- Vertex Specification
- Vertex Processing
- Vertex Post-Processing
- Primitive Assembly
- Fragment Shader
- Per-Sample Processing
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:
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.
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:
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.
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 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.
To set the stencil bitmask, use this function:
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.