WebGL EXT_texture_compression_bptc Extension Specification

Name

EXT_texture_compression_bptc

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: 6

Number

WebGL extension #38

Dependencies

Written against the WebGL API 1.0 specification.

Overview

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

Sampling from textures in the COMPRESSED_SRGB_ALPHA_BPTC_UNORM_EXT format performs a color space conversion as specified for SRGB textures in the EXT_sRGB OpenGL ES 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 format COMPRESSED_RGBA_BPTC_UNORM_EXT, COMPRESSED_SRGB_ALPHA_BPTC_UNORM_EXT, COMPRESSED_RGB_BPTC_SIGNED_FLOAT_EXT, and COMPRESSED_RGB_BPTC_UNSIGNED_FLOAT_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.
If the internalformat is one of the BPTC internal formats from this specification, 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.
The following format-specific restrictions must be enforced:

COMPRESSED_RGBA_BPTC_UNORM_EXT

COMPRESSED_SRGB_ALPHA_BPTC_UNORM_EXT

COMPRESSED_RGB_BPTC_SIGNED_FLOAT_EXT

COMPRESSED_RGB_BPTC_UNSIGNED_FLOAT_EXT

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_bptc {
    const GLenum COMPRESSED_RGBA_BPTC_UNORM_EXT = 0x8E8C;
    const GLenum COMPRESSED_SRGB_ALPHA_BPTC_UNORM_EXT = 0x8E8D;
    const GLenum COMPRESSED_RGB_BPTC_SIGNED_FLOAT_EXT = 0x8E8E;
    const GLenum COMPRESSED_RGB_BPTC_UNSIGNED_FLOAT_EXT = 0x8E8F;
};

New Tokens

compressedTexImage2D(GLenum internalformat): Accepted by the internalformat parameter: COMPRESSED_RGBA_BPTC_UNORM_EXT, COMPRESSED_SRGB_ALPHA_BPTC_UNORM_EXT, COMPRESSED_RGB_BPTC_SIGNED_FLOAT_EXT, COMPRESSED_RGB_BPTC_UNSIGNED_FLOAT_EXT
compressedTexSubImage2D(GLenum internalformat): Accepted by the internalformat parameter: COMPRESSED_RGBA_BPTC_UNORM_EXT, COMPRESSED_SRGB_ALPHA_BPTC_UNORM_EXT, COMPRESSED_RGB_BPTC_SIGNED_FLOAT_EXT, COMPRESSED_RGB_BPTC_UNSIGNED_FLOAT_EXT

Errors

The error INVALID_VALUE is generated by compressedTexImage2D and compressedTexSubImage2D if the internalformat parameter is one of the BPTC internal formats from this extension 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 width << level and height << level are not multiples of 4.

Revision History

Revision 1, 2018/09/14

Initial revision.

Revision 2, 2018/09/18

Moved to draft status.

Revision 3, 2018/09/26

Retarget against EXT_texture_compression_bptc instead of ARB_texture_compression_bptc
Rename to EXT_texture_compression_bptc from WEBGL_compressed_texture_bptc

Revision 4, 2018/11/05

Moved to Community Approved status.

Revision 5, 2020/06/25

Clarified texImage2D entry point.

Revision 6, 2020/07/07

Clarified WebGL-specific dimensions restriction.