GLSELECTBUFFER

Section: Misc. Reference Manual Pages (3G)
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NAME

glSelectBuffer - establish a buffer for selection mode values

C SPECIFICATION

void glSelectBuffer( GLsizei size,


                       GLuint *buffer )

PARAMETERS

size: Specifies the size of buffer.
buffer: Returns the selection data.

DESCRIPTION

glSelectBuffer has two arguments: buffer is a pointer to an array of unsigned integers, and size indicates the size of the array. buffer returns values from the name stack (see glInitNames, glLoadName, glPushName) when the rendering mode is GL_SELECT (see glRenderMode). glSelectBuffer must be issued before selection mode is enabled, and it must not be issued while the rendering mode is GL_SELECT. A programmer can use selection to determine which primitives are drawn into some region of a window. The region is defined by the current modelview and perspective matrices. In selection mode, no pixel fragments are produced from rasterization. Instead, if a primitive or a raster position intersects the clipping volume defined by the viewing frustum and the user-defined clipping planes, this primitive causes a selection hit. (With polygons, no hit occurs if the polygon is culled.) When a change is made to the name stack, or when glRenderMode is called, a hit record is copied to buffer if any hits have occurred since the last such event (name stack change or glRenderMode call). The hit record consists of the number of names in the name stack at the time of the event, followed by the minimum and maximum depth values of all vertices that hit since the previous event, followed by the name stack contents, bottom name first. Depth values (which are in the range [0,1]) are multiplied by 2^32 - 1, before being placed in the hit record. An internal index into buffer is reset to 0 whenever selection mode is entered. Each time a hit record is copied into buffer, the index is incremented to point to the cell just past the end of the block of names - that is, to the next available cell. If the hit record is larger than the number of remaining locations in buffer, as much data as can fit is copied, and the overflow flag is set. If the name stack is empty when a hit record is copied, that record consists of 0 followed by the minimum and maximum depth values. To exit selection mode, call glRenderMode with an argument other than GL_SELECT. Whenever glRenderMode is called while the render mode is GL_SELECT, it returns the number of hit records copied to buffer, resets the overflow flag and the selection buffer pointer, and initializes the name stack to be empty. If the overflow bit was set when glRenderMode was called, a negative hit record count is returned.

NOTES

The contents of buffer is undefined until glRenderMode is called with an argument other than GL_SELECT. glBegin/glEnd primitives and calls to glRasterPos can result in hits.

ERRORS

GL_INVALID_VALUE is generated if size is negative. GL_INVALID_OPERATION is generated if glSelectBuffer is called while the render mode is GL_SELECT, or if glRenderMode is called with argument GL_SELECT before glSelectBuffer is called at least once. GL_INVALID_OPERATION is generated if glSelectBuffer is executed between the execution of glBegin and the corresponding execution of glEnd.

ASSOCIATED GETS

glGet with argument GL_NAME_STACK_DEPTH
glGet with argument GL_SELECTION_BUFFER_SIZE
glGetPointerv with argument GL_SELECTION_BUFFER_POINTER