gldrawpixels - Man Page
glDrawPixels(3G) OpenGL Reference glDrawPixels(3G)
NAME
glDrawPixels - write a block of pixels to the frame buffer
C SPECIFICATION
void glDrawPixels( GLsizei width,
GLsizei height,
GLenum format,
GLenum type,
const GLvoid *pixels )
PARAMETERS
width, height Specify the dimensions of the pixel rectangle that will be
written into the frame buffer.
format Specifies the format of the pixel data. Symbolic constants
GL_COLOR_INDEX, GL_STENCIL_INDEX, GL_DEPTH_COMPONENT,
GL_RGBA, GL_ABGR_EXT, GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA,
GL_RGB, GL_LUMINANCE, and GL_LUMINANCE_ALPHA are accepted.
type Specifies the data type for pixels. Symbolic constants
GL_UNSIGNED_BYTE, GL_BYTE, GL_BITMAP, GL_UNSIGNED_SHORT,
GL_SHORT, GL_UNSIGNED_INT, GL_INT, GL_FLOAT,
GL_UNSIGNED_BYTE_3_3_2_EXT, GL_UNSIGNED_SHORT_4_4_4_4_EXT,
GL_UNSIGNED_SHORT_5_5_5_1_EXT, GL_UNSIGNED_INT_8_8_8_8_EXT,
and GL_UNSIGNED_INT_10_10_10_2_EXT are accepted.
pixels Specifies a pointer to the pixel data.
DESCRIPTION
glDrawPixels reads pixel data from memory and writes it into the frame
buffer relative to the current raster position. Use glRasterPos to set
the current raster position, and use glGet with argument
GL_CURRENT_RASTER_POSITION to query the raster position.
Several parameters define the encoding of pixel data in memory and
control the processing of the pixel data before it is placed in the frame
buffer. These parameters are set with the commands glPixelStore,
glPixelTransfer, glPixelMap, and glPixelZoom. The state of
GL_INTERLACE_SGIX (controlled by glEnable and glDisable) and the state
for the EXT_convolution, SGI_color_matrix, EXT_histogram, and
SGI_color_table extensions (see the reference pages for glPixelTransfer,
glConvolutionFilter2DEXT, glColorTableSGI, glHistogramEXT, and
glMinmaxEXT) also affect the results of glDrawPixels. This reference
page describes the effects on glDrawPixels of some, but not all, of these
parameters.
type specifies the data type for pixels. The following table summarizes
the meaning of the valid constants for type:
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glDrawPixels(3G) OpenGL Reference glDrawPixels(3G)
__________________________________________________________________________
|_______________________________|________________________________________|
| GL_UNSIGNED_BYTE | unsigned 8-bit integer |
| GL_BYTE | signed 8-bit integer |
| GL_BITMAP | single bits in unsigned 8-bit integers |
| GL_UNSIGNED_SHORT | unsigned 16-bit integer |
| GL_SHORT | signed 16-bit integer |
| GL_UNSIGNED_INT | unsigned 32-bit integer |
| GL_INT | 32-bit integer |
| GL_FLOAT | single-precision floating-point |
| GL_UNSIGNED_BYTE_3_3_2_EXT | unsigned 8-bit integer |
|GL_UNSIGNED_SHORT_4_4_4_4_EXT | unsigned 16-bit integer |
|GL_UNSIGNED_SHORT_5_5_5_1_EXT | unsigned 16-bit integer |
| GL_UNSIGNED_INT_8_8_8_8_EXT | unsigned 32-bit integer |
|GL_UNSIGNED_INT_10_10_10_2_EXT | unsigned 32-bit integer |
|_______________________________|________________________________________|
If type is GL_UNSIGNED_BYTE, GL_BYTE, GL_UNSIGNED_SHORT, GL_SHORT,
GL_UNSIGNED_INT, GL_INT, or GL_FLOAT, then data is read as a sequence of
signed or unsigned bytes, shorts, or integers, or single-precision
floating-point values. Each of these bytes, shorts, integers, or
floating-point values is interpreted as one color component, one depth
component, or one index, depending on format. Indices are always treated
individually. Colors are treated as groups of one, two, three, or four
elements, again based on format. Both individual indices and groups of
components are referred to as pixels.
