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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 )


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)