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 )
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 format 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 cor-ner corner
ner is at location (x, y), into client memory starting at location pixels. Several parameters con-trol control
trol 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 opera-tions operations
tions including color table lookup, color matrix tranformations, 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 con-
verted 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 compo-nent component
nent 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^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^n, it must be masked against 2^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 sin-gle-component single-component
gle-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 singleprecision
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 val-ues values
ues 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)
GLREADPIXELS(3G)
|