glPixelTransfer Subroutine

OpenGL 1.1 for AIX: Reference Manual

glPixelTransfer Subroutine

Purpose

Sets pixel transfer modes.

Library

OpenGL C bindings library: libGL.a

C Syntax

void glPixelTransferf(GLenum pName, 
        GLfloat Parameter)

void glPixelTransferi(GLenum pName, 
        GLint Parameter)

Parameters

pName	Specifies the symbolic name of the pixel transfer parameter to be set. Must be one of the following: GL_MAP_COLOR GL_MAP_STENCIL GL_INDEX_SHIFT GL_INDEX_OFFSET GL_RED_SCALE GL_RED_BIAS GL_GREEN_SCALE GL_GREEN_BIAS GL_BLUE_SCALE GL_BLUE_BIAS GL_ALPHA_SCALE GL_ALPHA_BIAS GL_DEPTH_SCALE GL_DEPTH_BIAS
Parameter	Specifies the value to which pName is set.

Description

The glPixelTransfer subroutine sets pixel transfer modes that affect the operation of subsequent glDrawPixels, glReadPixels, glCopyPixels, glCopyTexImage1D, glCopyTexImage2D, glCopyTexSubImage1D, glCopyTexSubImage2D, glCopyTexSubImage3DEXT, glTexImage1D, glTexImage2D, glTexImage3DEXT, glTexSubImage1D, glTexSubImage2D, and glTexSubImage3DEXT subroutines. The algorithms that are specified by pixel transfer modes operate on pixels after they are read from the frame buffer (glReadPixels and glCopyPixels) or unpacked from client memory (glDrawPixels, glReadPixels, glCopyPixels, glCopyTexImage1D, glCopyTexImage2D, glCopyTexSubImage1D, glCopyTexSubImage2D, glCopyTexSubImage3DEXT, glTexImage1D, glTexImage2D, glTexImage3DEXT, glTexSubImage1D, glTexSubImage2D, and glTexSubImage3DEXT subroutines). Pixel transfer operations happen in the same order, and in the same manner, regardless of the command that resulted in the pixel operation. Pixel storage modes control the unpacking of pixels being read from client memory and the packing of pixels being written back into client memory. (See the glPixelStore subroutine for information on setting pixel storage modes.)

Pixel transfer operations handle four fundamental pixel types: color, color index, depth, and stencil. Color pixels are made up of four floating-point values with unspecified mantissa and exponent sizes, scaled such that 0.0 represents 0 (zero) intensity and 1.0 represents full intensity. Color indexes comprise a single fixed-point value, with unspecified precision to the right of the binary point. Depth pixels comprise a single floating-point value, with unspecified mantissa and exponent sizes, scaled such that 0.0 represents the minimum depth buffer value and 1.0 represents the maximum depth buffer value. Finally, stencil pixels comprise a single fixed-point value, with unspecified precision to the right of the binary point.

The pixel transfer operations performed on the four basic pixel types are as follows:

color	Each of the four color components is multiplied by a scale factor, then added to a bias factor. That is, the red component is multiplied by GL_RED_SCALE, then added to GL_RED_BIAS; the green component is multiplied by GL_GREEN_SCALE, then added to GL_GREEN_BIAS; the blue component is multiplied by GL_BLUE_SCALE, then added to GL_BLUE_BIAS; and the alpha component is multiplied by GL_ALPHA_SCALE, then added to GL_ALPHA_BIAS. After all four color components are scaled and biased, each is clamped to the range [0,1]. All color scale and bias values are specified with glPixelTransfer. If GL_MAP_COLOR is True, each color component is scaled by the size of the corresponding color-to-color map, then replaced by the contents of that map indexed by the scaled component. That is, the red component is scaled by GL_PIXEL_MAP_R_TO_R_SIZE, then replaced by the contents of GL_PIXEL_MAP_R_TO_R indexed by itself. The green component is scaled by GL_PIXEL_MAP_G_TO_G_SIZE, then replaced by the contents of GL_PIXEL_MAP_G_TO_G indexed by itself. The blue component is scaled by GL_PIXEL_MAP_B_TO_B_SIZE, then replaced by the contents of GL_PIXEL_MAP_B_TO_B indexed by itself. The alpha component is scaled by GL_PIXEL_MAP_A_TO_A_SIZE, then replaced by the contents of GL_PIXEL_MAP_A_TO_A indexed by itself. All components taken from the maps are then clamped to the range [0,1]. GL_MAP_COLOR is specified with glPixelTransfer. The contents of the various maps are specified with the glPixelMap subroutine.
color index	Each color index is shifted left by GL_INDEX_SHIFT bits, and any bits beyond the number of fraction bits carried by the fixed-point index are filled with 0s. If GL_INDEX_SHIFT is negative, the shift is to the right, again 0 filled. Then GL_INDEX_OFFSET is added to the index. GL_INDEX_SHIFT and GL_INDEX_OFFSET are specified with glPixelTransfer. From this point, operation diverges depending on the required format of the resulting pixels. If the resulting pixels are to be written to a color index buffer, or if they are being read back to client memory in GL_COLOR_INDEX format, the pixels continue to be treated as indexes. If GL_MAP_COLOR is True, each index is masked by 2n-1, where n is GL_PIXEL_MAP_I_TO_I_SIZE, then replaced by the contents of GL_PIXEL_MAP_I_TO_I indexed by the masked value. GL_MAP_COLOR is specified with glPixelTransfer. The contents of the index map are specified with the glPixelMap subroutine. If the resulting pixels are to be written to a red, green, blue, alpha (RGBA) color buffer, or if they are being read back to client memory in a format other than GL_COLOR_INDEX, the pixels are converted from indexes to colors by referencing the four maps 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. Before being dereferenced, the index is masked by 2n-1, where n is GL_PIXEL_MAP_I_TO_R_SIZE for the red map, GL_PIXEL_MAP_I_TO_G_SIZE for the green map, GL_PIXEL_MAP_I_TO_B_SIZE for the blue map, and GL_PIXEL_MAP_I_TO_A_SIZE for the alpha map. All components taken from the maps are then clamped to the range [0,1]. The contents of the four maps are specified with the glPixelMap subroutine.
depth	Each depth value is multiplied by GL_DEPTH_SCALE, added to GL_DEPTH_BIAS, then clamped to the range [0,1].
stencil	Each index is shifted GL_INDEX_SHIFT bits just as a color index is, then added to GL_INDEX_OFFSET. If GL_MAP_STENCIL is True, each index is masked by 2n-1, where n is GL_PIXEL_MAP_S_TO_S_SIZE, then replaced by the contents of GL_PIXEL_MAP_S_TO_S indexed by the masked value.

