glAccum(3gl)

1GLACCUM(3G)                                                        GLACCUM(3G)
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NAME

6       glAccum - operate on the accumulation buffer
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C SPECIFICATION

10       void glAccum( GLenum op,
11                     GLfloat value )
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PARAMETERS

15       op     Specifies the accumulation buffer operation.  Symbolic constants
16              GL_ACCUM, GL_LOAD, GL_ADD, GL_MULT, and GL_RETURN are accepted.
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18       value  Specifies a floating-point value used in the accumulation buffer
19              operation.  op determines how value is used.
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DESCRIPTION

22       The  accumulation buffer is an extended-range color buffer.  Images are
23       not rendered into it.  Rather, images rendered into one  of  the  color
24       buffers are added to the contents of the accumulation buffer after ren‐
25       dering.  Effects such as antialiasing (of points, lines, and polygons),
26       motion  blur,  and depth of field can be created by accumulating images
27       generated with different transformation matrices.
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29       Each pixel in the accumulation buffer consists of red, green, blue, and
30       alpha  values.   The  number  of bits per component in the accumulation
31       buffer depends on the implementation. You can examine  this  number  by
32       calling  glGetIntegerv  four  times,  with arguments GL_ACCUM_RED_BITS,
33       GL_ACCUM_GREEN_BITS,   GL_ACCUM_BLUE_BITS,   and   GL_ACCUM_ALPHA_BITS.
34       Regardless  of  the  number  of bits per component, the range of values
35       stored by each component is [-1, 1].  The  accumulation  buffer  pixels
36       are mapped one-to-one with frame buffer pixels.
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38       glAccum  operates  on the accumulation buffer.  The first argument, op,
39       is a symbolic constant that selects an accumulation  buffer  operation.
40       The  second  argument,  value,  is a floating-point value to be used in
41       that operation.  Five  operations  are  specified:  GL_ACCUM,  GL_LOAD,
42       GL_ADD, GL_MULT, and GL_RETURN.
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44       All  accumulation buffer operations are limited to the area of the cur‐
45       rent scissor box and applied identically to the red, green,  blue,  and
46       alpha  components  of  each pixel.  If a glAccum operation results in a
47       value outside the range [-1, 1], the contents of an accumulation buffer
48       pixel component are undefined.
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50       The operations are as follows:
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52       GL_ACCUM      Obtains  R,  G, B, and A values from the buffer currently
53                     selected for reading (see glReadBuffer).  Each  component
54                     value  is  divided by 2n−1, where n is the number of bits
55                     allocated  to  each  color  component  in  the  currently
56                     selected buffer.  The result is a floating-point value in
57                     the range [0, 1], which is multiplied by value and  added
58                     to  the corresponding pixel component in the accumulation
59                     buffer, thereby updating the accumulation buffer.
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61       GL_LOAD       Similar to GL_ACCUM, except that the current value in the
62                     accumulation buffer is not used in the calculation of the
63                     new value.  That is, the R, G, B, and A values  from  the
64                     currently selected buffer are divided by 2n−1, multiplied
65                     by value, and then stored in the corresponding  accumula‐
66                     tion buffer cell, overwriting the current value.
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68       GL_ADD        Adds  value  to  each  R, G, B, and A in the accumulation
69                     buffer.
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71       GL_MULT       Multiplies each R, G, B, and A in the accumulation buffer
72                     by  value  and returns the scaled component to its corre‐
73                     sponding accumulation buffer location.
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75       GL_RETURN     Transfers accumulation buffer values to the color  buffer
76                     or buffers currently selected for writing.  Each R, G, B,
77                     and A component is multiplied by value,  then  multiplied
78                     by 2n−1, clamped to the range [0,2n−1], and stored in the
79                     corresponding display buffer  cell.   The  only  fragment
80                     operations  that  are  applied to this transfer are pixel
81                     ownership, scissor, dithering, and color writemasks.
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83       To clear the accumulation buffer, call glClearAccum with R, G, B, and A
84       values  to  set  it  to, then call glClear with the accumulation buffer
85       enabled.
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NOTES

88       Only pixels within the current scissor box are  updated  by  a  glAccum
89       operation.
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ERRORS

92       GL_INVALID_ENUM is generated if op is not an accepted value.
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94       GL_INVALID_OPERATION is generated if there is no accumulation buffer.
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96       GL_INVALID_OPERATION  is  generated  if glAccum is executed between the
97       execution of glBegin and the corresponding execution of glEnd.
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ASSOCIATED GETS

100       glGet with argument GL_ACCUM_RED_BITS
101       glGet with argument GL_ACCUM_GREEN_BITS
102       glGet with argument GL_ACCUM_BLUE_BITS
103       glGet with argument GL_ACCUM_ALPHA_BITS
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