Gsk.GLShader
Description: Reference for Gsk.GLShader

### Class

# GskGLShader

#### Description [src]

    final class Gsk.GLShader : GObject.Object {
      /* No available fields */
    }

A `GskGLShader` is a snippet of GLSL that is meant to run in the fragment shader of the rendering pipeline.

A fragment shader gets the coordinates being rendered as input and produces the pixel values for that particular pixel. Additionally, the shader can declare a set of other input arguments, called uniforms (as they are uniform over all the calls to your shader in each instance of use). A shader can also receive up to 4 textures that it can use as input when producing the pixel data.

`GskGLShader` is usually used with `gtk_snapshot_push_gl_shader()` to produce a `GskGLShaderNode` in the rendering hierarchy, and then its input textures are constructed by rendering the child nodes to textures before rendering the shader node itself. (You can pass texture nodes as children if you want to directly use a texture as input).

The actual shader code is GLSL code that gets combined with some other code into the fragment shader. Since the exact capabilities of the GPU driver differs between different OpenGL drivers and hardware, GTK adds some defines that you can use to ensure your GLSL code runs on as many drivers as it can.

If the OpenGL driver is GLES, then the shader language version is set to 100, and GSK_GLES will be defined in the shader.

Otherwise, if the OpenGL driver does not support the 3.2 core profile, then the shader will run with language version 110 for GL2 and 130 for GL3, and GSK_LEGACY will be defined in the shader.

If the OpenGL driver supports the 3.2 code profile, it will be used, the shader language version is set to 150, and GSK_GL3 will be defined in the shader.

The main function the shader must implement is:

     void mainImage(out vec4 fragColor,
                    in vec2 fragCoord,
                    in vec2 resolution,
                    in vec2 uv)

Where the input `fragCoord` is the coordinate of the pixel we’re currently rendering, relative to the boundary rectangle that was specified in the `GskGLShaderNode`, and `resolution` is the width and height of that rectangle. This is in the typical GTK coordinate system with the origin in the top left. `uv` contains the u and v coordinates that can be used to index a texture at the corresponding point. These coordinates are in the [0..1]x[0..1] region, with 0, 0 being in the lower left corder (which is typical for OpenGL).

The output `fragColor` should be a RGBA color (with premultiplied alpha) that will be used as the output for the specified pixel location. Note that this output will be automatically clipped to the clip region of the glshader node.

In addition to the function arguments the shader can define up to 4 uniforms for textures which must be called u_textureN (i.e. u_texture1 to u_texture4) as well as any custom uniforms you want of types int, uint, bool, float, vec2, vec3 or vec4.

All textures sources contain premultiplied alpha colors, but if some there are outer sources of colors there is a `gsk_premultiply()` helper to compute premultiplication when needed.

Note that GTK parses the uniform declarations, so each uniform has to be on a line by itself with no other code, like so:

    uniform float u_time;
    uniform vec3 u_color;
    uniform sampler2D u_texture1;
    uniform sampler2D u_texture2;

GTK uses the “gsk” namespace in the symbols it uses in the shader, so your code should not use any symbols with the prefix gsk or GSK. There are some helper functions declared that you can use:

    vec4 GskTexture(sampler2D sampler, vec2 texCoords);

This samples a texture (e.g. u_texture1) at the specified coordinates, and contains some helper ifdefs to ensure that it works on all OpenGL versions.

You can compile the shader yourself using `gsk_gl_shader_compile()`, otherwise the GSK renderer will do it when it handling the glshader node. If errors occurs, the returned `error` will include the glsl sources, so you can see what GSK was passing to the compiler. You can also set GSK_DEBUG=shaders in the environment to see the sources and other relevant information about all shaders that GSK is handling.

# An example shader

    uniform float position;
    uniform sampler2D u_texture1;
    uniform sampler2D u_texture2;

    void mainImage(out vec4 fragColor,
                   in vec2 fragCoord,
                   in vec2 resolution,
                   in vec2 uv) {
      vec4 source1 = GskTexture(u_texture1, uv);
      vec4 source2 = GskTexture(u_texture2, uv);

      fragColor = position * source1 + (1.0 - position) * source2;
    }

#### Ancestors

- GObject

#### Constructors

###### gsk_gl_shader_new_from_bytes

Creates a `GskGLShader` that will render pixels using the specified code.

###### gsk_gl_shader_new_from_resource

Creates a `GskGLShader` that will render pixels using the specified code.

#### Instance methods

###### gsk_gl_shader_compile

Tries to compile the `shader` for the given `renderer`.

###### gsk_gl_shader_find_uniform_by_name

Looks for a uniform by the name `name`, and returns the index of the uniform, or -1 if it was not found.

###### gsk_gl_shader_format_args

Formats the uniform data as needed for feeding the named uniforms values into the shader.

###### gsk_gl_shader_format_args_va

Formats the uniform data as needed for feeding the named uniforms values into the shader.

###### gsk_gl_shader_get_arg_bool

Gets the value of the uniform `idx` in the `args` block.

###### gsk_gl_shader_get_arg_float

Gets the value of the uniform `idx` in the `args` block.

###### gsk_gl_shader_get_arg_int

Gets the value of the uniform `idx` in the `args` block.

###### gsk_gl_shader_get_arg_uint

Gets the value of the uniform `idx` in the `args` block.

###### gsk_gl_shader_get_arg_vec2

Gets the value of the uniform `idx` in the `args` block.

###### gsk_gl_shader_get_arg_vec3

Gets the value of the uniform `idx` in the `args` block.

###### gsk_gl_shader_get_arg_vec4

Gets the value of the uniform `idx` in the `args` block.

###### gsk_gl_shader_get_args_size

Get the size of the data block used to specify arguments for this shader.

###### gsk_gl_shader_get_n_textures

Returns the number of textures that the shader requires.

###### gsk_gl_shader_get_n_uniforms

Get the number of declared uniforms for this shader.

###### gsk_gl_shader_get_resource

Gets the resource path for the GLSL sourcecode being used to render this shader.

###### gsk_gl_shader_get_source

Gets the GLSL sourcecode being used to render this shader.

###### gsk_gl_shader_get_uniform_name

Get the name of the declared uniform for this shader at index `idx`.

###### gsk_gl_shader_get_uniform_offset

Get the offset into the data block where data for this uniforms is stored.

###### gsk_gl_shader_get_uniform_type

Get the type of the declared uniform for this shader at index `idx`.

##### Methods inherited from GObject (43)

Please see GObject for a full list of methods.

#### Properties

###### Gsk.GLShader:resource

Resource containing the source code for the shader.

###### Gsk.GLShader:source

No description available.

#### Signals

##### Signals inherited from GObject (1)

###### GObject::notify

The notify signal is emitted on an object when one of its properties has its value set through g_object_set_property(), g_object_set(), et al.

#### Class structure

    struct GskGLShaderClass {
      GObjectClass parent_class;

    }

No description available.

###### Class members

`parent_class: GObjectClass`
  No description available.