If type is GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_SHORT_4_4_4_4,
GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_INT_8_8_8_8, or
GL_UNSIGNED_INT_10_10_10_2 then all the elements of each group are read
from a single unsigned byte, unsigned short, or unsigned int. The number
of elements per packed pixel is fixed by type, and must match the number
of elements per group indicated by format.
The following table shows which values of format are valid for each of
the packed pixel types:
________________________________________________________________________
| type number elements format |
|_______________________________________________________________________|
| GL_UNSIGNED_BYTE_3_3_2_EXT 3 GL_RGB |
|GL_UNSIGNED_SHORT_4_4_4_4_EXT 4 GL_RGBA,GL_ABGR_EXT |
|GL_UNSIGNED_SHORT_5_5_5_1_EXT 4 GL_RGBA,GL_ABGR_EXT |
| GL_UNSIGNED_INT_8_8_8_8_EXT 4 GL_RGBA,GL_ABGR_EXT |
|GL_UNSIGNED_INT_10_10_10_2_EXT 4 GL_RGBA,GL_ABGR_EXT |
|_______________________________________________________________________|
The elements in a packed pixel are ordered such that the first element is
in the most significant bits, followed by the second element, etc. For
example, if type is set to GL_UNSIGNED_SHORT_4_4_4_4_EXT then element 1
is read from bits 15-12, element 2 is read from bits 11-8, element 3 is
read from bits 7-4 and element 4 is read from bits 3-0.
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glDrawPixels(3G) OpenGL Reference glDrawPixels(3G)
The assignment of elements to fields in the packed pixel is as described
in the table below:
_____________________________________________________________
|____________________________________________________________|
| GL_RGB red green blue |
| GL_RGBA red green blue alpha |
|GL_ABGR_EXT alpha blue green red |
|____________________________________________________________|
If type is GL_BITMAP, the data must be unsigned bytes, and format must be
either GL_COLOR_INDEX or GL_STENCIL_INDEX. Each unsigned byte is treated
as eight 1-bit pixels, with bit ordering determined by
GL_UNPACK_LSB_FIRST. If GL_UNPACK_LSB_FIRST is enabled then the 8
single-bit elements are ordered from most significant to least
significant; otherwise the ordering is from least significant to most
significant. (see glPixelStore).
If GL_UNPACK_SWAP_BYTES is enabled (see glPixelStore), byte swapping is
performed as the data is read from memory. For pixels that aren't packed,
the 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. In the case of the packed pixel types, byte swapping is
performed before the elements are extracted from each pixel.
widthxheight pixels are read from memory, starting at location pixels.
By default, these pixels are taken from adjacent memory locations, except
that after all width pixels are read, the read pointer is advanced to the
next four-byte boundary. The four-byte row alignment is specified by
glPixelStore with argument GL_UNPACK_ALIGNMENT, and it can be set to one,
two, four, or eight bytes. Other pixel store parameters specify
different read pointer advancements, both before the first pixel is read,
and after all width pixels are read. Refer to the glPixelStore reference
page for details on these options.
The widthxheight pixels that are read from memory are each operated on in
the same way, based on the values of several parameters specified by
glPixelTransfer and glPixelMap. The details of these operations, as well
as the target buffer into which the pixels are drawn, are specific to the
format of the pixels, as specified by format. These operations generate
various fragment values as described below for each format. The number
of fragments generated for each source pixel and the assignment of x and
y window coordinates to these fragments are common to all formats and are
as follows.
If the current raster position is invalid, no fragments are generated for
any of the source pixels. Else, let (xr, yr) be the current raster
position, and let n and m be the column and row of the source pixel,
respectively. For each source pixel, fragments with values computed
according to that pixel are generated for destination pixels whose
centers are in the rectangle with corners at
(xr + n x zoomx, yr + m x interlace x zoomy) and
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glDrawPixels(3G) OpenGL Reference glDrawPixels(3G)
(xr + (n + 1) x zoomx,yr + (m x interlace + 1) x zoomy). zoomx and zoomy
are the values of GL_ZOOM_X and GL_ZOOM_Y, respectively, and interlace is
1 or 2 depending on whether GL_INTERLACE_SGIX is GL_FALSE or GL_TRUE,
respectively. GL_ZOOM_X and GL_ZOOM_Y, initially set to 1.0, are set
using glPixelZoom; GL_INTERLACE_SGIX, initially disabled, is controlled
by glEnable and glDisable. See the NOTES section for additional
discussion of GL_INTERLACE_SGIX.
