glReadPixels man page on OpenBSD

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GLREADPIXELS(3G)					      GLREADPIXELS(3G)

NAME
       glReadPixels - read a block of pixels from the frame buffer

C SPECIFICATION
       void glReadPixels( GLint x,
	    GLint y,
	    GLsizei width,
	    GLsizei height,
	    GLenum format,
	    GLenum type,
	    GLvoid *pixels )

       delim $$

PARAMETERS
       x, y   Specify the window coordinates of the first pixel that is read
	      from the frame buffer.  This location is the lower left corner
	      of a rectangular block of pixels.

       width, height
	      Specify the dimensions of the pixel rectangle.  width and height
	      of one correspond to a single pixel.

       format Specifies the  of the pixel data.	 The following symbolic values
	      are accepted: GL_COLOR_INDEX, GL_STENCIL_INDEX,
	      GL_DEPTH_COMPONENT, GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB,
	      GL_BGR, GL_RGBA, GL_BGRA, GL_LUMINANCE, and GL_LUMINANCE_ALPHA.

       type   Specifies the data type of the pixel data.  Must be one of
	      GL_UNSIGNED_BYTE, GL_BYTE, GL_BITMAP, GL_UNSIGNED_SHORT,
	      GL_SHORT, GL_UNSIGNED_INT, GL_INT, GL_FLOAT,
	      GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV,
	      GL_UNSIGNED_SHORT_5_6_5, GL_UNSIGNED_SHORT_5_6_5_REV,
	      GL_UNSIGNED_SHORT_4_4_4_4, GL_UNSIGNED_SHORT_4_4_4_4_REV,
	      GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV,
	      GL_UNSIGNED_INT_8_8_8_8, GL_UNSIGNED_INT_8_8_8_8_REV,
	      GL_UNSIGNED_INT_10_10_10_2, or GL_UNSIGNED_INT_2_10_10_10_REV.

       pixels Returns the pixel data.

DESCRIPTION
       glReadPixels returns pixel data from the frame buffer, starting with
       the pixel whose lower left corner is at location (x, y), into client
       memory starting at location pixels.  Several parameters control the
       processing of the pixel data before it is placed into client memory.
       These parameters are set with three commands: glPixelStore,
       glPixelTransfer, and glPixelMap.	 This reference page describes the
       effects on glReadPixels of most, but not all of the parameters
       specified by these three commands.

       When the GL_ARB_imaging extension is supported, the pixel data may be
       processed by additional operations including color table lookup, color
       matrix transformations, convolutions, histograms and minimum and
       maximun pixel value computations.

       glReadPixels returns values from each pixel with lower left corner at
       (x + $i$, y + $j$) for 0 <= $i$ < width and 0 <= $j$ < height.  This
       pixel is said to be the $i$th pixel in the $j$th row.  Pixels are
       returned in row order from the lowest to the highest row, left to right
       in each row.

       format specifies the  for the returned pixel values; accepted values
       are:

       GL_COLOR_INDEX
		 Color indices are read from the color buffer selected by
		 glReadBuffer.	Each index is converted to fixed point,
		 shifted left or right depending on the value and sign of
		 GL_INDEX_SHIFT, and added to GL_INDEX_OFFSET.	If
		 GL_MAP_COLOR is GL_TRUE, indices are replaced by their
		 mappings in the table GL_PIXEL_MAP_I_TO_I.

       GL_STENCIL_INDEX
		 Stencil values are read from the stencil buffer.  Each index
		 is converted to fixed point, shifted left or right depending
		 on the value and sign of GL_INDEX_SHIFT, and added to
		 GL_INDEX_OFFSET.  If GL_MAP_STENCIL is GL_TRUE, indices are
		 replaced by their mappings in the table GL_PIXEL_MAP_S_TO_S.

