Shader snippets

## Shader snippets Shader snippets — Functions for creating and manipulating shader snippets |

## Synopsis

                        CoglSnippet;
    enum                CoglSnippetHook;
    CoglSnippet *       cogl_snippet_new                    (CoglSnippetHook hook,
                                                             const char *declarations,
                                                             const char *post);
    CoglSnippetHook     cogl_snippet_get_hook               (CoglSnippet *snippet);
    CoglBool            cogl_is_snippet                     (void *object);
    void                cogl_snippet_set_declarations       (CoglSnippet *snippet,
                                                             const char *declarations);
    const char *        cogl_snippet_get_declarations       (CoglSnippet *snippet);
    void                cogl_snippet_set_pre                (CoglSnippet *snippet,
                                                             const char *pre);
    const char *        cogl_snippet_get_pre                (CoglSnippet *snippet);
    void                cogl_snippet_set_replace            (CoglSnippet *snippet,
                                                             const char *replace);
    const char *        cogl_snippet_get_replace            (CoglSnippet *snippet);
    void                cogl_snippet_set_post               (CoglSnippet *snippet,
                                                             const char *post);
    const char *        cogl_snippet_get_post               (CoglSnippet *snippet);

## Description

CoglSnippets are used to modify or replace parts of a CoglPipeline using GLSL. GLSL is a programming language supported by OpenGL on programmable hardware to provide a more flexible description of what should be rendered. A description of GLSL itself is outside the scope of this documentation but any good OpenGL book should help to describe it.

Unlike in OpenGL, when using GLSL with Cogl it is possible to write short snippets to replace small sections of the pipeline instead of having to replace the whole of either the vertex or fragment pipelines. Of course it is also possible to replace the whole of the pipeline if needed.

Each snippet is a standalone chunk of code which would attach to the pipeline at a particular point. The code is split into four separate strings (all of which are optional):

declarations

  The code in this string will be inserted outside of any function in the global scope of the shader. This can be used to declare uniforms, attributes, varyings and functions to be used by the snippet.

pre

  The code in this string will be inserted before the hook point.

post

  The code in this string will be inserted after the hook point. This can be used to modify the results of the builtin generated code for that hook point.

replace

  If present the code in this string will replace the generated code for the hook point.

All of the strings apart from the declarations string of a pipeline are generated in a single function so they can share variables declared from one string in another. The scope of the code is limited to each snippet so local variables declared in the snippet will not collide with variables declared in another snippet. However, code in the 'declarations' string is global to the shader so it is the application's responsibility to ensure that variables declared here will not collide with those from other snippets.

The snippets can be added to a pipeline with `cogl_pipeline_add_snippet()` or `cogl_pipeline_add_layer_snippet()`. Which function to use depends on which hook the snippet is targetting. The snippets are all generated in the order they are added to the pipeline. That is, the post strings are executed in the order they are added to the pipeline and the pre strings are executed in reverse order. If any replace strings are given for a snippet then any other snippets with the same hook added before that snippet will be ignored. The different hooks are documented under CoglSnippetHook.

For portability with GLES2, it is recommended not to use the GLSL builtin names such as gl_FragColor. Instead there are replacement names under the cogl_* namespace which can be used instead. These are:

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 layer 0. This is an alternative name for cogl_tex_coord0_in.

attribute vec4 cogl_tex_coord0_in

  The texture coordinate for the layer 0. This is equivalent to gl_MultiTexCoord0. There will also be cogl_tex_coord1_in and so on if more layers are added to the pipeline.

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_in

  The incoming point size from the cogl_point_size_in attribute. This is only available if `cogl_pipeline_set_per_vertex_point_size()` is set on the pipeline.

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_coord0_out

  The calculated texture coordinate for layer 0 of the pipeline. This is equivalent to gl_TexCoord[0]. There will also be cogl_tex_coord1_out and so on if more layers are added to the pipeline. In the fragment shader, this varying is called cogl_tex_coord0_in.

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_coord0_in

  The texture coordinate for layer 0. This is equivalent to gl_TexCoord[0]. There will also be cogl_tex_coord1_in and so on if more layers are added to the pipeline.

