xna - How much memory does a texture take up on the GPU?

A large png on disk may only take up a couple megabytes but I imagine that on the gpu the same png is stored in an uncompressed format which takes up much more space. Is this true? If it is true, how much space?

Answer

JPG and PNG files will almost always be smaller on-disk than in memory; they need to be decompressed on-the-fly to acquire raw RGB data, thus requiring more processing power for the loading and more RAM afterwards. So many modern engines opt to store the same format on disk as they do in memory, leading to files that are the same size as the texture's memory requirements (but also larger than a PNG or JPG). RGB/RGBA and S3TC/DXTn/BCn are the most widely used formats, because they are read straight into memory without any processing (DXT textures are precompressed).

So, these are the sizes for different common texture formats:

• L (luminance, e.g. greyscale): width * height * 1 byte.


• LA (luminance and alpha, common for fonts): width * height * 2 bytes.


• RGB (color, no alpha): width * height * 3 bytes.


• RGBA (color with alpha): width * height * 4 bytes.


• DXT1/BC1 (color, binary alpha): (width * height * 4 bytes) / 8 (8:1 compression ratio).


• DXT3/BC2 (color, sharp alpha)/DXT5/BC3 (color, gradient alpha): (width * height * 4 bytes) / 4 (4:1 compression ratio).

If you use a image with mipmaps, the texture will require 4/3 as much memory. Additionally, the texture width and height may be rounded up internally to be a power of two on old or less capable hardware, and on some very limited hardware, also forced to be a square.

More info on DXT: it's a lossy compression; this means, some color data is lost when compressing the texture. This has a negative impact on your texture, distorting sharp borders and creating "blocks" on gradients; but the benefits are far better than the disadvantages (if you have a texture that looks horribly bad in DXT, just keep it uncompressed; the other ones will make up for the size loss). Also, since the pixels are compressed by fixed-size blocks, the texture width and height must be a multiple of four.