       GL_DEPTH_COMPONENT
		 Depth values are read from the depth buffer.  Each component
		 is converted to floating point such that the minimum depth
		 value maps to 0 and the maximum value maps to 1.  Each
		 component is then multiplied by GL_DEPTH_SCALE, added to
		 GL_DEPTH_BIAS, and finally clamped to the range [0,1].

       GL_RED

       GL_GREEN

       GL_BLUE

       GL_ALPHA

       GL_RGB

       GL_BGR

       GL_RGBA

       GL_BGRA

       GL_LUMINANCE

       GL_LUMINANCE_ALPHA
		 Processing differs depending on whether color buffers store
		 color indices or RGBA color components.  If color indices are
		 stored, they are read from the color buffer selected by
		 glReadBuffer.	Each index is converted to fixed point,
		 shifted left or right depending on the value and sign of
		 GL_INDEX_SHIFT, and added to GL_INDEX_OFFSET.	Indices are
		 then replaced by the red, green, blue, and alpha values
		 obtained by indexing the tables GL_PIXEL_MAP_I_TO_R,
		 GL_PIXEL_MAP_I_TO_G, GL_PIXEL_MAP_I_TO_B, and
		 GL_PIXEL_MAP_I_TO_A.  Each table must be of size $2 sup n$,
		 but $n$ may be different for different tables.	 Before an
		 index is used to look up a value in a table of size $2 sup
		 n$, it must be masked against $2 sup n - 1$.

		 If RGBA color components are stored in the color buffers,
		 they are read from the color buffer selected by glReadBuffer.
		 Each color component is converted to floating point such that
		 zero intensity maps to 0.0 and full intensity maps to 1.0.
		 Each component is then multiplied by GL_c_SCALE and added to
		 GL_c_BIAS, where c is RED, GREEN, BLUE, or ALPHA.  Finally,
		 if GL_MAP_COLOR is GL_TRUE, each component is clamped to the
		 range [0, 1], scaled to the size of its corresponding table,
		 and is then replaced by its mapping in the table
		 GL_PIXEL_MAP_c_TO_c, where c is R, G, B, or A.

		 Unneeded data is then discarded.  For example, GL_RED
		 discards the green, blue, and alpha components, while GL_RGB
		 discards only the alpha component.  GL_LUMINANCE computes a
		 single-component value as the sum of the red, green, and blue
		 components, and GL_LUMINANCE_ALPHA does the same, while
		 keeping alpha as a second value.  The final values are
		 clamped to the range [0, 1].

       The shift, scale, bias, and lookup factors just described are all
       specified by
       glPixelTransfer.	 The lookup table contents themselves are specified by
       glPixelMap.

       Finally, the indices or components are converted to the proper , as
       specified by type.  If format is GL_COLOR_INDEX or GL_STENCIL_INDEX and
       type is not GL_FLOAT, each index is masked with the mask value given in
       the following table.  If type is GL_FLOAT, then each integer index is
       converted to single-precision floating-point .

       If format is GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_BGR,
       GL_RGBA, GL_BGRA, GL_LUMINANCE, or GL_LUMINANCE_ALPHA and type is not
       GL_FLOAT, each component is multiplied by the multiplier shown in the
       following table.	 If type is GL_FLOAT, then each component is passed as
       is (or converted to the client's single-precision floating-point	 if it
       is different from the one used by the GL).

       --------------------------------------------------------------
       type		    index mask	     component conversion
       --------------------------------------------------------------
       GL_UNSIGNED_BYTE	   $2"^"8 - 1$	       $(2"^"8 - 1)  c$
       GL_BYTE		   $2"^"7 - 1$	  $[(2"^"8 - 1) c - 1] / 2$
       GL_BITMAP	       $1$		     $1$
       GL_UNSIGNED_SHORT   $2"^"16 - 1$	       $(2"^"16 - 1) c$
       GL_SHORT		   $2"^"15 - 1$	  $[(2"^"16 - 1) c - 1] / 2$
       GL_UNSIGNED_INT	   $2"^"32 - 1$	       $(2"^"32 - 1) c$
       GL_INT		   $2"^"31 - 1$	  $[(2"^"32 - 1) c - 1] / 2$
       GL_FLOAT		       none		     $c$
       --------------------------------------------------------------