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.

vec2 cogl_point_coord

  When rendering points, this will contain a vec2 which represents the position within the point of the current fragment. vec2(0.0,0.0) will be the topleft of the point and vec2(1.0,1.0) will be the bottom right. Note that there is currently a bug in Cogl where when rendering to an offscreen buffer these coordinates will be upside-down. The value is undefined when not rendering points. This builtin can only be used if the `COGL_FEATURE_ID_POINT_SPRITE` feature is available.

Here is an example of using a snippet to add a desaturate effect to the generated color on a pipeline.

      CoglPipeline *pipeline = cogl_pipeline_new ();

      /* Set up the pipeline here, ie by adding a texture or other
         layers */

      /* Create the snippet. The first string is the declarations which
         we will use to add a uniform. The second is the 'post' string which
         will contain the code to perform the desaturation. */
      CoglSnippet *snippet =
        cogl_snippet_new (COGL_SNIPPET_HOOK_FRAGMENT,
                          "uniform float factor;",
                          "float gray = dot (vec3 (0.299, 0.587, 0.114), "
                          "                  cogl_color_out.rgb);"
                          "cogl_color_out.rgb = mix (vec3 (gray),"
                          "                          cogl_color_out.rgb,"
                          "                          factor);");

      /* Add it to the pipeline */
      cogl_pipeline_add_snippet (pipeline, snippet);
      /* The pipeline keeps a reference to the snippet
         so we don't need to */
      cogl_object_unref (snippet);

      /* Update the custom uniform on the pipeline */
      int location = cogl_pipeline_get_uniform_location (pipeline, "factor");
      cogl_pipeline_set_uniform_1f (pipeline, location, 0.5f);

      /* Now we can render with the snippet as usual */
      cogl_push_source (pipeline);
      cogl_rectangle (0, 0, 10, 10);
      cogl_pop_source ();

## Details

### CoglSnippet

    typedef struct _CoglSnippet CoglSnippet;

### enum CoglSnippetHook

    typedef enum {
      /* Per pipeline vertex hooks */
      COGL_SNIPPET_HOOK_VERTEX = 0,
      COGL_SNIPPET_HOOK_VERTEX_TRANSFORM,
      COGL_SNIPPET_HOOK_VERTEX_GLOBALS,
      COGL_SNIPPET_HOOK_POINT_SIZE,

      /* Per pipeline fragment hooks */
      COGL_SNIPPET_HOOK_FRAGMENT = 2048,
      COGL_SNIPPET_HOOK_FRAGMENT_GLOBALS,

      /* Per layer vertex hooks */
      COGL_SNIPPET_HOOK_TEXTURE_COORD_TRANSFORM = 4096,

      /* Per layer fragment hooks */
      COGL_SNIPPET_HOOK_LAYER_FRAGMENT = 6144,
      COGL_SNIPPET_HOOK_TEXTURE_LOOKUP
    } CoglSnippetHook;

CoglSnippetHook is used to specify a location within a CoglPipeline where the code of the snippet should be used when it is attached to a pipeline.

`COGL_SNIPPET_HOOK_VERTEX_GLOBALS`

  Adds a shader snippet at the beginning of the global section of the shader for the vertex processing. Any declarations here can be shared with all other snippets that are attached to a vertex hook. Only the ‘declarations’ string is used and the other strings are ignored.

`COGL_SNIPPET_HOOK_FRAGMENT_GLOBALS`

  Adds a shader snippet at the beginning of the global section of the shader for the fragment processing. Any declarations here can be shared with all other snippets that are attached to a fragment hook. Only the ‘declarations’ string is used and the other strings are ignored.

`COGL_SNIPPET_HOOK_VERTEX`

  Adds a shader snippet that will hook on to the vertex processing stage of the pipeline. This gives a chance for the application to modify the vertex attributes generated by the shader. Typically the snippet will modify cogl_color_out or cogl_position_out builtins.

  The ‘declarations’ string in `snippet` will be inserted in the global scope of the shader. Use this to declare any uniforms, attributes or functions that the snippet requires.

  The ‘pre’ string in `snippet` will be inserted at the top of the `main()` function before any vertex processing is done.

