Fragment Shader

A Fragment Shader is a user-supplied program that, when executed, will process a Fragment from the rasterization process into a set of colors and a single depth value.

Overview

The fragment shader is the OpenGL pipeline stage after a primitive is rasterized. For each sample of the pixels covered by a primitive, a "fragment" is generated. Each fragment has a Window Space position, a few other values, and it contains all of the interpolated per-vertex output values from the last Vertex Processing stage.

The output of a fragment shader is a depth value, a possible stencil value (unmodified by the fragment shader), and zero or more color values to be potentially written to the buffers in the current framebuffers.

Inputs

The inputs to the fragment shader are either generated by the system or passed from prior fixed-function operations and potentially interpolated across the surface of the primitive.

The user-defined outputs from the last Vertex Processing stage will be interpolated according to their assigned interpolation qualifiers. The fragment shader must define its user-defined inputs exactly as they were defined in the last Vertex Processing stage (with the exception of the centroid qualifier).

System inputs

Fragment shaders have a lot of input variables who's values are generated primarily by the rendering system.

in vec4 gl_FragCoord;
in bool gl_FrontFacing;
in vec2 gl_PointCoord;

gl_FragCoord

The location of the fragment in window space. The X, Y and Z components are the window-space position of the fragment. The Z value will be written to the depth buffer if gl_FragDepth is not written to by this shader stage. The W component of gl_FragCoord is 1/W_clip, where W_clip is the interpolated W component of the clip-space vertex position output to gl_Position from the last Vertex Processing stage.

The space of gl_FragCoord can be modified by redeclaring gl_FragCoord with special input layout qualifiers:

layout(origin_upper_left) in vec4 gl_FragCoord;

This means that the origin for gl_FragCoord's window-space will be the upper-left of the screen, rather than the usual lower-left.

layout(pixel_center_integer​) in vec4 gl_FragCoord;

OpenGL window space is defined such that pixel centers are on half-integer boundaries. So the center of the lower-left pixel is (0.5, 0.5). Using pixel_center_integer​ adjust {{code|gl_FragCoord)) such that whole integer values represent pixel centers.

Both of these exist to be compatible with D3D's window space. Unless you need your shaders to have this compatibility, you are advised not to use these features.

gl_FrontFacing

This is true if this fragment was generated by the front-face of the primitive; it is false if it was generated by the back-face.

gl_PointCoord

The location within a point primitive that defines the position of the fragment relative to the side of the point. Points are effectively rasterized as window-space squares of a certain pixel size. Since points are defined by a single vertex, the only way to tell where in that square a particular fragment is is with gl_PointCoord.

The values of gl_PointCoord's coordinates range from [0, 1]. Standard OpenGL convention uses a upper-left origin for point-coordinates, so (0, 0) is the bottom left. However, the origin can be switched to a bottom-left origin by calling glPointParameteri(GL_POINT_SPRITE_COORD_ORIGIN, GL_LOWER_LEFT);

OpenGL 4.0 and above define additional system-generated input values:

in int gl_SampleID;
in vec2 gl_SamplePosition;
in int gl_SampleMaskIn[];

gl_SampleID

This is an integer identifier for the current sample that this fragment is rasterized for.

Warning: Any use of this variable at all will force this shader to be evaluated per-sample. Since much of the point of multisampling is to avoid that, you should use it only when you must.

gl_SamplePosition

This is the location of the current sample for the fragment within the pixel's area, with values on the range [0, 1]. The origin is the bottom-left of the pixel area.

Warning: Any use of this variable at all will force this shader to be evaluated per-sample. Since much of the point of multisampling is to avoid that, you should use it only when you must.

gl_SampleMaskIn

When using multisampling, this variable contains a bitfield for the sample mask of the fragment being generated. The array is as long as needed to fill in the number of samples supported by the GL implementation.