       Return values are placed in memory as follows.  If format is
       GL_COLOR_INDEX, GL_STENCIL_INDEX, GL_DEPTH_COMPONENT, GL_RED, GL_GREEN,
       GL_BLUE, GL_ALPHA, or GL_LUMINANCE, a single value is returned and the
       data for the $i$th pixel in the $j$th row is placed in location
       $(j)~"width"~+~i$.  GL_RGB and GL_BGR return three values, GL_RGBA and
       GL_BGRA return four values, and GL_LUMINANCE_ALPHA returns two values
       for each pixel, with all values corresponding to a single pixel
       occupying contiguous space in pixels.  Storage parameters set by
       glPixelStore, such as GL_PACK_LSB_FIRST and GL_PACK_SWAP_BYTES, affect
       the way that data is written into memory.  See glPixelStore for a
       description.

NOTES
       Values for pixels that lie outside the window connected to the current
       GL context are undefined.

       If an error is generated, no change is made to the contents of pixels.

ERRORS
       GL_INVALID_ENUM is generated if format or type is not an accepted
       value.

       GL_INVALID_ENUM is generated if type is GL_BITMAP and format is not
       GL_COLOR_INDEX or GL_STENCIL_INDEX.

       GL_INVALID_VALUE is generated if either width or height is negative.

       GL_INVALID_OPERATION is generated if format is GL_COLOR_INDEX and the
       color buffers store RGBA color components.

       GL_INVALID_OPERATION is generated if format is GL_STENCIL_INDEX and
       there is no stencil buffer.

       GL_INVALID_OPERATION is generated if format is GL_DEPTH_COMPONENT and
       there is no depth buffer.

       GL_INVALID_OPERATION is generated if glReadPixels is executed between
       the execution of glBegin and the corresponding execution of glEnd.

       GL_INVALID_OPERATION is generated if type is one of
       GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV,
       GL_UNSIGNED_SHORT_5_6_5, or GL_UNSIGNED_SHORT_5_6_5_REV and format is
       not GL_RGB.

       GL_INVALID_OPERATION is generated if type is one of
       GL_UNSIGNED_SHORT_4_4_4_4, GL_UNSIGNED_SHORT_4_4_4_4_REV,
       GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV,
       GL_UNSIGNED_INT_8_8_8_8, GL_UNSIGNED_INT_8_8_8_8_REV,
       GL_UNSIGNED_INT_10_10_10_2, or GL_UNSIGNED_INT_2_10_10_10_REV and
       format is neither GL_RGBA nor GL_BGRA.

       The formats GL_BGR, and GL_BGRA and types GL_UNSIGNED_BYTE_3_3_2,
       GL_UNSIGNED_BYTE_2_3_3_REV, GL_UNSIGNED_SHORT_5_6_5,
       GL_UNSIGNED_SHORT_5_6_5_REV, GL_UNSIGNED_SHORT_4_4_4_4,
       GL_UNSIGNED_SHORT_4_4_4_4_REV, GL_UNSIGNED_SHORT_5_5_5_1,
       GL_UNSIGNED_SHORT_1_5_5_5_REV, GL_UNSIGNED_INT_8_8_8_8,
       GL_UNSIGNED_INT_8_8_8_8_REV, GL_UNSIGNED_INT_10_10_10_2, and
       GL_UNSIGNED_INT_2_10_10_10_REV are available only if the GL version is
       1.2 or greater.

ASSOCIATED GETS
       glGet with argument GL_INDEX_MODE

SEE ALSO
       glCopyPixels(3G), glDrawPixels(3G), glPixelMap(3G), glPixelStore(3G),
       glPixelTransfer(3G),
       glReadBuffer(3G)

								 March 1, 2011
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