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Manual Reference Pages  - GLREADPIXELS (3G)

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glReadPixels - read a block of pixels from the frame buffer

CONTENTS

C Specification
Parameters
Description
Notes
Errors
Associated Gets
See Also

void glReadPixels( GLint x,

                 GLint y,
                 GLsizei width,
                 GLsizei height,
                 GLenum format,
                 GLenum type,
                 GLvoid *pixels )

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.

 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 ith pixel in the jth 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 2n, but n may be different for different tables. Before an index is used to look up a value in a table of size 2n, it must be masked against 2n-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 ith pixel in the jth 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.

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.

 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.

 glGet with argument  GL_INDEX_MODE

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

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