glpixelstore - Man Page
glPixelStore(3G) OpenGL Reference glPixelStore(3G)
NAME
glPixelStoref, glPixelStorei - set pixel storage modes
C SPECIFICATION
void glPixelStoref( GLenum pname,
GLfloat param )
void glPixelStorei( GLenum pname,
GLint param )
PARAMETERS
pname Specifies the symbolic name of the parameter to be set. Eight
values affect the packing of pixel data into memory:
GL_PACK_SWAP_BYTES, GL_PACK_LSB_FIRST, GL_PACK_ROW_LENGTH,
GL_PACK_SKIP_PIXELS, GL_PACK_SKIP_ROWS, GL_PACK_ALIGNMENT,
GL_PACK_IMAGE_HEIGHT_EXT, GL_PACK_IMAGE_DEPTH_SGIS,
GL_PACK_SKIP_IMAGES_EXT, and GL_PACK_SKIP_VOLUMES_SGIS. Eight
more affect the unpacking of pixel data from memory:
GL_UNPACK_SWAP_BYTES, GL_UNPACK_LSB_FIRST, GL_UNPACK_ROW_LENGTH,
GL_UNPACK_SKIP_PIXELS, GL_UNPACK_SKIP_ROWS, GL_UNPACK_ALIGNMENT,
GL_UNPACK_IMAGE_HEIGHT_EXT, GL_UNPACK_IMAGE_DEPTH_SGIS,
GL_UNPACK_SKIP_IMAGES_EXT, and GL_UNPACK_SKIP_VOLUMES_SGIS.
param Specifies the value that pname is set to.
DESCRIPTION
glPixelStore sets pixel storage modes that affect the operation of
subsequent glDrawPixels and glReadPixels as well as the unpacking of
polygon stipple patterns (see glPolygonStipple), bitmaps (see glBitmap),
and texture patterns (see glTexImage1D, glTexImage2D, glTexImage3DEXT,
and glTexImage4DSGIS).
When glDrawPixels, glTexImage1D, glTexImage2D, glTexImage3DEXT, or
glTexImage4DSGIS is called, pixels are read from memory and then written
to either the framebuffer or to texture memory. Each pixel consists of a
single index value, a single depth component, or one, two, three or four
color components, depending on the pixel format. The pixels may be packed
into memory in which case all the elements of each pixel group are read
from a single unsigned byte, unsigned short, or unsigned int. If the
pixels are not packed, then the elements for each pixel group are read
from memory as a sequence of signed or unsigned bytes, shorts, or
integers, or single-precision floating-point values. The pixel type
determines the data type for the packed pixels or pixel elements and
whether or not the pixels are packed into memory. (See glDrawPixels for
more information.)
When glReadPixels or glGetTexImage is called, pixels are read from either
the framebuffer or from texture memory and then written into client
memory. Similiar to glDrawPixels the pixel format determines the format
of the pixel data and the pixel type indicates the data type and whether
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glPixelStore(3G) OpenGL Reference glPixelStore(3G)
or not the pixels are packed into memory. (See glReadPixels for more
information.)
pname is a symbolic constant indicating the parameter to be set, and
param is the new value. Eight of the sixteen storage parameters affect
how pixel data is returned to client memory, and are therefore
significant only for glReadPixels and glGetTexImage commands. They are
as follows:
GL_PACK_SWAP_BYTES
If true, byte swapping is performed as the data is written to
client memory. For pixels that aren't packed, byte ordering
for multibyte color components, depth components, color
indices, or stencil indices is reversed. That is, if a four-
byte component is made up of bytes b0, b1, b2, b3, it is stored
in memory as b3, b2, b1, b0 In the case of the packed pixel
types, byte swapping is performed before the elements are
extracted from each pixel.
GL_PACK_LSB_FIRST
If true, bits are ordered within a byte from least significant
to most significant; otherwise, the first bit in each byte is
the most significant one. This parameter is significant for
bitmap data only.
