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c++opengl-estextures

DXT Texture working despite S3TC not being supported

The topic involves OpenGL ES 2.0.

I have a device that when queried on the extensions via 

glGetString(GL_EXTENSIONS)

Returns a list of supported extensions, none of which is GL_EXT_texture_compression_s3tc .

AFAIK , not haveing GL_EXT_texture_compression_s3tc shouldn't allow using DXT compressed textures.

However, when DXT compressed texture are used on the device , they render without any problems.

Texture data is commited using glCompressedTexImage2D .

Tried for DXT1 , DXT3 and DXT5 .

Why does it work ? Is it safe to use a texture compression although the compression seems to not be supported ?

Solution

  • I think, that missing support for GL_EXT_texture_compression_s3tc does not mean, that you can't use compressed formats. They may be supported anyway.

    Quote from glCompressedTexImage2D doc page for ES2:

    The texture image is decoded according to the extension specification defining the specified internalformat. OpenGL ES (...) provide a mechanism to obtain symbolic constants for such formats provided by extensions. The number of compressed texture formats supported can be obtained by querying the value of GL_NUM_COMPRESSED_TEXTURE_FORMATS. The list of specific compressed texture formats supported can be obtained by querying the value of GL_COMPRESSED_TEXTURE_FORMATS.

    Note, that there is nothing about GL_EXT_texture_compression_s3tc. Support for various capabilities may be implemented even though their 'pseudo-standardized' (I mean - extensionized) substitutes are not listed as supported.

    You should probably query those constants (GL_NUM_COMPRESSED_TEXTURE_FORMATS and GL_COMPRESSED_TEXTURE_FORMATS) using glGetIntegerv() to learn, which compressed formats are actually supported.