'''OpenGL extension APPLE.rgb_422

This module customises the behaviour of the 
OpenGL.raw.GL.APPLE.rgb_422 to provide a more 
Python-friendly API

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
	both pixels.
	
	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 write their own fragment shader/program 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:
http://www.opengl.org/registry/specs/APPLE/rgb_422.txt
'''
from OpenGL import platform, constant, arrays
from OpenGL import extensions, wrapper
import ctypes
from OpenGL.raw.GL import _types, _glgets
from OpenGL.raw.GL.APPLE.rgb_422 import *
from OpenGL.raw.GL.APPLE.rgb_422 import _EXTENSION_NAME

def glInitRgb422APPLE():
    '''Return boolean indicating whether this extension is available'''
    from OpenGL import extensions
    return extensions.hasGLExtension( _EXTENSION_NAME )


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