CoglShader

Functions for accessing the programmable GL pipeline

Cogl allows accessing the GL programmable pipeline in order to create vertex and fragment shaders.

When using GLSL Cogl provides replacement names for most of the builtin varyings and uniforms. It is recommended to use these names wherever possible to increase portability between OpenGL 2.0 and GLES 2.0. GLES 2.0 does not have most of the builtins under their original names so they will only work with the Cogl names.

For use in all GLSL shaders, the Cogl builtins are as follows:

• uniform mat4 cogl_modelview_matrix The current modelview matrix. This is equivalent to #gl_ModelViewMatrix.
• uniform mat4 cogl_projection_matrix The current projection matrix. This is equivalent to #gl_ProjectionMatrix.
• uniform mat4 cogl_modelview_projection_matrix The combined modelview and projection matrix. A vertex shader would typically use this to transform the incoming vertex position. The separate modelview and projection matrices are usually only needed for lighting calculations. This is equivalent to #gl_ModelViewProjectionMatrix.
• uniform mat4 cogl_texture_matrix[] An array of matrices for transforming the texture coordinates. This is equivalent to #gl_TextureMatrix.

In a vertex shader, the following are also available:

• attribute vec4 cogl_position_in The incoming vertex position. This is equivalent to #gl_Vertex.
• attribute vec4 cogl_color_in The incoming vertex color. This is equivalent to #gl_Color.
• attribute vec4 cogl_tex_coord_in The texture coordinate for the first texture unit. This is equivalent to #gl_MultiTexCoord0.
• attribute vec4 cogl_tex_coord0_in The texture coordinate for the first texture unit. This is equivalent to #gl_MultiTexCoord0. There is also #cogl_tex_coord1_in and so on.
• attribute vec3 cogl_normal_in The normal of the vertex. This is equivalent to #gl_Normal.
• vec4 cogl_position_out The calculated position of the vertex. This must be written to in all vertex shaders. This is equivalent to #gl_Position.
• float cogl_point_size_out The calculated size of a point. This is equivalent to #gl_PointSize.
• varying vec4 cogl_color_out The calculated color of a vertex. This is equivalent to #gl_FrontColor.
• varying vec4 cogl_tex_coord_out[] An array of calculated texture coordinates for a vertex. This is equivalent to #gl_TexCoord.

In a fragment shader, the following are also available:

• varying vec4 cogl_color_in The calculated color of a vertex. This is equivalent to #gl_FrontColor.
• varying vec4 cogl_tex_coord_in[] An array of calculated texture coordinates for a vertex. This is equivalent to #gl_TexCoord.
• vec4 cogl_color_out The final calculated color of the fragment. All fragment shaders must write to this variable. This is equivalent to #gl_FrontColor.
• float cogl_depth_out An optional output variable specifying the depth value to use for this fragment. This is equivalent to #gl_FragDepth.
• bool cogl_front_facing A readonly variable that will be true if the current primitive is front facing. This can be used to implement two-sided coloring algorithms. This is equivalent to #gl_FrontFacing.

It’s worth nothing that this API isn’t what Cogl would like to have in the long term and it may be removed in Cogl 2.0. The experimental CoglShader API is the proposed replacement.