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question about glDrawPixels()

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In the following code, I am having a problem with the "pixel" argument. I am usure what value to pass into the function and I assume it deals directly with the pixel color. glDrawPixels(nCols, nRows, GL_RGB, GL_UNSIGNED_BYTE, pixel); I can assume that the GL_UNSIGNED_BYTE argument sets the type of variable to be pased by "pixel". I would rather use some sort of int to pass the value if at all possible. Thanks for your help. Brian Anttila banttila@vt.edu

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It is a pointer to data formated as the "type" parameter that you pass, and it is interpreted as information that you specify in the "format" parameter:

glDrawPixels
The glDrawPixels function writes a block of pixels to the frame buffer.

void glDrawPixels(
GLsizei width,
GLsizei height,
GLenum format,
GLenum type,
const GLvoid *pixels
);

Parameters
width, height
The dimensions of the pixel rectangle that will be written into the frame buffer.
format
The format of the pixel data. Acceptable symbolic constants are:
GL_COLOR_INDEX
Each pixel is a single value, a color index.
1.The glDrawPixels function converts each pixel 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. The glDrawPixels function converts signed and unsigned integer data with all fraction bits set to zero. The function converts bitmap data to either 0.0 or 1.0.

2.The glDrawPixels function shifts each fixed-point index left by GL_INDEX_SHIFT bits and adds it 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.

3.When in RGBA mode, glDrawPixels converts the resulting index 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. When in the color-index mode and GL_MAP_COLOR is true, the index is replaced with the value that glDrawPixels references in lookup table GL_PIXEL_MAP_I_TO_I.

4.Whether the lookup replacement of the index is done or not, the integer part of the index is ANDed with 2b – 1, where b is the number of bits in a color-index buffer.

5.The resulting indexes or RGBA colors are then converted to fragments by attaching the current raster position z-coordinate and texture coordinates to each pixel, and then assigning x and y window coordinates to the nth fragment such that

x (n) = x (r) + n mod width

y (n) = y (r) + ë n/width û

where (x (r) , y (r) ) is the current raster position.

6.The glDrawPixels function treats these pixel fragments just like the fragments generated by rasterizing points, lines, or polygons. It applies texture mapping, fog, and all the fragment operations before writing the fragments to the frame buffer.

GL_STENCIL_INDEX
Each pixel is a single value, a stencil index.
1.The glDrawPixels function converts it 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. The glDrawPixels function converts signed and unsigned integer data with all fraction bits set to zero. Bitmap data converts to either 0.0 or 1.0.

2.The glDrawPixels function shifts each fixed-point index left by GL_INDEX_SHIFT bits, and adds it 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.

3.If GL_MAP_STENCIL is true, the index is replaced with the value that glDrawPixels references in lookup table GL_PIXEL_MAP_S_TO_S.

4.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 indexes are then written to the stencil buffer such that the nth index is written to location

x (n) = x (r) + n mod width

y (n) = y (r) + ë n /width û

where (x (r) , y (r) ) is the current raster position. Only the pixel ownership test, the scissor test, and the stencil writemask affect these writes.

GL_DEPTH_COMPONENT
Each pixel is a single-depth component.
1.The glDrawPixels function converts floating-point data 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.

2.The glDrawPixels function multiplies the resulting floating-point depth value by GL_DEPTH_SCALE and adds it to GL_DEPTH_BIAS. The result is clamped to the range [0,1].

3.The glDrawPixels function converts the resulting depth components to fragments by attaching the current raster position color or color index and texture coordinates to each pixel, and then assigning x and y window coordinates to the nth fragment such that

x (n) = x (r) + n mod width

y (n) = y (r) + ë n /width û

where (x (r) , y (r) ) is the current raster position.

4.These pixel fragments are then treated just like the fragments generated by rasterizing points, lines, or polygons. The glDrawPixels function applies texture mapping, fog, and all the fragment operations before writing the fragments to the frame buffer.

GL_RGBA
Each pixel is a four-component group in this order: red, green, blue, alpha.
1.The glDrawPixels function converts floating-point values 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.

2.The glDrawPixels function multiplies the resulting floating-point color values by GL_c_SCALE and adds them to GL_c_BIAS, where c is RED, GREEN, BLUE, and ALPHA for the respective color components. The results are clamped to the range [0,1].

3.If GL_MAP_COLOR is true, glDrawPixels scales each color component by the size of lookup table GL_PIXEL_MAP_c_TO_c, and then replaces the component by the value that it references in that table; c is R, G, B, or A, respectively.

4.The glDrawPixels function converts the resulting RGBA colors to fragments by attaching the current raster position z-coordinate and texture coordinates to each pixel, then assigning x and y window coordinates to the nth fragment such that

x (n) = x (r) + n mod width

y (n) = y (r) + ë n /width û

where (x (r) , y (r) ) is the current raster position.

5.These pixel fragments are then treated just like the fragments generated by rasterizing points, lines, or polygons. The glDrawPixels function applies texture mapping, fog, and all the fragment operations before writing the fragments to the frame buffer.

