WebGL WEBGL_clip_cull_distance Extension Draft Specification

Name

WEBGL_clip_cull_distance

Contact

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

Contributors

Members of the WebGL working group

Kevin Rogovin (kevinrogovin 'at' invisionapp.com)

Version

Last modified date: December 01, 2022
Revision: 4

Number

WebGL extension #43

Dependencies

Written against the WebGL API 2.0 specification.

Overview

This extension exposes the EXT_clip_cull_distance functionality to WebGL.

The following WebGL-specific behavioral changes apply:

• Cull distance support is optional. This extension may be implemented on top of ANGLE_clip_cull_distance.
• Although the extension name is WEBGL_clip_cull_distance, the extension must be enabled with the #extension GL_ANGLE_clip_cull_distance directive, as shown in the sample code, to use the built-ins in shaders. Likewise the shading language preprocessor #define GL_ANGLE_clip_cull_distance, will be defined to 1 if the extension is supported.

Consult the above extension for documentation, issues and new functions and enumerants.

When this extension is enabled:

• Clip and cull distance built-ins are supported in ESSL 3.00 shaders.

  If the platform does not support cull distances:

  • Querying MAX_CULL_DISTANCES_WEBGL and MAX_COMBINED_CLIP_AND_CULL_DISTANCES_WEBGL must return zero.
  • The built-in shader constants gl_MaxCullDistances and gl_MaxCombinedClipAndCullDistances are defined as zero.
  • The gl_CullDistance built-in must not be redeclared and may only be used conditionally, see the sample code below.

• When a vertex or fragment shader enables, requires, or warns GL_ANGLE_clip_cull_distance with an #extension directive:
  • highp float gl_ClipDistance[] is a built-in input.
  • highp float gl_CullDistance[] is a built-in input.
  • highp float gl_ClipDistance[] is a built-in output.
  • highp float gl_CullDistance[] is a built-in output.
• The GLSL macro GL_ANGLE_clip_cull_distance is defined as 1.

IDL

    [Exposed=(Window,Worker), LegacyNoInterfaceObject]
    interface WEBGL_clip_cull_distance {
      const GLenum MAX_CLIP_DISTANCES_WEBGL                   = 0x0D32;
      const GLenum MAX_CULL_DISTANCES_WEBGL                   = 0x82F9;
      const GLenum MAX_COMBINED_CLIP_AND_CULL_DISTANCES_WEBGL = 0x82FA;

      const GLenum CLIP_DISTANCE0_WEBGL                       = 0x3000;
      const GLenum CLIP_DISTANCE1_WEBGL                       = 0x3001;
      const GLenum CLIP_DISTANCE2_WEBGL                       = 0x3002;
      const GLenum CLIP_DISTANCE3_WEBGL                       = 0x3003;
      const GLenum CLIP_DISTANCE4_WEBGL                       = 0x3004;
      const GLenum CLIP_DISTANCE5_WEBGL                       = 0x3005;
      const GLenum CLIP_DISTANCE6_WEBGL                       = 0x3006;
      const GLenum CLIP_DISTANCE7_WEBGL                       = 0x3007;
    };
  

Sample Code

    #extension GL_ANGLE_clip_cull_distance : enable

    void main() {
        // Compute the clip distances for the current vertex
        gl_ClipDistance[0] = ...;
        gl_ClipDistance[1] = ...;

        // For better portability, check if cull distances are supported.
        // This should compile even when the platform does not support cull distances.
        if (gl_MaxCullDistances > 0) {
            gl_CullDistance[0] = ...;
            gl_CullDistance[1] = ...;
        }
    }
    

Revision History

Revision 1, 2016/08/22

• Initial revision.

Revision 2, 2019/09/25

• Promoted to Draft.

Revision 3, 2020/07/01

• Added addendum that allows for the number of cull distances to be 0.

Revision 4, 2022/12/01

• Renamed to WEBGL_clip_cull_distance.