glReadPixels function

Article
12/11/2020

The glReadPixels function reads a block of pixels from the framebuffer.

Syntax

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

Parameters

x

The window x coordinate of the first pixel that is read from the framebuffer. Together with the y coordinate, specifies the location of the lower-left corner of a rectangular block of pixels.

y

The window y coordinates of the first pixel that is read from the framebuffer. Together with the x coordinate, specifies the location of the lower-left corner of a rectangular block of pixels.

width

The width of the pixel rectangle.

height

The height of the pixel rectangle. Width and height parameters of value "1" correspond to a single pixel.

format

The format of the pixel data. The following symbolic values are accepted:

Value	Meaning
GL_COLOR_INDEX	Color indexes 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, indexes 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, indexes 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 0.0 and the maximum value maps to 1.0. 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_RGBA, GL_BGR_EXT, GL_BGRA_EXT, GL_LUMINANCE, GL_LUMINANCE_ALPHA	Processing differs depending on whether color buffers store color indexes or RGBA color components. If color indexes 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. Indexes are then replaced by the red, green, blue, and alpha values obtained by indexing the 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 tables. 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 GL_RED, GL_GREEN, GL_BLUE, and GL_ALPHA. Each component is clamped to the range [0,1]. Finally, if GL_MAP_COLOR is GL_TRUE, each color component c is replaced by its mapping in the table GL_PIXEL_MAP_c_TO_c, where c again is GL_RED, GL_GREEN, GL_BLUE, and GL_ALPHA. Each component is scaled to the size of its corresponding table before the lookup is performed. Finally, unneeded data is 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.

type

The data type of the pixel data. Must be one of the following values.

Type	Index mask	Component conversion
GL_UNSIGNED_BYTE	281	(281)c
GL_BYTE	271	[(271)c-1]/2
GL_BITMAP	1	1
GL_UNSIGNED_SHORT	2 61	(2 61)c
GL_SHORT	2 51	[(2 51)c1]/2
GL_UNSIGNED_INT_	2 1	(2 1)c
GL_INT	2 1	[(2 1)c1]/2
GL_FLOAT	none	c

pixels

Returns the pixel data.

Return value

This function does not return a value.

Error codes

The following error codes can be retrieved by the glGetError function.

Name	Meaning
GL_INVALID_ENUM	format or type was not an accepted value.
GL_INVALID_VALUE	Either width or height was negative.
GL_INVALID_OPERATION	format was GL_COLOR_INDEX, and the color buffers stored RGBA or BGRA color components.
GL_INVALID_OPERATION	format was GL_STENCIL_INDEX, and there was no stencil buffer.
GL_INVALID_OPERATION	format was GL_DEPTH_COMPONENT, and there was no depth buffer.
GL_INVALID_OPERATION	The function was called between a call to glBegin and the corresponding call to glEnd.

Remarks

The glReadPixels function returns pixel data from the framebuffer, 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 topic describes the effects on glReadPixels of most, but not all of the parameters specified by these three commands.

The glReadPixels function 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.

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

The final step involves converting the indexes or components to the proper format, 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 format.

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

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_EXT return three values, GL_RGBA and GL_BGRA_EXT 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_SWAP_BYTES and GL_PACK_LSB_FIRST, 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 OpenGL context are undefined.

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

The following function retrieves information related to glReadPixels:

glGet with argument GL_INDEX_MODE

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Header	Gl.h
Library	Opengl32.lib
DLL	Opengl32.dll

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