Write Mask

OpenGL Rendering Pipeline

Rendering Pipeline Overview

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• Vertex Specification
  • Vertex Rendering
  • Primitive
• Vertex Processing
  • Vertex Shader
  • Tessellation
  • Geometry Shader
• Vertex Post-Processing
  • Transform Feedback
• Primitive Assembly
  • Face Culling
• Rasterization
• Fragment Shader
• Per-Sample Processing
  • Scissor Test
  • Stencil Test
  • Depth Test
  • Blending
  • Logical Operation
  • Write Mask

The Write Mask is the part of the rendering pipeline that allows or prevents color, depth, or stencil components from being written to the current framebuffer.

Fragments can contain a number of color values, a depth value, and a stencil value. All but the stencil can be generated by a Fragment Shader. After various testing operations, the fragment's values can be written to a sample in the current Framebuffer. Write masking state can prevent certain components of data from being written.

Masking state affects these operations that modify the current Framebuffer:

• All Drawing Commands.
• Framebuffer clearing calls, glClear and glClearBuffer.

Note: This does not apply to operations that affect images that just so happen to be part of the current framebuffer. For example, if you use Image Load Store to modify an image's data, the write mask will not affect that write even if that image is also part of the current Framebuffer Object (of course, trying to write to the same image in two ways also yields undefined behavior). The write mask only applies to write operations that go through to the current Framebuffer.

Even though the mask only applies to writes to a framebuffer, the mask state is not Framebuffer state. So it is not part of a Framebuffer Object or the Default Framebuffer. Binding a new framebuffer will not affect the mask.

Color Mask

The color(s) output from the fragment shader can be masked. Each individual color component, R, G, B, and A, has a separate mask. So it is possible to prevent certain color components from being written.

Each separate color buffer, as defined by the glDrawBuffers function, has a separate mask.

To set the mask for a particular draw buffer, use the glColorMaski function:

void glColorMaski(GLuint buf​, GLboolean red​, GLboolean green​, GLboolean blue​, GLboolean alpha​);

The buf​ parameter specifies which of the color buffers to set the mask for. This can be any value on the range [0, GL_MAX_DRAW_BUFFERS). The other values are set to GL_TRUE to enable writing that component and GL_FALSE to disable writing it.

The function glColorMask can be used to set the mask for all draw buffers. This is useful for quickly re-enabling write masks on all output buffers.

Depth Mask

The depth buffer write can be masked, thus preventing the depth buffer from being updated. This useful for implementing transparency. Masking is controlled by this function:

 void glDepthMask(GLboolean flag​);

Setting flag​ to GL_TRUE means that the depth is written.

Stencil Mask

Stencil write masking works slightly differently, in two ways. First, the Stencil Buffer is always an unsigned integer (of some bitdepth), so the mask here is an actual bitfield, rather than simply being on/off.

The other difference is that the Stencil Test can be different for the two sides of triangles. Therefore, the stencil write mask can be set for either side, or both at once.

To set the stencil bitmask, use this function:

void glStencilMaskSeparate(GLenum face​, GLuint mask​);

The face​ determines which stencil bitmask facing is being set. It can be GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK to set the bitmask for both sides at once. The bits set in mask​ are the only stencil bits that will be affected by any subsequent rendering commands.

The glStencilMask function can be used as shorthand for GL_FRONT_AND_BACK.