WebGL WEBGL_compressed_texture_s3tc_srgb Extension Specification

Name

WEBGL_compressed_texture_s3tc_srgb

Contact

WebGL working group (public_webgl 'at' khronos.org)

Contributors

Members of the WebGL working group

Version

Last modified date: October 24, 2019
Revision: 4

Number

WebGL extension #32

Dependencies

Written against the WebGL API 1.0 specification.

Overview

This extension exposes the sRGB compressed texture formats defined in the EXT_texture_compression_s3tc_srgb OpenGL extension to WebGL.

When this extension is enabled:

• Compression formats COMPRESSED_SRGB_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, and COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT may be passed to the compressedTexImage2D and compressedTexSubImage2D entry points.
  Some platforms do not have native support for COMPRESSED_SRGB_S3TC_DXT1_EXT format, so WebGL implementations may map it to COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT. This means the alpha channel is not guaranteed to be opaque (0xFF); it may be transparent (0x0) in some implementations. Applications expecting maximum portability should avoid authoring and uploading textures of COMPRESSED_SRGB_S3TC_DXT1_EXT format. See the corresponding OpenGL extension for technical details about format differences.
• Calling getParameter with the argument COMPRESSED_TEXTURE_FORMATS will include the 4 formats from this specification.

• The following format specific restrictions must be enforced:

  COMPRESSED_SRGB_S3TC_DXT1_EXT

  COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT

  The byteLength of the ArrayBufferView, pixels, passed to either compressedTexImage2D or compressedTexSubImage2D must match the following equation:

  floor((width + 3) / 4) * floor((height + 3) / 4) * 8

  If it is not an INVALID_VALUE error is generated.

  When level equals zero width and height must be a multiple of 4. When level is greater than 0 width and height must be 0, 1, 2 or a multiple of 4.

  If they are not an INVALID_OPERATION error is generated.

  For compressedTexSubImage2D xoffset and yoffset must be a multiple of 4 and width must be a multiple of 4 or equal to the original width of the level. height must be a multiple of 4 or equal to the original height of the level. If they are not an INVALID_OPERATION error is generated.

  COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT

  COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT

  The byteLength of the ArrayBufferView, pixels, passed to either compressedTexImage2D or compressedTexSubImage2D must match the following equation:

  floor((width + 3) / 4) * floor((height + 3) / 4) * 16

  If it is not an INVALID_VALUE error is generated.

  When level equals zero width and height must be a multiple of 4. When level is greater than 0 width and height must be 0, 1, 2 or a multiple of 4.

  If they are not an INVALID_OPERATION error is generated.

  For compressedTexSubImage2D xoffset and yoffset must be a multiple of 4 and width must be a multiple of 4 or equal to the original width of the level. height must be a multiple of 4 or equal to the original height of the level. If they are not an INVALID_OPERATION error is generated.

IDL

[Exposed=(Window,Worker), LegacyNoInterfaceObject]
interface WEBGL_compressed_texture_s3tc_srgb {
    /* Compressed Texture Formats */
    const GLenum COMPRESSED_SRGB_S3TC_DXT1_EXT        = 0x8C4C;
    const GLenum COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT  = 0x8C4D;
    const GLenum COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT  = 0x8C4E;
    const GLenum COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT  = 0x8C4F;
};
  

Revision History

Revision 1, 2016/06/10

• Initial revision.

Revision 2, 2016/07/21

• Moved to draft after discussion in working group, and study of possible dependencies.

Revision 3, 2017/05/31

• Moved to community approved after Mozilla voiced support.

Revision 4, 2019/10/24

• Added a note about COMPRESSED_SRGB_S3TC_DXT1_EXT support.