标题：glAccum
NAME
glAccum - operate on the accumulation buffer 
C SPECIFICATION
void glAccum( GLenum op, GLfloat value )
PARAMETERS
op ? Specifies the accumulation buffer operation. Symbolic constants GL_ACCUM, GL_LOAD, GL_ADD, GL_MULT, and GL_RETURN are accepted. 
value ? Specifies a floating-point value used in the accumulation buffer operation. op determines how value is used.
DESCRIPTION
The accumulation buffer is an extended-range color buffer. Images are not rendered into it. Rather, images rendered into one of the color buffers are added to the contents of the accumulation buffer after rendering. Effects such as antialiasing (of points, lines, and polygons), motion blur, and depth of field can be created by accumulating images generated with different transformation matrices. 
Each pixel in the accumulation buffer consists of red, green, blue, and alpha values. The number of bits per component in the accumulation buffer depends on the implementation. You can examine this number by calling glGetIntegerv four times, with arguments GL_ACCUM_RED_BITS, GL_ACCUM_GREEN_BITS, GL_ACCUM_BLUE_BITS, and GL_ACCUM_ALPHA_BITS, respectively. Regardless of the number of bits per component, however, the range of values stored by each component is [-1, 1]. The accumulation buffer pixels are mapped one-to-one with frame buffer pixels. 
glAccum operates on the accumulation buffer. The first argument, op, is a symbolic constant that selects an accumulation buffer operation. The second argument, value, is a floating-point value to be used in that operation. Five operations are specified: GL_ACCUM, GL_LOAD, GL_ADD, GL_MULT, and GL_RETURN. 
All accumulation buffer operations are limited to the area of the current scissor box and are applied identically to the red, green, blue, and alpha components of each pixel. The contents of an accumulation buffer pixel component are undefined if the glAccum operation results in a value outside the range [-1,1]. The operations are as follows: 
GL_ACCUM ? Obtains R, G, B, and A values from the buffer currently selected for reading (see "glReadBuffer" .) Each component value is divided by 2n - 1, where n is the number of bits allocated to each color component in the currently selected buffer. The result is a floating-point value in the range [0,1], which is multiplied by value and added to the corresponding pixel component in the accumulation buffer, thereby updating the accumulation buffer. 
GL_LOAD  ? Similar to GL_ACCUM, except that the current value in the accumulation buffer is not used in the calculation of the new value. That is, the R, G, B, and A values from the currently selected buffer are divided by 2n - 1, multiplied by value, and then stored in the corresponding accumulation buffer cell, overwriting the current value. 
GL_ADD  ? Adds value to each R, G, B, and A in the accumulation buffer. 
GL_MULT  ? Multiplies each R, G, B, and A in the accumulation buffer by value and returns the scaled component to its corresponding accumulation buffer location. 
GL_RETURN  ? Transfers accumulation buffer values to the color buffer or buffers currently selected for writing. Each R, G, B, and A component is multiplied by value, then multiplied by 2n - 1, clamped to the range [0, 2n - 1 ], and stored in the corresponding display buffer cell. The only fragment operations that are applied to this transfer are pixel ownership, scissor, dithering, and color writemasks. 
The accumulation buffer is cleared by specifying R, G, B, and A values to set it to with the glClearAccum directive, and issuing a glClear command with the accumulation buffer enabled.
NOTES
Only those pixels within the current scissor box are updated by any glAccum operation. 
ERRORS
GL_INVALID_ENUM is generated if op is not an accepted value. 
GL_INVALID_OPERATION is generated if there is no accumulation buffer. 
GL_INVALID_OPERATION is generated if glAccum is called between a call to glBegin and the corresponding call to glEnd. 
ASSOCIATED GETS
glGet with argument GL_ACCUM_RED_BITS 
glGet with argument GL_ACCUM_GREEN_BITS 
glGet with argument GL_ACCUM_BLUE_BITS 
glGet with argument GL_ACCUM_ALPHA_BITS 
函数功能描述:这是OpenGL中的一个库函数，在累积缓冲区中进行操作
函数原形：
void glAccum(GLenum op,GLfloat value)
函数参数：
op:    指定累积缓存操作,主要有以下几个值
GL_ACCUM , GL_LOAD , GL_ADD , GL_MULT , GL_RETURN
op是一个在累积缓冲器操作中使用的指针值,它确定了value的使用方式.
value: 指定一个用于累积缓冲区操作的浮点值。
返回值:无
使用方法：（C描述）
void glAccum(
GLenum op,      
GLfloat value   
);
备注:
累积缓冲区是一种扩展了范围的颜色缓冲区。
只有在当前裁剪箱范转内的像素才可以被glAccum操作更新
使用环境:
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.
必须包含的头文件:
Gl.h.
动态链接库: 
Use Opengl32.lib.