GL_RED
Each pixel is a single red component.
The glDrawPixels function converts this component to the internal floating-point format in the same way that the red component of an RGBA pixel is, and then converts it 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.
The glDrawPixels function converts this component to the internal floating-point format in the same way that the green component of an RGBA pixel is, and then converts it 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.
The glDrawPixels function converts this component to the internal floating-point format in the same way that the blue component of an RGBA pixel is, and then converts it 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.
The glDrawPixels function converts this component to the internal floating-point format in the same way that the alpha component of an RGBA pixel is, and then converts it 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 group of three components in this order: red, green, blue.
The glDrawPixels function converts each component to the internal floating-point format in the same way that 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.
The glDrawPixels function converts this component to the internal floating-point format in the same way that the red component of an RGBA pixel is, and then converts it 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.

GL_LUMINANCE_ALPHA
Each pixel is a group of two components in this order: luminance, alpha.
The glDrawPixels function converts the two components to the internal floating-point format in the same way that the red component of an RGBA pixel is, and then converts them 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.

GL_BGR_EXT
Each pixel is a group of three components in this order: blue, green, red.
GL_BGR_EXT provides a format that matches the memory layout of Windows device-independent bitmaps (DIBs). Thus your applications can use the same data with Win32 function calls and OpenGL pixel function calls.

GL_BGRA_EXT
Each pixel is a group of four components in this order: blue, green, red, alpha.
GL_BGRA_EXT provides a format that matches the memory layout of Windows device-independent bitmaps (DIBs). Thus your applications can use the same data with Win32 function calls and OpenGL pixel function calls.

type
The data type for pixels. The following symbolic constants are accepted: GL_UNSIGNED_BYTE, GL_BYTE, GL_BITMAP, GL_UNSIGNED_SHORT, GL_SHORT, GL_UNSIGNED_INT, GL_INT, and GL_FLOAT.
The following table summarizes the meaning of the valid constants for the type parameter. Type Meaning
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



pixels
A pointer to the pixel data.
Remarks
The glDrawPixels function 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 four functions: glPixelStore, glPixelTransfer, glPixelMap, and glPixelZoom. This topic describes the effects on glDrawPixels of many, but not all, of the parameters specified by these four functions.

Data is read from pixels as a sequence of signed or unsigned bytes, signed or unsigned shorts, signed or unsigned integers, or single-precision floating-point values, depending on type. Each of these bytes, shorts, integers, or floating-point values is interpreted as one color or depth component, or one index, depending on format. Indexes are always treated individually. Color components are treated as groups of one, two, three, or four values, again based on format. Both individual indexes and groups of components are referred to as pixels. 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 (see glPixelStore).

The width by height 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 4-byte boundary. The glPixelStore function specifies the 4-byte row alignment with argument GL_UNPACK_ALIGNMENT, and you can set it to 1, 2, 4, or 8 bytes. Other pixel store parameters specify different read pointer advancements, both before the first pixel is read, and after all width pixels are read.

The glPixelStore function operates on each of the width-by-height pixels that it reads from memory 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.

The rasterization described thus far assumes pixel zoom factors of 1.0. If you use glPixelZoom to change the x and y pixel zoom factors, pixels are converted to fragments as follows. If (x (r) , y (r) ) is the current raster position, and a given pixel is in the n column and m row of the pixel rectangle, then fragments are generated for pixels whose centers are in the rectangle with corners at

(x (r) + zoom (x) n, y (r) + zoom (y) m)

(x (r) + zoom (x) (n + 1), y (r) + zoom (y) (m + 1))

where zoom (x) is the value of GL_ZOOM_X and zoom (y) is the value of GL_ZOOM_Y.

The following functions retrieve information related to glDrawPixels:

glGet with argument GL_CURRENT_RASTER_POSITION

glGet with argument GL_CURRENT_RASTER_POSITION_VALID

Error Codes
The following are the error codes generated and their conditions.

Error Code Condition
GL_INVALID_VALUE Either width or height was negative.
GL_INVALID_ENUM Either format or type was not an accepted value.
GL_INVALID_OPERATION format was GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA, GL_BGR_EXT, GL_BGRA_EXT, GL_LUMINANCE, or GL_LUMINANCE_ALPHA, and OpenGL was in color-index mode.
GL_INVALID_ENUM type was GL_BITMAP and format was not either GL_COLOR_INDEX or GL_STENCIL_INDEX.
GL_INVALID_OPERATION format was GL_STENCIL_INDEX and there was no stencil buffer.
GL_INVALID_OPERATION glDrawPixels was called between a call to glBegin and the corresponding call to glEnd.

QuickInfo
Windows NT: Use version 3.5 and later.
Windows: Use Windows 95 and later.
Windows CE: Unsupported.
Header: Declared in gl.h.
Import Library: Link with opengl32.lib.

See Also
glAlphaFunc, glBegin, glBlendFunc, glCopyPixels, glDepthFunc, glEnd, glGet, glLogicOp, glPixelMap, glPixelStore, glPixelTransfer, glPixelZoom, glRasterPos, glReadPixels, glScissor, glStencilFunc

"Finger to spiritual emptiness underlying everything." -- How a C manual referred to a "pointer to void." --Things People Said
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Thanks a lot. That info was very helpful. I found most of the this information on MSDN, so I''m installing that today. Again, thanks!


Brian Anttila
banttila@vt.edu

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