DXT5 stores 16 pixels in 128 bits.

[This message has been edited by GPSnoopy (edited 11-12-2002).]

Thanks a lot!

You simply might ask OpenGL. See

If you call glGet…() with parameter GL_TEXTURE_COMPRESSED_IMAGE_SIZE_ARB it returns “the size (in ubytes) of the compressed texture image” [from the mentioned spec].


Just what I need! Thanks!