Except for when format is GL_STENCIL_INDEX the generated fragments are
treated just like the fragments generated by rasterizing points, lines,
or polygons. Texture mapping, fog, and all the fragment operations are
applied before the fragments are written to the frame buffer. For
GL_STENCIL_INDEX only the pixel ownership test, the scissor test, and the
stencil writemask affect the writes to the stencil buffer.
format can assume the following symbolic values:
GL_COLOR_INDEX
Each pixel is a single value, a color index. It is converted to
fixed-point format, with an unspecified number of bits to the right
of the binary point, regardless of the memory data type. Floating-
point values convert to true fixed-point values. Signed and
unsigned integer data is converted with all fraction bits set to
zero. Bitmap data convert to either 0.0 or 1.0.
Each fixed-point index is then shifted left by GL_INDEX_SHIFT bits
and added to GL_INDEX_OFFSET. If GL_INDEX_SHIFT is negative, the
shift is to the right. In either case, zero bits fill otherwise
unspecified bit locations in the result.
If the GL is in RGBA mode, the resulting index is converted to an
RGBA pixel using the 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 tables. If the GL is
in color index mode, and if GL_MAP_COLOR is true, the index is
replaced with the value that it references in lookup table
GL_PIXEL_MAP_I_TO_I. Whether the lookup replacement of the index is
done or not, the integer part of the index is then ANDed with 2b-1,
where b is the number of bits in a color index buffer.
The resulting index or RGBA color components, and the current raster
position z and texture coordinates are assigned to each of the
fragments generated for the source pixel.
GL_STENCIL_INDEX
Each pixel is a single value, a stencil index. It is converted to
fixed-point format, with an unspecified number of bits to the right
of the binary point, regardless of the memory data type. Floating-
point values convert to true fixed-point values. Signed and
unsigned integer data is converted with all fraction bits set to
zero. Bitmap data convert to either 0.0 or 1.0.
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glDrawPixels(3G) OpenGL Reference glDrawPixels(3G)
Each fixed-point index is then shifted left by GL_INDEX_SHIFT bits,
and added to GL_INDEX_OFFSET. If GL_INDEX_SHIFT is negative, the
shift is to the right. In either case, zero bits fill otherwise
unspecified bit locations in the result. If GL_MAP_STENCIL is true,
the index is replaced with the value that it references in lookup
table GL_PIXEL_MAP_S_TO_S. Whether the lookup replacement of the
index is done or not, the integer part of the index is then ANDed
with 2b-1, where b is the number of bits in the stencil buffer.
The resulting stencil value is assigned to each of the fragments
generated for the source pixel.
GL_DEPTH_COMPONENT
Each pixel is a single-depth component. Floating-point data is
converted directly to an internal floating-point format with
unspecified precision. Signed integer data is mapped linearly to
the internal floating-point format such that the most positive
representable integer value maps to 1.0, and the most negative
representable value maps to -1.0. Unsigned integer data is mapped
similarly: the largest integer value maps to 1.0, and zero maps to
0.0. The resulting floating-point depth value is then multiplied by
GL_DEPTH_SCALE and added to GL_DEPTH_BIAS. The result is clamped to
the range [0,1].
The resulting depth component, and the current raster position color
or color index and texture coordinates are assigned to each of the
fragments generated for the source pixel.
GL_RGBA
GL_ABGR_EXT
Each pixel is a four-component group; for GL_RGBA, the red component
is first, followed by green, followed by blue, followed by alpha;
for GL_ABGR_EXT the order is alpha, blue, green, and then red.
Floating-point values are converted directly to an internal
floating-point format with unspecified precision. Signed integer
values are mapped linearly to the internal floating-point format
such that the most positive representable integer value maps to 1.0,
and the most negative representable value maps to -1.0. Unsigned
integer data is mapped similarly: the largest integer value maps to
1.0, and zero maps to 0.0. The resulting floating-point color
values are then multiplied by GL_c_SCALE and added to GL_c_BIAS,
where c is RED, GREEN, BLUE, or ALPHA for the respective color
components. The results are clamped to the range [0,1].
