Vertex Shader

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The Vertex Shader is the programmable Shader stage in the rendering pipeline that handles the processing of individual vertices. Vertex shaders are fed Vertex Attribute data, as specified from a vertex array object by a rendering command. A vertex shader receives a single vertex from the vertex stream and generates a single vertex to the output vertex stream. There must be a 1:1 mapping from input vertices to output vertices.

Vertex shaders typically perform transformations to post-projection space, for consumption by the Vertex Post-Processing stage. They can also be used to do per-vertex lighting, or to perform setup work for later shader stages.

Inputs

User-defined input values to the vertex shader are provided by issuing a rendering command while an appropriate vertex array object is bound.

Vertex shader input variables are defined as normal for shader stages, using the in​ type qualifier. Vertex Shader inputs cannot be aggregated into Interface Blocks.

Each user-defined input variable is assigned one or more vertex attribute indices. These can be explicitly assigned in one of three ways. The methods for assigning these are listed in priority order, with the highest priority first. The higher priority methods take precedence over the later ones.

In-shader specification
The shader defines the attribute index. This is done using the layout(location = #)​ syntax:
layout(location = 2) in vec4 a_vec;
This assigns the attribute a_vec​ the index 2.
Pre-link specification
Before linking a program that includes a vertex shader, the user may tell OpenGL to assign a particular attribute to a particular index. This is done with the following function:
 void glBindAttribLocation(GLuint program​, GLuint index​, const GLchar *name​);
index​ is the attribute index to assign. name​ is the name of the vertex shader input to assign the given index to.
Note that it is perfectly legal to assign names to indices that are not mentioned in the vertex shader. The linking process will only use the names that are actually mentioned in the vertex shader. Because of that, it is also perfectly legal to assign multiple names to the same index; this is only an error if you attempt to link a program that uses both names.
Automatic assignment
If neither of the prior two methods assign an input to an attribute index, then the index is automatically assigned by OpenGL when the program is linked. The index assigned is completely arbitrary and may be different for different programs that are linked, even if they use the exact same vertex shader code.

Note that like uniforms, vertex inputs can be "active" and non-active. Active inputs are those that the compiler/linker detects are actually in use. The vertex shader and GLSL program linking process can decide that some input are not in use and therefore they are not active.

The number of active input variables can queried with glGetProgram using GL_ACTIVE_ATTRIBUTES​. The names of these inputs can be queried with glGetActiveAttrib, and the attribute index can be queried by providing the attribute name to glGetAttribLocation.

Multiple attributes

User-defined vertex shader inputs may be arrays, matrices, and double-precision types (if GL 4.1/ARB_vertex_attrib_64bit is available). Or combinations of any of these. Some of these types require that the input variable be assigned to multiple attribute indices.

Matrix inputs take up one attribute index for every column. Array inputs take one attribute index for every .

Double-precision input variables of double​ or dvec​ types always take up one attribute. Even if they are dvec4​.

These combine with each other. A mat2x4[2]​ array is broken up into four vec4​ values, each of which is assigned an index. Thus, it takes up 4 indices; the first two indices specify the two columns of array index 0, and the next two indices specify the two columns of array index 1.

When an input requires multiple indices, it will always be assigned sequential indices starting from the given index. Consider:

layout(location = 3) in mat4 a_matrix;

a_matrix​ will be assigned attribute indices 3, 4, 5, and 6. This works regardless of what methods you use to assign vertex attribute indices to input variables.

Linking will fail if any input names collide when they are assigned a range of attribute indices. Thus, this will fail to link:

layout(location = 0) in mat4 a_matrix;
layout(location = 3) in vec4 a_vec;

Because a_matrix​ has four columns, it will take up the attribute indices on the range [0, 3]. That overlaps with a_vec​'s attributes, and thus a linking error will occur.

This will also fail:

layout(location = 0) in mat4 a_matrix1;
layout(location = 5) in mat4 a_matrix2;
layout(location = 10) in mat4 a_matrix3;
in mat4 bad_matrix;

Even though there are 4 available attribute indices after the a_matrix*​ indices are assigned, they cannot be assigned sequentially. There is no attribute index for bad_matrix​ that will allow it to get 4 attribute indices in a row. Thus, the linker will fail.

Attribute limits

In general, the number of attribute indices are the limitation on them. No attribute can be assigned an index higher than GL_MAX_VERTEX_ATTRIBS​.

There is a case which makes this more complex: double-precision attributes (if GL 4.1/ARB_vertex_attrib_64bit is available). dvec3​ and dvec4​ only take up one attribute index. But implementations are allowed to count them twice when determining the limits on the number of attributes. Thus, while a dmat2x3[4]​ will only take up 8 attribute indices (4 arrays of 2 column dvec3​s, the implementation is allowed to consider this as taking up 16 indices when determining if a shader is using up too many attribute indices. As such, a dmat2x3[5]​ may fail to link even though it only uses 10 attribute indices.

Note the use of the word "allowed". The implementation is free to count them only once, but you can't rely on it. So you need to assume that these will consume two input resources, even though they only use one index. Since there is no way to query whether any particular implementation will count them twice, you must assume that they will take up two resources.

Other inputs

V · E

Vertex Shaders have the following built-in input variables.

in int gl_VertexID;
in int gl_InstanceID;
gl_VertexID​
the index of the vertex currently being processed. When using non-indexed rendering, it is the effective index of the current vertex (the number of vertices processed + the first​ value). For indexed rendering, it is the index used to fetch this vertex from the buffer.
Note: gl_VertexID​ will have the base vertex applied to it.
gl_InstanceID​
the index of the current instance when doing some form of instanced rendering. The instance count always starts at 0, even when using base instance calls. When not using instanced rendering, this value will be 0.

Outputs

Output variables from the vertex shader are passed to the next section of the pipeline. Many of the next stages are optional, so if they are not present, then the outputs are passed to the next one that is. They are in this order:

  1. Tessellation. If no Tessellation Control Shader is present, the Tessellation Evaluation Shader will get them.
  2. Geometry Shader
  3. Vertex Post-Processing

User-defined output variables can have interpolation qualifiers (though these only matter if the output is being passed directly to the Vertex Post-Processing stage). Vertex shader outputs can also be aggregated into Interface Blocks.


V · E

Vertex Shaders have the following predefined outputs.

out gl_PerVertex
{
  vec4 gl_Position;
  float gl_PointSize;
  float gl_ClipDistance[];
}

gl_PerVertex​ defines an interface block for outputs. The block is defined without an instance name, so that prefixing the names is not required.

These variables only take on the meanings below if this shader is the last active Vertex Processing stage, and if rasterization is still active (ie: GL_RASTERIZER_DISCARD​ is not enabled). The text below explains how the Vertex Post-Processing system uses the variables. These variables may not be redeclared with interpolation qualifiers.

gl_Position​
the clip-space output position of the current vertex.
gl_PointSize​
the pixel width/height of the point being rasterized. It only has a meaning when rendering point primitives.
gl_ClipDistance​
allows the shader to set the distance from the vertex to each clip plane. A positive distance means that the vertex is inside/behind the clip plane, and a negative distance means it is outside/in front of the clip plane. Each element in the array is one clip plane. In order to use this variable, the user must manually redeclare it with an explicit size.

See also