glPixelStoref, glPixelStorei

  • Article

The glPixelStoref and glPixelStorei functions set pixel storage modes.

void glPixelStoref(
  GLenum pname,  GLfloat param);void glPixelStorei(  GLenum pname,  GLint param);

Parameters

  • pname
    The symbolic name of the parameter to be set. Six of the storage parameters affect how pixel data is returned to client memory, and are therefore significant only for glReadPixels commands. They are as follows:
    • GL_PACK_SWAP_BYTES
      If true, byte ordering for multibyte color components, depth components, color indexes, or stencil indexes 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 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 indexes. 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_PACK_SWAP_BYTES.
    • 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 by skipping

        

components or indexes, where n is the number of components or indexes 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

        

components or indexes.

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 simply by incrementing the pointer passed to glReadPixels. Setting GL_PACK_SKIP_PIXELS to i is equivalent to incrementing the pointer by i n components or indexes, where n is the number of components or indexes in each pixel. Setting GL_PACK_SKIP_ROWS to j is equivalent to incrementing the pointer by j k components or indexes, where k is the number of components or indexes per row, as computed above 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-alignment), 2 (rows aligned to even-numbered bytes), 4 (word alignment), and 8 (rows start on double-word boundaries).

    The other six 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 indexes, or stencil indexes 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 indexes. 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

        

components or indexes, where n is the number of components or indexes 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

        

components or indexes.

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 by i n components or indexes, where n is the number of components or indexes in each pixel. Setting GL_UNPACK_SKIP_ROWS to j is equivalent to incrementing the pointer by j k components or indexes, where k is the number of components or indexes per row, as computed above 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).
  • param
    The value that pname is set to.

Remarks

The glPixelStore function 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, glTexSubImage1D, and glTexSubImage2D).

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.

Pname Type Initial Value Valid Range
GL_PACK_SWAP_BYTES Boolean false true or false
GL_PACK_SWAP_BYTES Boolean false true or false
GL_PACK_ROW_LENGTH integer 0 [0,&infin;)
GL_PACK_SKIP_ROWS integer 0 [0,&infin;)
GL_PACK_SKIP_PIXELS integer 0 [0,&infin;)
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,&infin;)
GL_UNPACK_SKIP_ROWS integer 0 [0,&infin;)
GL_UNPACK_SKIP_PIXELS integer 0 [0,&infin;)
GL_UNPACK_ALIGNMENT integer 4 1, 2, 4, or 8

The glPixelStoref function can be used to set any pixel store parameter. If the parameter type is Boolean, and if param is 0.0, then the parameter is false; otherwise it is set to true. If pname is an integer type parameter, then param is rounded to the nearest integer.

Likewise, the glPixelStorei function 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. The param parameter is converted to floating point before being assigned to real-valued parameters.

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 following functions retrieve information related to glPixelStore:

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

Error Codes

The following are the error codes generated and their conditions.

Error code Condition
GL_INVALID_ENUM pname was not an accepted value.
GL_INVALID_VALUE A negative row length, pixel skip, or row skip value was specified, or if alignment was specified as other than 1, 2, 4, or 8.
GL_INVALID_OPERATION glPixelStore was called between a call to glBegin and the corresponding call to glEnd.

Requirements

**  Windows NT/2000:** Requires Windows NT 3.5 or later.
**  Windows 95/98:** Requires Windows 95 or later. Available as a redistributable for Windows 95.
**  Header:** Declared in Gl.h.
**  Library:** Use Opengl32.lib.

See Also

glBegin, glBitmap, glDrawPixels, glEnd, glPixelMap, glPixelTransfer, glPixelZoom, glPolygonStipple, glReadPixels, glTexImage1D, glTexImage2D