  The ‘replace’ string in `snippet` will be used instead of the generated vertex processing if it is present. This can be used if the application wants to provide a complete vertex shader and doesn't need the generated output from Cogl.

  The ‘post’ string in `snippet` will be inserted after all of the standard vertex processing is done. This can be used to modify the outputs.

`COGL_SNIPPET_HOOK_VERTEX_TRANSFORM`

  Adds a shader snippet that will hook on to the vertex transform stage. Typically the snippet will use the cogl_modelview_matrix, cogl_projection_matrix and cogl_modelview_projection_matrix matrices and the cogl_position_in attribute. The hook must write to cogl_position_out. The default processing for this hook will multiply cogl_position_in by the combined modelview-projection matrix and store it on cogl_position_out.

  The ‘declarations’ string in `snippet` will be inserted in the global scope of the shader. Use this to declare any uniforms, attributes or functions that the snippet requires.

  The ‘pre’ string in `snippet` will be inserted at the top of the `main()` function before the vertex transform is done.

  The ‘replace’ string in `snippet` will be used instead of the generated vertex transform if it is present.

  The ‘post’ string in `snippet` will be inserted after all of the standard vertex transformation is done. This can be used to modify the cogl_position_out in addition to the default processing.

`COGL_SNIPPET_HOOK_POINT_SIZE`

  Adds a shader snippet that will hook on to the point size calculation step within the vertex shader stage. The snippet should write to the builtin cogl_point_size_out with the new point size. The snippet can either read cogl_point_size_in directly and write a new value or first read an existing value in cogl_point_size_out that would be set by a previous snippet. Note that this hook is only used if `cogl_pipeline_set_per_vertex_point_size()` is enabled on the pipeline.

  The ‘declarations’ string in `snippet` will be inserted in the global scope of the shader. Use this to declare any uniforms, attributes or functions that the snippet requires.

  The ‘pre’ string in `snippet` will be inserted just before calculating the point size.

  The ‘replace’ string in `snippet` will be used instead of the generated point size calculation if it is present.

  The ‘post’ string in `snippet` will be inserted after the standard point size calculation is done. This can be used to modify cogl_point_size_out in addition to the default processing.

`COGL_SNIPPET_HOOK_FRAGMENT`

  Adds a shader snippet that will hook on to the fragment processing stage of the pipeline. This gives a chance for the application to modify the fragment color generated by the shader. Typically the snippet will modify cogl_color_out.

  The ‘declarations’ string in `snippet` will be inserted in the global scope of the shader. Use this to declare any uniforms, attributes or functions that the snippet requires.

  The ‘pre’ string in `snippet` will be inserted at the top of the `main()` function before any fragment processing is done.

  The ‘replace’ string in `snippet` will be used instead of the generated fragment processing if it is present. This can be used if the application wants to provide a complete fragment shader and doesn't need the generated output from Cogl.

  The ‘post’ string in `snippet` will be inserted after all of the standard fragment processing is done. At this point the generated value for the rest of the pipeline state will already be in cogl_color_out so the application can modify the result by altering this variable.

`COGL_SNIPPET_HOOK_TEXTURE_COORD_TRANSFORM`

  Adds a shader snippet that will hook on to the texture coordinate transformation of a particular layer. This can be used to replace the processing for a layer or to modify the results.

  Within the snippet code for this hook there are two extra variables. The first is a mat4 called cogl_matrix which represents the user matrix for this layer. The second is called cogl_tex_coord and represents the incoming and outgoing texture coordinate. On entry to the hook, cogl_tex_coord contains the value of the corresponding texture coordinate attribute for this layer. The hook is expected to modify this variable. The output will be passed as a varying to the fragment processing stage. The default code will just multiply cogl_matrix by cogl_tex_coord and store the result in cogl_tex_coord.

  The ‘declarations’ string in `snippet` will be inserted in the global scope of the shader. Use this to declare any uniforms, attributes or functions that the snippet requires.

  The ‘pre’ string in `snippet` will be inserted just before the fragment processing for this layer. At this point cogl_tex_coord still contains the value of the texture coordinate attribute.

  If a ‘replace’ string is given then this will be used instead of the default fragment processing for this layer. The snippet can modify cogl_tex_coord or leave it as is to apply no transformation.

