glPixelTransfer

C Specification

void glPixelTransferf(
    GLenum	 pname,
    GLfloat	 param)
void glPixelTransferi(
    GLenum	 pname,
    GLint	 param)

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, or GL_DEPTH_BIAS.

param

Specifies the value that pname is set to.

Description

Pixel transfer operations handle four fundamental pixel types: color, color index, depth, and stencil. Color pixels consist of four floating-point values with unspecified mantissa and exponent sizes, scaled such that 0 represents zero intensity and 1 represents full intensity. Color indices 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. And 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 glPixelMap.

Color index

Each color index is shifted left by GL_INDEX_SHIFT bits; any bits beyond the number of fraction bits carried by the fixed-point index are filled with zeros. If GL_INDEX_SHIFT is negative, the shift is to the right, again zero 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 indices. 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 is specified with glPixelMap.

If the resulting pixels are to be written to an RGBA color buffer, or if they are read back to client memory in a format other than GL_COLOR_INDEX, the pixels are converted from indices 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 is specified with glPixelMap.

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.

pname	type	initial value	valid range
GL_MAP_COLOR	boolean	false	true/false
GL_MAP_STENCIL	boolean	false	true/false
GL_INDEX_SHIFT	integer	0	(, )
GL_INDEX_OFFSET	integer	0	(, )
GL_RED_SCALE	float	1	(, )
GL_GREEN_SCALE	float	1	(, )
GL_BLUE_SCALE	float	1	(, )
GL_ALPHA_SCALE	float	1	(, )
GL_DEPTH_SCALE	float	1	(, )
GL_RED_BIAS	float	0	(, )
GL_GREEN_BIAS	float	0	(, )
GL_BLUE_BIAS	float	0	(, )
GL_ALPHA_BIAS	float	0	(, )
GL_DEPTH_BIAS	float	0	(, )

glPixelTransferf can be used to set any pixel transfer parameter. If the parameter type is boolean, 0 implies false and any other value implies true. If pname is an integer parameter, param 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 param is 0 and to true otherwise. param is converted to floating point before being assigned to real-valued parameters.

Notes

Errors

• GL_INVALID_ENUM is generated if pname is not an accepted value.
• GL_INVALID_OPERATION is generated if glPixelTransfer is executed between the execution of glBegin and the corresponding execution of glEnd.

Associated Gets

See Also