GL_PACK_ROW_LENGTH
If greater than zero, GL_PACK_ROW_LENGTH defines the number of
pixels in a row. If the first pixel of a row is placed at
location p in memory, then the location of the first pixel of
the next row is obtained b( skipping
|nl | s>a
k=| |___| s<a
|_| a |
components or indices, where|n is the number of components or
indices in a pixel, l is the number of pixels in a row
(GL_PACK_ROW_LENGTH if it is greater than zero, the width
argument to the pixel routine otherwise), a is the value of
GL_PACK_ALIGNMENT, and s is the size, in bytes, of a single
component (if a<s, then it is as if a=s). In the case of 1-bit
values, the location of the next row is obtained by skipping
|
|__|
k=8a|8a|
|
components or indices.
The word component in this description refers to the nonindex
values red, green, blue, alpha, and depth. Storage format
GL_RGB, for example, has three components per pixel: first
red, then green, and finally blue.
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glPixelStore(3G) OpenGL Reference glPixelStore(3G)
GL_PACK_SKIP_PIXELS and GL_PACK_SKIP_ROWS
These values are provided as a convenience to the programmer;
they provide no functionality that cannot be duplicated simply
by incrementing the pointer passed to glReadPixels. Setting
GL_PACK_SKIP_PIXELS to i is equivalent to incrementing the
pointer just once by in components or indices, where n is the
number of components or indices in each pixel. Setting
GL_PACK_SKIP_ROWS to j is equivalent to incrementing the
pointer just once by jk components or indices, where k is the
number of components or indices per row, as computed above in
the GL_PACK_ROW_LENGTH section. Note that the adjustment to
the pointer takes place just once, before the start of the
pixel transfer; specifying a value of one for GL_PACK_SKIP_ROWS
does not cause every other row of the image to be skipped.
GL_PACK_ALIGNMENT
Specifies the alignment requirements for the start of each
pixel row in memory. The allowable values are 1 (byte-
alignment), 2 (rows aligned to even-numbered bytes), 4 (word
alignment), and 8 (rows start on double-word boundaries).
The other six of the twelve storage parameters affect how pixel data is
read from client memory. These values are significant for glDrawPixels,
glTexImage1D, glTexImage2D, glBitmap, and glPolygonStipple. They are as
follows:
GL_UNPACK_SWAP_BYTES
If true, byte ordering for multibyte color components, depth
components, color indices, or stencil indices is reversed. That is,
if a four-byte component is made up of bytes b0, b1, b2, b3, it is
taken from memory as b3, b2, b1, b0 if GL_UNPACK_SWAP_BYTES is true.
GL_UNPACK_SWAP_BYTES has no effect on the memory order of components
within a pixel, only on the order of bytes within components or
indices. For example, the three components of a GL_RGB format pixel
are always stored with red first, green second, and blue third,
regardless of the value of GL_UNPACK_SWAP_BYTES.
GL_UNPACK_LSB_FIRST
If true, bits are ordered within a byte from least significant to
most significant; otherwise, the first bit in each byte is the most
significant one. This is significant for bitmap data only.
GL_UNPACK_ROW_LENGTH
If greater than zero, GL_UNPACK_ROW_LENGTH defines the number of
pixels in a row. If the first pixel of a row is placed at location
p in memory, then the location of the first pixel of the next row is
obtained by skipping
(nl s>a
| | | s<a
k=|_|___|
|s| a |
(
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glPixelStore(3G) OpenGL Reference glPixelStore(3G)
components or indices, where n is the number of components or
indices in a pixel, l is the number of pixels in a row
(GL_UNPACK_ROW_LENGTH if it is greater than zero, the width argument
to the pixel routine otherwise), a is the value of
GL_UNPACK_ALIGNMENT, and s is the size, in bytes, of a single
component (if a<s, then it is as if a=s). In the case of 1-bit
values, the location of the next row is obtained by skipping
| |
k=8a|__|
|8a|
components or indices.
The word component in this description refers to the nonindex values
red, green, blue, alpha, and depth. Storage format GL_RGB, for
example, has three components per pixel: first red, then green, and
finally blue.
