GLPIXELTRANSFER()GLPIXELTRANSFER()NAME
glPixelTransferf, glPixelTransferi - set pixel transfer modes
C SPECIFICATION
void glPixelTransferf( GLenum pname,
GLfloat param )
void glPixelTransferi( GLenum pname,
GLint param )
delim $$
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
glPixelTransfer sets pixel transfer modes that affect the operation of
subsequent glCopyPixels, glCopyTexImage1D, glCopyTexImage2D, glCopyTex‐
SubImage1D, glCopyTexSubImage2D, glDrawPixels, glReadPixels, glTexIm‐
age1D, glTexImage2D, glTexSubImage1D, and glTexSubImage2D commands.
The algorithms that are specified by pixel transfer modes operate on
pixels after they are read from the frame buffer (glCopyPixels glCopy‐
TexImage1D, glCopyTexImage2D, glCopyTexSubImage1D, glCopyTexSubImage2D,
and glReadPixels), or unpacked from client memory (glDrawPixels, glTex‐
Image1D, glTexImage2D, glTexSubImage1D, and glTexSubImage2D). 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 (see glPixelStore) control the unpacking of pixels being
read from client memory, and the packing of pixels being written back
into client memory.
Pixel transfer operations handle four fundamental pixel types: color,
color index, depth, and stencil. Color pixels consist of four float‐
ing-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 com‐
ponent 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 com‐
ponent 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 con‐
tents 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 $2 sup n ~-~
1$, 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 for‐
mat 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 $2 sup n ~-~ 1$, 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 $2 sup n ~-~ 1$, 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/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
If a glCopyPixels, glCopyTexImage1D, glCopyTexImage2D, glCopyTexSubIm‐
age1D, glCopyTexSubImage2D, glDrawPixels, glReadPixels, glTexImage1D,
glTexImage2D, glTexSubImage1D, or glTexSubImage2D command is placed in
a display list (see glNewList and glCallList), 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 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
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
SEE ALSO
glCallList, glCopyPixels, glCopyTexImage1D, glCopyTexImage2D, glCopy‐
TexSubImage1D, glCopyTexSubImage2D, glDrawPixels, glNewList, glPix‐
elMap, glPixelStore, glPixelZoom, glReadPixels, glTexImage1D, glTexIm‐
age2D, glTexSubImage1D, glTexSubImage2D
GLPIXELTRANSFER()
