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GLPIXELSTORE() GLPIXELSTORE() NAME glPixelStoref, glPixelStorei - set pixel storage modes C SPECIFICATION void glPixelStoref( GLenum pname, GLfloat param ) void glPixelStorei( GLenum pname, GLint param ) delim $$ PARAMETERS pname Specifies the symbolic name of the parameter to be set. Six 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, and GL_PACK_ALIGNMENT. Six more affect the unpacking of pixel data from memory: GL_UN- PACK_SWAP_BYTES, GL_UNPACK_LSB_FIRST, GL_UNPACK_ROW_LENGTH, GL_UNPACK_SKIP_PIXELS, GL_UNPACK_SKIP_ROWS, and GL_UNPACK_ALIGN- MENT. param Specifies the value that pname is set to. DESCRIPTION glPixelStore sets pixel storage modes that affect the operation of sub- sequent glDrawPixels and glReadPixels as well as the unpacking of poly- gon stipple patterns (see glPolygonStipple), bitmaps (see glBitmap), and texture patterns (see glTexImage1D, glTexImage2D, glTexSubImage1D, and glTexSubImage2D). pname is a symbolic constant indicating the parameter to be set, and param is the new value. Six of the twelve storage parameters affect how pixel data is returned to client memory, and are therefore signifi- cant only for glReadPixels commands. They are as follows: GL_PACK_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 consists of bytes $b sub 0$, $b sub 1$, $b sub 2$, $b sub 3$, it is stored in memory as $b sub 3$, $b sub 2$, $b sub 1$, $b sub 0$ if GL_PACK_SWAP_BYTES is true. GL_PACK_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, regard- less of the value of GL_PACK_SWAP_BYTES. GL_PACK_LSB_FIRST If true, bits are ordered within a byte from least signifi- cant to most significant; otherwise, the first bit in each byte is the most significant one. This parameter is signifi- cant for bitmap data only. GL_PACK_ROW_LENGTH If greater than 0, 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 by skipping $k ~=~ left { lpile { n l above {a over s left ceiling { s n l } over a right ceiling}} ~~ lpile {s >= a above s < a }$ 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 0, the $width$ ar- gument 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 ob- tained by skipping $k ~=~ 8 a left ceiling { n l } over { 8 a } right ceiling$ 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_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 sim- ply by incrementing the pointer passed to glReadPixels. Set- ting GL_PACK_SKIP_PIXELS to $i$ is equivalent to incrementing the pointer by $i n$ 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 by $j k$ components or indices, where $k$ is the num- ber of components or indices per row, as just computed in the GL_PACK_ROW_LENGTH section. GL_PACK_ALIGNMENT Specifies the alignment requirements for the start of each pixel row in memory. The allowable values are 1 (byte-align- ment), 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 glDrawPix- els, glTexImage1D, glTexImage2D, glTexSubImage1D, glTexSubImage2D, 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 consists of bytes $b sub 0$, $b sub 1$, $b sub 2$, $b sub 3$, it is taken from memory as $b sub 3$, $b sub 2$, $b sub 1$, $b sub 0$ if GL_UNPACK_SWAP_BYTES is true. GL_UNPACK_SWAP_BYTES has no effect on the memory order of compo- nents within a pixel, only on the order of bytes within compo- nents 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 relevant only for bitmap data. GL_UNPACK_ROW_LENGTH If greater than 0, GL_UNPACK_ROW_LENGTH defines the number of pixels in a row. If the first pixel of a row is placed at loca- tion $p$ in memory, then the location of the first pixel of the next row is obtained by skipping $k ~=~ left { lpile { n l above {a over s left ceiling { s n l } over a right ceiling}} ~~ lpile {s >= a above s < a }$ 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_UN- PACK_ROW_LENGTH if it is greater than 0, the $width$ argument to the pixel routine otherwise), $a$ is the value of GL_UN- PACK_ALIGNMENT, and $s$ is the size, in bytes, of a single com- ponent (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 skip- ping $k ~=~ 8 a left ceiling { n l } over { 8 a } right ceiling$ 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 by in- crementing the pointer passed to glDrawPixels, glTexImage1D, gl- TexImage2D, glTexSubImage1D, glTexSubImage2D, glBitmap, or glPolygonStipple. Setting GL_UNPACK_SKIP_PIXELS to $i$ is equivalent to incrementing the pointer by $i n$ 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 by $j k$ components or indices, where $k$ is the number of components or indices per row, as just com- puted in the GL_UNPACK_ROW_LENGTH section. 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 storage parameter that can be set with glPixelStore. +-----------------------+---------+---------------+----------------+ | pname | type | initial value | valid range | +-----------------------+---------+---------------+----------------+ | 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,<infinity>) | | GL_PACK_SKIP_ROWS | integer | 0 | [0,<infinity>) | | GL_PACK_SKIP_PIXELS | integer | 0 | [0,<infinity>) | | 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,<infinity>) | | GL_UNPACK_SKIP_ROWS | integer | 0 | [0,<infinity>) | | GL_UNPACK_SKIP_PIXELS | integer | 0 | [0,<infinity>) | | GL_UNPACK_ALIGNMENT | integer | 4 | 1, 2, 4, or 8 | +-----------------------+---------+---------------+----------------+ glPixelStoref can be used to set any pixel store parameter. If the pa- rameter type is boolean, then if param is 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, gl- TexImage1D, glTexImage2D, glTexSubImage1D, glTexSubImage2D, glBitmap, or glPolygonStipple is placed in a display list control the interpreta- tion of memory data. The pixel storage modes in effect when a display list is executed are not significant. Pixel storage modes are client state and must be pushed and restored using glPushClientAttrib and glPopClientAttrib. 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 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, glPushClientAttrib, glReadPixels, glTexImage1D, gl- TexImage2D, glTexSubImage1D, glTexSubImage2D GLPIXELSTORE()
NAME | C SPECIFICATION | PARAMETERS | DESCRIPTION | NOTES | ERRORS | ASSOCIATED GETS | SEE ALSO
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