OpenGL.GL.EXT.packed_depth_stencil

OpenGL extension EXT.packed_depth_stencil

This module customises the behaviour of the OpenGL.raw.GL.EXT.packed_depth_stencil to provide a more Python-friendly API

Overview (from the spec)

Many OpenGL implementations have chosen to interleave the depth and stencil buffers into one buffer, often with 24 bits of depth precision and 8 bits of stencil data. 32 bits is more than is needed for the depth buffer much of the time; a 24-bit depth buffer, on the other hand, requires that reads and writes of depth data be unaligned with respect to power-of-two boundaries. On the other hand, 8 bits of stencil data is more than sufficient for most applications, so it is only natural to pack the two buffers into a single buffer with both depth and stencil data. OpenGL never provides direct access to the buffers, so the OpenGL implementation can provide an interface to applications where it appears the one merged buffer is composed of two logical buffers.

One disadvantage of this scheme is that OpenGL lacks any means by which this packed data can be handled efficiently. For example, when an application reads from the 24-bit depth buffer, using the type GL_UNSIGNED_SHORT will lose 8 bits of data, while GL_UNSIGNED_INT has 8 too many. Both require expensive format conversion operations. A 24-bit format would be no more suitable, because it would also suffer from the unaligned memory accesses that made the standalone 24-bit depth buffer an unattractive proposition in the first place.

Many applications, such as parallel rendering applications, may also wish to draw to or read back from both the depth and stencil buffers at the same time. Currently this requires two separate operations, reducing performance. Since the buffers are interleaved, drawing to or reading from both should be no more expensive than using just one; in some cases, it may even be cheaper.

This extension provides a new data format, GL_DEPTH_STENCIL_EXT, that can be used with the glDrawPixels, glReadPixels, and glCopyPixels commands, as well as a packed data type, GL_UNSIGNED_INT_24_8_EXT, that is meant to be used with GL_DEPTH_STENCIL_EXT. No other data types are supported with GL_DEPTH_STENCIL_EXT. If ARB_depth_texture or SGIX_depth_texture is supported, GL_DEPTH_STENCIL_EXT/GL_UNSIGNED_INT_24_8_EXT data can also be used for textures; this provides a more efficient way to supply data for a 24-bit depth texture.

GL_DEPTH_STENCIL_EXT data, when passed through the pixel path, undergoes both depth and stencil operations. The depth data is scaled and biased by the current GL_DEPTH_SCALE and GL_DEPTH_BIAS, while the stencil data is shifted and offset by the current GL_INDEX_SHIFT and GL_INDEX_OFFSET. The stencil data is also put through the stencil-to-stencil pixel map.

glDrawPixels of GL_DEPTH_STENCIL_EXT data operates similarly to that of GL_STENCIL_INDEX data, bypassing the OpenGL fragment pipeline entirely, unlike the treatment of GL_DEPTH_COMPONENT data. The stencil and depth masks are applied, as are the pixel ownership and scissor tests, but all other operations are skipped.

glReadPixels of GL_DEPTH_STENCIL_EXT data reads back a rectangle from both the depth and stencil buffers.

glCopyPixels of GL_DEPTH_STENCIL_EXT data copies a rectangle from both the depth and stencil buffers. Like glDrawPixels, it applies both the stencil and depth masks but skips the remainder of the OpenGL fragment pipeline.

glTex[Sub]Image[1,2,3]D of GL_DEPTH_STENCIL_EXT data loads depth and stencil data into a depth_stencil texture. glGetTexImage of GL_DEPTH_STENCIL_EXT data can be used to retrieve depth and stencil data from a depth/stencil texture.

In addition, a new base internal format, GL_DEPTH_STENCIL_EXT, can be used by both texture images and renderbuffer storage. When an image with a DEPTH_STENCIL_EXT internal format is attached to both the depth and stencil attachment points of a framebuffer object (see EXT_framebuffer_object), then it becomes both the depth and stencil buffers of the framebuffer. This fits nicely with hardware that interleaves both depth and stencil data into a single buffer. When a texture with DEPTH_STENCIL_EXT data is bound for texturing, only the depth component is accessible through the texture fetcher. The stencil data can be written with TexImage or CopyTexImage, and can be read with GetTexImage. When a DEPTH_STENCIL_EXT image is attached to the stencil attachment of the bound framebuffer object, the stencil data can be accessed through any operation that reads from or writes to the framebuffer's stencil buffer.

The official definition of this extension is available here: http://www.opengl.org/registry/specs/EXT/packed_depth_stencil.txt