The following table gives the type, initial value, and range of valid values for each of the pixel transfer parameters that are set with glPixelTransfer.

pName	Type	Initial Value	Valid Range
GL_MAP_COLOR	Boolean	False	True or False
GL_MAP_STENCIL	Boolean	False	True or False
GL_INDEX_SHIFT	integer	0	(-infinity,+infinity)
GL_INDEX_OFFSET	integer	0	(-infinity,+infinity)
GL_RED_SCALE	float	1.0	(-infinity,+infinity)
GL_GREEN_SCALE	float	1.0	(-infinity,+infinity)
GL_BLUE_SCALE	float	1.0	(-infinity,+infinity)
GL_ALPHA_SCALE	float	1.0	(-infinity,+infinity)
GL_DEPTH_SCALE	float	1.0	(-infinity,+infinity)
GL_RED_BIAS	float	0.0	(-infinity,+infinity)
GL_GREEN_BIAS	float	0.0	(-infinity,+infinity)
GL_BLUE_BIAS	float	0.0	(-infinity,+infinity)
GL_ALPHA_BIAS	float	0.0	(-infinity,+infinity)
GL_DEPTH_BIAS	float	0.0	(-infinity,+infinity)

The glPixelTransferf subroutine can be used to set any pixel transfer parameter. If the parameter type is Boolean, 0.0 implies False and any other value implies True. If pName is an integer parameter, Parameter is rounded to the nearest integer.

Likewise, glPixelTransferi can be used to set any of the pixel transfer parameters. Boolean parameters are set to False if Parameter is 0 and True otherwise. Parameter is converted to floating-point format before being assigned to real-valued parameters.

Notes

If a glDrawPixels, glReadPixels, glCopyPixels, glTexImage1D, or glTexImage2D subroutine is placed in a display list (see the glNewList subroutine and the glCallList subroutine for information about display lists), the pixel transfer mode settings in effect when the display list is executed are the ones that are used. They may be different from the settings when the command was compiled into the display list.

Errors

GL_INVALID_ENUM	pName is not an accepted value.
GL_INVALID_OPERATION	The glPixelTransfer subroutine is called between a call to glBegin and the corresponding call to glEnd.

Associated Gets

Associated gets for the glPixelTransfer subroutine are as follows. (See the glGet subroutine for more information.)

glGet with argument GL_MAP_COLOR

glGet with argument GL_MAP_STENCIL

glGet with argument GL_INDEX_SHIFT

glGet with argument GL_INDEX_OFFSET

glGet with argument GL_RED_SCALE

glGet with argument GL_RED_BIAS

glGet with argument GL_GREEN_SCALE

glGet with argument GL_GREEN_BIAS

glGet with argument GL_BLUE_SCALE

glGet with argument GL_BLUE_BIAS

glGet with argument GL_ALPHA_SCALE

glGet with argument GL_ALPHA_BIAS

glGet with argument GL_DEPTH_SCALE

glGet with argument GL_DEPTH_BIAS.

Files

/usr/include/GL/gl.h

Contains C language constants, variable type definitions, and ANSI function prototypes for OpenGL.

Related Information

The glBegin or glEnd subroutine, glCallList subroutine, glCopyPixels subroutine, glCopyTexImage1D subroutine, glCopyTexImage2D subroutine, glCopyTexSubImage1D subroutine, glCopyTexSubImage2D subroutine, glCopyTexSubImage3DEXT subroutine, glDrawPixels subroutine, glNewList subroutine, glPixelMap subroutine, glPixelStore subroutine, glPixelZoom subroutine, glReadPixels subroutine, glTexImage1D subroutine, glTexImage2D subroutine, glTexImage3DEXT subroutine, glTexSubImage1D subroutine, glTexSubImage2D subroutine, glTexSubImage3DEXT subroutine, .

OpenGL Overview.