WebGL EXT_texture_compression_rgtc Extension Specification

Name

EXT_texture_compression_rgtc

Contact

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

Contributors

Olli Etuaho, NVIDIA

Members of the WebGL working group

Version

Last modified date: July 07, 2020
Revision: 5

Number

WebGL extension #39

Dependencies

Written against the WebGL API 1.0 specification.

Overview

This extension exposes the compressed texture format defined in the EXT_texture_compression_rgtc OpenGL extension to WebGL. Consult that extension specification for behavioral definitions, including error behaviors.

Updates of partial tiles detailed in the "Implementation Note" section of the EXT_texture_compression_rgtc specification must be supported in an implementation of this WebGL extension.

Unlike the OpenGL ES extension, this WebGL extension does not extend texImage2D entry point.

For compatibility reasons, the width and the height of the top level mip map are required to be a multiple of 4.

When this extension is enabled:

Compression formats COMPRESSED_RED_RGTC1_EXT, COMPRESSED_SIGNED_RED_RGTC1_EXT, COMPRESSED_RED_GREEN_RGTC2_EXT, and COMPRESSED_SIGNED_RED_GREEN_RGTC2_EXT may be passed to the compressedTexImage2D and compressedTexSubImage2D entry points.
Calling getParameter with the argument COMPRESSED_TEXTURE_FORMATS will include the formats from this specification.
The following format-specific restrictions must be enforced:

COMPRESSED_RED_RGTC1_EXT

COMPRESSED_SIGNED_RED_RGTC1_EXT

The byteLength of the ArrayBufferView, pixels, passed to compressedTexImage2D or compressedTexSubImage2D must be equal to the following number of bytes:

ceil(width / 4) * ceil(height / 4) * 8

If it is not, an INVALID_VALUE error is generated.

width << level and height << level must be a multiple of 4.

If they are not an INVALID_OPERATION error is generated.

COMPRESSED_RED_GREEN_RGTC2_EXT

COMPRESSED_SIGNED_RED_GREEN_RGTC2_EXT

The byteLength of the ArrayBufferView, pixels, passed to compressedTexImage2D or compressedTexSubImage2D must be equal to the following number of bytes:

ceil(width / 4) * ceil(height / 4) * 16

If it is not, an INVALID_VALUE error is generated.

width << level and height << level must be a multiple of 4.

If they are not an INVALID_OPERATION error is generated.

IDL

[Exposed=(Window,Worker), LegacyNoInterfaceObject]
interface EXT_texture_compression_rgtc {
    const GLenum COMPRESSED_RED_RGTC1_EXT = 0x8DBB;
    const GLenum COMPRESSED_SIGNED_RED_RGTC1_EXT = 0x8DBC;
    const GLenum COMPRESSED_RED_GREEN_RGTC2_EXT = 0x8DBD;
    const GLenum COMPRESSED_SIGNED_RED_GREEN_RGTC2_EXT = 0x8DBE;
};

New Tokens

compressedTexImage2D(GLenum internalformat): Accepted by the internalformat parameter: COMPRESSED_RED_RGTC1_EXT, COMPRESSED_SIGNED_RED_RGTC1_EXT, COMPRESSED_RED_GREEN_RGTC2_EXT, COMPRESSED_SIGNED_RED_GREEN_RGTC2_EXT
compressedTexSubImage2D(GLenum internalformat): Accepted by the internalformat parameter: COMPRESSED_RED_RGTC1_EXT, COMPRESSED_SIGNED_RED_RGTC1_EXT, COMPRESSED_RED_GREEN_RGTC2_EXT, COMPRESSED_SIGNED_RED_GREEN_RGTC2_EXT

Errors

The error INVALID_VALUE is generated by compressedTexImage2D and compressedTexSubImage2D if the internalformat parameter is COMPRESSED_RED_RGTC1_EXT or COMPRESSED_SIGNED_RED_RGTC1_EXT and the byteLength of the ArrayBufferView is not:

ceil(width / 4) * ceil(height / 4) * 8

The error INVALID_VALUE is generated by compressedTexImage2D and compressedTexSubImage2D if the internalformat parameter is COMPRESSED_RED_GREEN_RGTC2_EXT or COMPRESSED_SIGNED_RED_GREEN_RGTC2_EXT and the byteLength of the ArrayBufferView is not:

ceil(width / 4) * ceil(height / 4) * 16

The error INVALID_OPERATION is generated by compressedTexImage2D and compressedTexSubImage2D if the level parameter is 0 and the width or height is not a multiple of 4.

Revision History

Revision 1, 2018/09/26

Initial revision.

Revision 2, 2018/09/26

Rename to EXT_texture_compression_rgtc from WEBGL_compressed_texture_rgtc

Revision 3, 2018/11/05

Moved to Community Approved status.

Revision 4, 2020/06/25

Clarified texImage2D entry point.

Revision 5, 2020/07/07

Clarified WebGL-specific dimensions restriction.