GLPIXELTRANSFER() MachTen Programmer’s Manual 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, glCopyTexSubImage1D, glCopyTexSubIm-
age2D, glDrawPixels, glReadPixels, glTexImage1D, glTexIm-
age2D, 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 glCopyTexImage1D, glCopyTexImage2D,
glCopyTexSubImage1D, glCopyTexSubImage2D, and glReadPix-
els), or unpacked from client memory (glDrawPixels, glTex-
Image1D, glTexImage2D, glTexSubImage1D, and glTexSubIm-
age2D). 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 pix-
els 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 unspeci-
fied 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, sten-
cil 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 multi-
plied by a scale factor, then added to a bias
factor. That is, the red component is multi-
plied 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 multi-
plied by GL_BLUE_SCALE, then added to
GL_BLUE_BIAS; and the alpha component is mul-
tiplied by GL_ALPHA_SCALE, then added to
GL_ALPHA_BIAS. After all four color compo-
nents are scaled and biased, each is clamped
to the range [0,1]. All color, scale, and
bias values are specified with glPixelTrans-
fer.

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 con-
tents of that map indexed by the scaled compo-
nent. 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 glPixelTrans-
fer. 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 num-
ber 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 speci-
fied with
glPixelTransfer.

From this point, operation diverges depending
on the required format of the resulting pix-
els. If the resulting pixels are to be writ-
ten 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 glPix-
elTransfer. 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 $2 sup n ~-~ 1$, 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.

center box tab(:); ci | ci | ci | ci c | c | c | c .
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:(-oo,oo)
GL_INDEX_OFFSET:integer:0:(-oo,oo)
GL_RED_SCALE:float:1:(-oo,oo)
GL_GREEN_SCALE:float:1:(-oo,oo)
GL_BLUE_SCALE:float:1:(-oo,oo)
GL_ALPHA_SCALE:float:1:(-oo,oo)
GL_DEPTH_SCALE:float:1:(-oo,oo)
GL_RED_BIAS:float:0:(-oo,oo)
GL_GREEN_BIAS:float:0:(-oo,oo)
GL_BLUE_BIAS:float:0:(-oo,oo)
GL_ALPHA_BIAS:float:0:(-oo,oo)
GL_DEPTH_BIAS:float:0:(-oo,oo)

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 inte-
ger.

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 con-
verted to floating point before being assigned to real-
valued parameters.

NOTES
If a glCopyPixels, glCopyTexImage1D, glCopyTexImage2D,
glCopyTexSubImage1D, 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 corre-
sponding 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, glCopyTexIm-
age2D, glCopyTexSubImage1D, glCopyTexSubImage2D, glDraw-
Pixels, glNewList, glPixelMap, glPixelStore, glPixelZoom,
glReadPixels, glTexImage1D, glTexImage2D, glTexSubImage1D,
glTexSubImage2D

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