OpenGL extension EXT.packed_depth_stencil
This module customises the behaviour of the
OpenGL.raw.GL.EXT.packed_depth_stencil to provide a more
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:
|EXTENSION_NAME = 'GL_EXT_packed_depth_stencil'|
GL_DEPTH24_STENCIL8_EXT = GL_DEPTH24_STENCIL8_EXT
GL_DEPTH_STENCIL_EXT = GL_DEPTH_STENCIL_EXT
GL_TEXTURE_STENCIL_SIZE_EXT = GL_TEXTURE_STENCIL_SIZE_EXT
GL_UNSIGNED_INT = GL_UNSIGNED_INT
GL_UNSIGNED_INT_24_8_EXT = GL_UNSIGNED_INT_24_8_EXT
__file__ = '/home/mcfletch/pylive/OpenGL/GL/EXT/packed_depth_stencil.pyc'
__name__ = 'OpenGL.GL.EXT.packed_depth_stencil'
__package__ = 'OpenGL.GL.EXT'