If GL_MAP_COLOR is true, each color component is scaled by the size
of lookup table GL_PIXEL_MAP_c_TO_c, then replaced by the value that
it references in that table. c is R, G, B, or A, respectively.
The resulting RGBA color components, and the current raster position
z and texture coordinates are assigned to each of the fragments
generated for the source pixel.
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glDrawPixels(3G) OpenGL Reference glDrawPixels(3G)
GL_RGBA may be faster or slower than GL_ABGR_EXT on a given platform
depending on the internal representation of the colors.
GL_RED
Each pixel is a single red component. This component is converted
to the internal floating-point format in the same way as the red
component of an RGBA pixel is, then it is converted to an RGBA pixel
with green and blue set to 0.0, and alpha set to 1.0. After this
conversion, the pixel is treated just as if it had been read as an
RGBA pixel.
GL_GREEN
Each pixel is a single green component. This component is converted
to the internal floating-point format in the same way as the green
component of an RGBA pixel is, then it is converted to an RGBA pixel
with red and blue set to 0.0, and alpha set to 1.0. After this
conversion, the pixel is treated just as if it had been read as an
RGBA pixel.
GL_BLUE
Each pixel is a single blue component. This component is converted
to the internal floating-point format in the same way as the blue
component of an RGBA pixel is, then it is converted to an RGBA pixel
with red and green set to 0.0, and alpha set to 1.0. After this
conversion, the pixel is treated just as if it had been read as an
RGBA pixel.
GL_ALPHA
Each pixel is a single alpha component. This component is converted
to the internal floating-point format in the same way as the alpha
component of an RGBA pixel is, then it is converted to an RGBA pixel
with red, green, and blue set to 0.0. After this conversion, the
pixel is treated just as if it had been read as an RGBA pixel.
GL_RGB
Each pixel is a three-component group: red first, followed by
green, followed by blue. Each component is converted to the
internal floating-point format in the same way as the red, green,
and blue components of an RGBA pixel are. The color triple is
converted to an RGBA pixel with alpha set to 1.0. After this
conversion, the pixel is treated just as if it had been read as an
RGBA pixel.
GL_LUMINANCE
Each pixel is a single luminance component. This component is
converted to the internal floating-point format in the same way as
the red component of an RGBA pixel is, then it is converted to an
RGBA pixel with red, green, and blue set to the converted luminance
value, and alpha set to 1.0. After this conversion, the pixel is
treated just as if it had been read as an RGBA pixel.
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glDrawPixels(3G) OpenGL Reference glDrawPixels(3G)
GL_LUMINANCE_ALPHA
Each pixel is a two-component group: luminance first, followed by
alpha. The two components are converted to the internal floating-
point format in the same way as the red component of an RGBA pixel
is, then they are converted to an RGBA pixel with red, green, and
blue set to the converted luminance value, and alpha set to the
converted alpha value. After this conversion, the pixel is treated
just as if it had been read as an RGBA pixel.
NOTES
Enabling GL_INTERLACE_SGIX is equivalent to having a source rectangle
with image height equal to 2xheight where every other row contains
"transparent" pixels that do not affect the corresponding destination
pixels in the frame buffer. For example:
glEnable(GL_INTERLACE_SGIX);
set current raster position to (xr,yr)
glDrawPixels(width, height, GL_RGBA, GL_UNSIGNED_BYTE, I0);
set raster position to (xr,yr+zoomy)
glDrawPixels(width, height, GL_RGBA, GL_UNSIGNED_BYTE, I1);
is equivalent to
glDisable( GL_INTERLACE_SGIX);
set current raster position to (xr,yr)
glDrawPixels(width, 2xheight, GL_RGBA, GL_UNSIGNED_BYTE, I2);
where pixel rows (0,2,4,...) of I2 are from image I0, and rows
(1,3,5,...) are from image I1.
If type is set to GL_UNSIGNED_BYTE_3_3_2_EXT,
GL_UNSIGNED_SHORT_4_4_4_4_EXT, GL_UNSIGNED_SHORT_5_5_5_1_EXT,
GL_UNSIGNED_INT_8_8_8_8_EXT, or GL_UNSIGNED_INT_10_10_10_2_EXT and the
EXT_packed_pixels extension is not supported then a GL_INVALID_ENUM error
is generated.
