OpenGL extension APPLE.rgb_422
This module customises the behaviour of the
OpenGL.raw.GL.APPLE.rgb_422 to provide a more
Overview (from the spec)
A common storage format for video data is 8-bit 422, with every four
bytes encoding two pixels. Within the four bytes there are two
luminance samples, and two chrominance samples that are shared between
There is a previous extension, namely GL_APPLE_ycbcr_422 that provided
transparent support for this kind of data. However, that extension
left the exact conversion from Y'CbCr to RGB undefined. In reality,
it really had always been based on the ITU-R BT.601 standard, which
meant it was not particularly useful for dealing with high definition
video data, which is encoded using the Rec. 709 standard.
In some cases the original extension was implemented via fixed function
hardware, but on more modern graphics processors this is done via
a combination of 422 sampling formats and fragment shader instructions.
This extension essentially exposes a "raw" 422 texture format that
allows developers to access the raw pre-converted Y'CbCr components
so that they have full control over the colorspace conversion.
In order to avoid defining entirely new color channels within GL,
the Y, Cb and Cr color channels within the 422 data are mapped into
the existing green, blue and red color channels, respectively. Developers
must essentially write their own fragment shader to perform the desired
color space transformation.
Note: Because of the use of the packed UNSIGNED_SHORT_8_8[_REV] types, the
correct type to use based on the layout of the data in memory (Cb Y Cr Y
versus Y Cb Y Cr) will necessarily be sensitive to host endianness.
This extension differs from the EXT_422_pixels extension in a couple of
ways. First, this extension defines only a single new format, while
relying on two new type arguments to differentiate between the two
component orderings. Second, this extension provides no defined method
of filtering the chroma values between adjacent pixels. And lastly,
the color channel assignments are slightly different, essentially to
match more closely the rough meanings of the Y, Cb and Cr values in
422 video data.
The official definition of this extension is available here:
|EXTENSION_NAME = 'GL_APPLE_rgb_422'|
GL_RGB_422_APPLE = GL_RGB_422_APPLE
__file__ = '/home/mcfletch/pylive/OpenGL/GL/APPLE/rgb_422.pyc'
__name__ = 'OpenGL.GL.APPLE.rgb_422'
__package__ = 'OpenGL.GL.APPLE'