Name
gl.SelectBuffer -- establish a buffer for selection mode values
Synopsis
buffer = gl.SelectBuffer(size)
Function
gl.SelectBuffer() allocates a memory buffer of the size specified in the size argument and returns a pointer to this buffer. Values from the name stack will be written to this buffer (see gl.InitNames(), gl.LoadName(), gl.PushName()) when the rendering mode is #GL_SELECT (See gl.RenderMode for details.). gl.SelectBuffer() must be issued before selection mode is enabled, and it must not be issued while the rendering mode is #GL_SELECT.

You can read values from the memory buffer returned by gl.SelectBuffer() by calling gl.GetSelectBuffer() or accessing the buffer directly using Hollywood's GetMemPointer() and Peek() functions. When accessing the buffer directly, please note that the first four bytes in the buffer contain the size of the selection buffer in bytes.

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 gl.RenderMode() is called, a hit record is copied to buffer if any hits have occurred since the last such event (name stack change or gl.RenderMode() 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 gl.RenderMode() with an argument other than #GL_SELECT. Whenever gl.RenderMode() 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 gl.RenderMode() was called, a negative hit record count is returned.

The contents of buffer is undefined until gl.RenderMode() is called with an argument other than #GL_SELECT.

gl.Begin() / gl.End() primitives and calls to gl.RasterPos() can result in hits.

To free a buffer allocated by this function, call gl.FreeSelectBuffer(). See gl.FreeSelectBuffer for details.

Please consult an OpenGL reference manual for more information.

Inputs
size
specifies the size of the buffer in bytes
Results
buffer
memory buffer to use in selection mode
Errors
#GL_INVALID_VALUE is generated if size is negative.

#GL_INVALID_OPERATION is generated if gl.SelectBuffer() is called while the render mode is #GL_SELECT, or if gl.RenderMode() is called with argument #GL_SELECT before gl.SelectBuffer() is called at least once.

#GL_INVALID_OPERATION is generated if gl.SelectBuffer() is executed between the execution of gl.Begin() and the corresponding execution of gl.End().

Associated gets
gl.Get() with argument #GL_NAME_STACK_DEPTH

gl.Get() with argument #GL_SELECTION_BUFFER_SIZE

gl.GetPointer() with argument #GL_SELECTION_BUFFER_POINTER


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