  The ‘post’ string in `snippet` will be inserted just after the transformation. At this point cogl_tex_coord will contain the results of the transformation but it can be further modified by the snippet.

`COGL_SNIPPET_HOOK_LAYER_FRAGMENT`

  Adds a shader snippet that will hook on to the fragment processing of a particular layer. This can be used to replace the processing for a layer or to modify the results.

  Within the snippet code for this hook there is an extra vec4 variable called ‘cogl_layer’. This contains the resulting color that will be used for the layer. This can be modified in the ‘post’ section or it the default processing can be replaced entirely using the ‘replace’ section.

  The ‘declarations’ string in `snippet` will be inserted in the global scope of the shader. Use this to declare any uniforms, attributes or functions that the snippet requires.

  The ‘pre’ string in `snippet` will be inserted just before the fragment processing for this layer.

  If a ‘replace’ string is given then this will be used instead of the default fragment processing for this layer. The snippet must write to the ‘cogl_layer’ variable in that case.

  The ‘post’ string in `snippet` will be inserted just after the fragment processing for the layer. The results can be modified by changing the value of the ‘cogl_layer’ variable.

`COGL_SNIPPET_HOOK_TEXTURE_LOOKUP`

  Adds a shader snippet that will hook on to the texture lookup part of a given layer. This gives a chance for the application to modify the coordinates that will be used for the texture lookup or to alter the returned texel.

  Within the snippet code for this hook there are three extra variables available. ‘cogl_sampler’ is a sampler object representing the sampler for the layer where the snippet is attached. ‘cogl_tex_coord’ is a vec4 which contains the texture coordinates that will be used for the texture lookup. This can be modified. ‘cogl_texel’ will contain the result of the texture lookup. This can also be modified.

  The ‘declarations’ string in `snippet` will be inserted in the global scope of the shader. Use this to declare any uniforms, attributes or functions that the snippet requires.

  The ‘pre’ string in `snippet` will be inserted at the top of the `main()` function before any fragment processing is done. This is a good place to modify the cogl_tex_coord variable.

  If a ‘replace’ string is given then this will be used instead of a the default texture lookup. The snippet would typically use its own sampler in this case.

  The ‘post’ string in `snippet` will be inserted after texture lookup has been preformed. Here the snippet can modify the cogl_texel variable to alter the returned texel.

`COGL_SNIPPET_HOOK_VERTEX` | A hook for the entire vertex processing stage of the pipeline.
`COGL_SNIPPET_HOOK_VERTEX_TRANSFORM` | A hook for the vertex transformation.
`COGL_SNIPPET_HOOK_VERTEX_GLOBALS` | A hook for declaring global data that can be shared with all other snippets that are on a vertex hook.
`COGL_SNIPPET_HOOK_POINT_SIZE` | A hook for manipulating the point size of a vertex. This is only used if `cogl_pipeline_set_per_vertex_point_size()` is enabled on the pipeline.
`COGL_SNIPPET_HOOK_FRAGMENT` | A hook for the entire fragment processing stage of the pipeline.
`COGL_SNIPPET_HOOK_FRAGMENT_GLOBALS` | A hook for declaring global data wthat can be shared with all other snippets that are on a fragment hook.
`COGL_SNIPPET_HOOK_TEXTURE_COORD_TRANSFORM` | A hook for applying the layer matrix to a texture coordinate for a layer.
`COGL_SNIPPET_HOOK_LAYER_FRAGMENT` | A hook for the fragment processing of a particular layer.
`COGL_SNIPPET_HOOK_TEXTURE_LOOKUP` | A hook for the texture lookup stage of a given layer in a pipeline.

Since 1.10

Stability Level: Unstable

### cogl_snippet_new ()

    CoglSnippet *       cogl_snippet_new                    (CoglSnippetHook hook,
                                                             const char *declarations,
                                                             const char *post);

Allocates and initializes a new snippet with the given source strings.