User overridden inputs

Some built-in inputs will take values specified by OpenGL, but these values can be overridden by user control.

in float gl_ClipDistance[];
in int gl_PrimitiveID;

gl_ClipDistance

This array contains the interpolated clipping plane half-spaces, as output for vertices from the last Vertex Processing stage.

gl_PrimitiveID

This value is the index of the current primitive being rendered by this rendering command. However, if a Geometry Shader is active, then the gl_PrimitiveID is what the GS provided as output. If the GS did not output a value, then the fragment shader gets an undefined value.

GL 4.3 provides the following additional inputs:

in int gl_Layer;
in int gl_ViewportIndex;

gl_Layer

This is either 0 or the layer number for this primitive output by the Geometry Shader.

gl_ViewportIndex

This is either 0 or the viewport index for this primitive output by the Geometry Shader.

Outputs

User-defined output values from the fragment are written to one or more color buffers in the current framebuffer.

There are some built-in output values with special meaning:

out float gl_FragDepth;

gl_FragDepth

This output is the fragment's depth. If the shader does not statically write this value, then it will take the value of gl_FragCoord.z.

To "statically write" to a variable means that you write to it anywhere in the program. Even if the writing code is technically unreachable for some reason, if there is a {{{1}}} expression anywhere in the shader, then it is statically written.

Warning: If the fragment shader statically writes to gl_FragDepth, then it is the responsibility of the shader to statically write to the value in all circumstances. No matter what branches may or may not be taken, the shader must ensure that the value is written. So, if you conditionally write to it in one place, you should at least make sure that there is a single non-conditional write sometime before that.

OpenGL 4.2/ARB_conservative_depth allows the user to specify that modifications to gl_FragDepth (relative to the gl_FragCoord.z value it would have otherwise had) will happen in certain ways. This allows the implementation the freedom to not turn off Early Depth Tests in certain situations.

This is done by re-declaring gl_FragDepth with a special layout qualifier:

layout (depth_<condition>) out float gl_FragDepth;

The condition​ can be one of the following:

any

The default. You may freely change the depth, but you lose the most potential performance.

greater

You will only make the depth larger, compared to gl_FragDepth.z.

less

You will only make the depth smaller, compared to gl_FragDepth.z.

unchanged

If you write to gl_FragDepth, you will write exactly gl_FragDepth.z.

Violating the condition​ yields undefined behavior.

GL 4.0/ARB_sample_shading brings us:

out int gl_SampleMask[];

gl_SampleMask

This defines the sample mask for the fragment when performing mutlisampled rendering. If a shader does not statically write to it, then it will be filled in by gl_SampleMaskIn.

Warning: Just as with gl_FragDepth, if a fragment shader writes to it at all, it must make sure to write to the value for all execution paths. But it must also make sure to write to each element in the array. The array is sized the same way as for gl_SampleMaskIn.

Special Operations

Unlike every other shader stage, fragment shaders have implicit derivatives generated. As such, they can use the majority of the texturing functions. You still need to watch out for non-uniform flow control.

Fragment shaders also have access to the discard command. When executed, this command causes the fragment's output values to be discarded in the same way that a fragment is discarded if the depth test failed. Thus, the fragment does not proceed on to the next pipeline stages.

Normally, early tests, performing the depth/etc test before the fragment shader, is a transparent optimization. However, with GL 4.2/ARB_shader_image_load_store, early depth tests can be forced for a fragment shader using this syntax:

layout(early_fragment_tests) in;

Warning: This does not mean that you can subvert the meaning of the depth test. You cannot, for example, perform the depth test with one value and then write a different value to the depth buffer than the one you tested. If you attempt to write to gl_FragDepth when you force early-fragment tests, then the value written will be ignored. The value written to the depth buffer will always be the value tested against the depth buffer.

See also

• The previous pipeline stages:
  • Vertex Post-Processing
  • Rasterization
• The next pipeline stage:
  • Per-Sample Processing