ERRORS
GL_INVALID_VALUE is generated if either width or height is negative.
GL_INVALID_ENUM is generated if format or type is not one of the accepted
values.
GL_INVALID_OPERATION is generated if type is set to
GL_UNSIGNED_BYTE_3_3_2_EXT, GL_UNSIGNED_SHORT_4_4_4_4_EXT,
GL_UNSIGNED_SHORT_5_5_5_1_EXT, GL_UNSIGNED_INT_8_8_8_8_EXT, or
GL_UNSIGNED_INT_10_10_10_2_EXT and the number of elements indicated by
type does not match the number indicated by format.
GL_INVALID_OPERATION is generated if format is GL_RED, GL_GREEN, GL_BLUE,
GL_ALPHA, GL_RGB, GL_RGBA, GL_ABGR_EXT, GL_LUMINANCE, or
GL_LUMINANCE_ALPHA, and the GL is in color index mode.
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glDrawPixels(3G) OpenGL Reference glDrawPixels(3G)
GL_INVALID_ENUM is generated if type is GL_BITMAP and format is not
either GL_COLOR_INDEX or GL_STENCIL_INDEX.
GL_INVALID_OPERATION is generated if format is GL_STENCIL_INDEX and there
is no stencil buffer.
GL_INVALID_OPERATION is generated if format is GL_DEPTH_COMPONENT and
there is no depth buffer.
GL_INVALID_OPERATION is generated if glDrawPixels is executed between the
execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGet with argument GL_CURRENT_RASTER_POSITION
glGet with argument GL_CURRENT_RASTER_POSITION_VALID
glGet with argument GL_INTERLACE_SGIX
MACHINE DEPENDENCIES
On RealityEngine, RealityEngine2, and VTX systems convolution may not be
used in the following circumstances:
1. When rendering to pixmaps.
2. When fragment processing (texturing, depth buffering, alpha
testing, multisampling, fog) is enabled.
3. When histogramming or minmax is enabled.
4. When either of the pixel zoom factors has a value other than 1.0
or -1.0.
In these cases, glDrawPixels and glCopyPixels report a
GL_INVALID_OPERATION error and do not transfer any pixels.
Loading a convolution filter with some combinations of pixel transfer
modes may be unreliable on InfiniteReality systems. Straightforward
transfers are known to work, though. The remaining problems will be
fixed in the next release.
Performance note for RealityEngine, RealityEngine2, and VTX systems:
Unsigned color types use the fastest pixel-drawing path. Smaller types
(e.g., GL_UNSIGNED_BYTE) require less host-to-graphics bandwidth, and are
therefore faster than larger types (e.g., GL_UNSIGNED_INT). Signed and
float types use the significantly slower floating-point pixel-drawing
path. The slower pixel-drawing path is also used when the format is
GL_DEPTH_COMPONENT and when fragment operations (i.e., depth or alpha
testing, texturing, fog, etc.) are enabled.
For best performance on XS, XZ, Elan, and Extreme systems set type to
GL_UNSIGNED_BYTE and, when drawing to the color buffer, set format to
GL_ABGR_EXT.
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glDrawPixels(3G) OpenGL Reference glDrawPixels(3G)
On InfiniteReality systems, signed color-index pixels written to
drawables with dual-personality (luminance + color-index) visuals will be
sign-extended into the high-order bits of the framebuffer. For example,
writing a signed byte value of 0x88 would yield 0xF88 in a 12-bit
drawable.
The SGIX_interlace extension is supported only on InfiniteReality systems
and on RealityEngine, RealityEngine2, and VTX systems.
The EXT_packed_pixels extension is not supported on RealityEngine,
RealityEngine2, and VTX systems; it will be supported on High Impact and
Maximum Impact systems in a future release.
SEE ALSO
glAlphaFunc, glBlendFunc, glColorTableSGI, glCopyPixels,
glConvolutionFilter2DEXT, glDepthFunc, glEnable, glHistogramEXT,
glLogicOp, glMinmaxEXT, glPixelMap, glPixelStore, glPixelTransfer,
glPixelZoom, glRasterPos, glReadPixels, glScissor,
glSeparableFilter2DEXT, glStencilFunc.
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