GL_UNPACK_SKIP_PIXELS and GL_UNPACK_SKIP_ROWS
These values are provided as a convenience to the programmer; they
provide no functionality that cannot be duplicated simply by
incrementing the pointer passed to glDrawPixels, glTexImage1D,
glTexImage2D, glBitmap, or glPolygonStipple. Setting
GL_UNPACK_SKIP_PIXELS to i is equivalent to incrementing the pointer
just once by in components or indices, where n is the number of
components or indices in each pixel. Setting GL_UNPACK_SKIP_ROWS to
j is equivalent to incrementing the pointer just once by jk
components or indices, where k is the number of components or
indices per row, as computed above in the GL_UNPACK_ROW_LENGTH
section. As with GL_PACK_SKIP_PIXELS and GL_PACK_SKIP_ROWS, the
pointer adjustment happens only once; setting GL_UNPACK_SKIP_ROWS to
one does not cause every other row of pixels to be skipped.
GL_UNPACK_ALIGNMENT
Specifies the alignment requirements for the start of each pixel row
in memory. The allowable values are 1 (byte-alignment), 2 (rows
aligned to even-numbered bytes), 4 (word alignment), and 8 (rows
start on double-word boundaries).
The following table gives the type, initial value, and range of valid
values for each of the storage parameters that can be set with
glPixelStore.
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glPixelStore(3G) OpenGL Reference glPixelStore(3G)
__________________________________________________________________
|______________________|_________|_______________|_______________|
| GL_PACK_SWAP_BYTES | Boolean | false | true or false |
| GL_PACK_LSB_FIRST | Boolean | false | true or false |
| GL_PACK_ROW_LENGTH | integer | 0 | [0,oo) |
| GL_PACK_SKIP_ROWS | integer | 0 | [0,oo) |
| GL_PACK_SKIP_PIXELS | integer | 0 | [0,oo) |
| GL_PACK_ALIGNMENT | integer | 4 | 1, 2, 4, or 8 |
|______________________|_________|_______________|_______________|
|GL_UNPACK_SWAP_BYTES | Boolean | false | true or false |
| GL_UNPACK_LSB_FIRST | Boolean | false | true or false |
|GL_UNPACK_ROW_LENGTH | integer | 0 | [0,oo) |
| GL_UNPACK_SKIP_ROWS | integer | 0 | [0,oo) |
|GL_UNPACK_SKIP_PIXELS | integer | 0 | [0,oo) |
|______________________|_________|_______________|_______________|
glPixelStoref can be used to set any pixel store parameter. If the
parameter type is Boolean, then if param is 0.0, the parameter is false;
otherwise it is set to true. If pname is a integer type parameter, param
is rounded to the nearest integer.
Likewise, glPixelStorei can also be used to set any of the pixel store
parameters. Boolean parameters are set to false if param is 0 and true
otherwise.
NOTES
The pixel storage modes in effect when glDrawPixels, glReadPixels,
glTexImage1D, glTexImage2D, glBitmap, or glPolygonStipple is placed in a
display list control the interpretation of memory data. The pixel
storage modes in effect when a display list is executed are not
significant.
The formulas in this man page are incorrect for 3 dimensional images.
ERRORS
GL_INVALID_ENUM is generated if pname is not an accepted value.
GL_INVALID_VALUE is generated if a negative row length, pixel skip, or
row skip value is specified, or if alignment is specified as other than
1, 2, 4, or 8.
GL_INVALID_OPERATION is generated if glPixelStore is executed between the
execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGet with argument GL_PACK_SWAP_BYTES
glGet with argument GL_PACK_LSB_FIRST
glGet with argument GL_PACK_ROW_LENGTH
glGet with argument GL_PACK_SKIP_ROWS
glGet with argument GL_PACK_SKIP_PIXELS
glGet with argument GL_PACK_ALIGNMENT
glGet with argument GL_UNPACK_SWAP_BYTES
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glPixelStore(3G) OpenGL Reference glPixelStore(3G)
glGet with argument GL_UNPACK_LSB_FIRST
glGet with argument GL_UNPACK_ROW_LENGTH
glGet with argument GL_UNPACK_SKIP_ROWS
glGet with argument GL_UNPACK_SKIP_PIXELS
glGet with argument GL_UNPACK_ALIGNMENT
SEE ALSO
glBitmap, glDrawPixels, glPixelMap, glPixelTransfer, glPixelZoom,
glPolygonStipple, glReadPixels, glTexImage1D, glTexImage2D
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