`hook` : | The point in the pipeline that this snippet will wrap around or replace.
`declarations` : | The source code for the declarations for this snippet or `NULL`. See `cogl_snippet_set_declarations()`.
`post` : | The source code to run after the hook point where this shader snippet is attached or `NULL`. See `cogl_snippet_set_post()`.
Returns : | a pointer to a new CoglSnippet

Since 1.10

Stability Level: Unstable

### cogl_snippet_get_hook ()

    CoglSnippetHook     cogl_snippet_get_hook               (CoglSnippet *snippet);

`snippet` : | A CoglSnippet
Returns : | the hook that was set when `cogl_snippet_new()` was called.

Since 1.10

Stability Level: Unstable

### cogl_is_snippet ()

    CoglBool            cogl_is_snippet                     (void *object);

Gets whether the given `object` references an existing snippet object.

`object` : | A CoglObject pointer
Returns : | `TRUE` if the `object` references a CoglSnippet, `FALSE` otherwise

Since 1.10

Stability Level: Unstable

### cogl_snippet_set_declarations ()

    void                cogl_snippet_set_declarations       (CoglSnippet *snippet,
                                                             const char *declarations);

Sets a source string that will be inserted in the global scope of the generated shader when this snippet is used on a pipeline. This string is typically used to declare uniforms, attributes or functions that will be used by the other parts of the snippets.

This function should only be called before the snippet is attached to its first pipeline. After that the snippet should be considered immutable.

`snippet` : | A CoglSnippet
`declarations` : | The new source string for the declarations section of this snippet.

Since 1.10

Stability Level: Unstable

### cogl_snippet_get_declarations ()

    const char *        cogl_snippet_get_declarations       (CoglSnippet *snippet);

`snippet` : | A CoglSnippet
Returns : | the source string that was set with `cogl_snippet_set_declarations()` or `NULL` if none was set.

Since 1.10

Stability Level: Unstable

### cogl_snippet_set_pre ()

    void                cogl_snippet_set_pre                (CoglSnippet *snippet,
                                                             const char *pre);

Sets a source string that will be inserted before the hook point in the generated shader for the pipeline that this snippet is attached to. Please see the documentation of each hook point in CoglPipeline for a description of how this string should be used.

This function should only be called before the snippet is attached to its first pipeline. After that the snippet should be considered immutable.

`snippet` : | A CoglSnippet
`pre` : | The new source string for the pre section of this snippet.

Since 1.10

Stability Level: Unstable

### cogl_snippet_get_pre ()

    const char *        cogl_snippet_get_pre                (CoglSnippet *snippet);

`snippet` : | A CoglSnippet
Returns : | the source string that was set with `cogl_snippet_set_pre()` or `NULL` if none was set.

Since 1.10

Stability Level: Unstable

### cogl_snippet_set_replace ()

    void                cogl_snippet_set_replace            (CoglSnippet *snippet,
                                                             const char *replace);

Sets a source string that will be used instead of any generated source code or any previous snippets for this hook point. Please see the documentation of each hook point in CoglPipeline for a description of how this string should be used.

This function should only be called before the snippet is attached to its first pipeline. After that the snippet should be considered immutable.

`snippet` : | A CoglSnippet
`replace` : | The new source string for the replace section of this snippet.

Since 1.10

Stability Level: Unstable

### cogl_snippet_get_replace ()

    const char *        cogl_snippet_get_replace            (CoglSnippet *snippet);

`snippet` : | A CoglSnippet
Returns : | the source string that was set with `cogl_snippet_set_replace()` or `NULL` if none was set.

Since 1.10

Stability Level: Unstable

### cogl_snippet_set_post ()

    void                cogl_snippet_set_post               (CoglSnippet *snippet,
                                                             const char *post);

Sets a source string that will be inserted after the hook point in the generated shader for the pipeline that this snippet is attached to. Please see the documentation of each hook point in CoglPipeline for a description of how this string should be used.

This function should only be called before the snippet is attached to its first pipeline. After that the snippet should be considered immutable.

`snippet` : | A CoglSnippet
`post` : | The new source string for the post section of this snippet.

Since 1.10

Stability Level: Unstable

### cogl_snippet_get_post ()

    const char *        cogl_snippet_get_post               (CoglSnippet *snippet);

`snippet` : | A CoglSnippet
Returns : | the source string that was set with `cogl_snippet_set_post()` or `NULL` if none was set.

Since 1.10

Stability Level: Unstable