[MacRuby-devel] Vertex buffer objects in OpenGL

[Dave Baldwin]

I have an openGL program that works if I use vertex arrays to store the geometry data, but if I convert it to use an openGL vertex buffer object then nothing is drawn.  Has anyone got vertex buffer objects to work with MacRuby?  I am using 0.8 on 10.6.4

(In OpenGL a vertex buffer object just holds geometry but on the graphics card so for large amounts of geometry will be significantly faster than a vertex array where the geometry is held in the client's memory and copied to the graphics card every time it is drawn).

The code is:

			@vbo_buffer = Pointer.new('f', 1000)
			...     # initialise with vertex data (BYTES_IN_BUFFER set up with the size of the vertex data)

			# Get a buffer object id
			cubeVBO = Pointer.new_with_type('I', 1)
			glGenBuffers(1, cubeVBO)
			@cubeVBO = cubeVBO[0]

			glBindBuffer(GL_ARRAY_BUFFER, @cubeVBO)

			# The vertex buffer object can be initialised in several ways
			case vbo_method
				when :writeOnAllocate
					# This call will allocate the vertex buffer object and initialise it from @vbo_buffer
					glBufferData(GL_ARRAY_BUFFER, BYTES_IN_BUFFER, @vbo_buffer, GL_STATIC_DRAW)
				when :writeAfterAllocate
					nullPtr = Pointer.new("^I")
					# This call will allocate the vertex buffer object but not initialise it.
					glBufferData(GL_ARRAY_BUFFER, BYTES_IN_BUFFER, nullPtr, GL_STATIC_DRAW)
					# Now initialise it.	
					glBufferSubData(GL_ARRAY_BUFFER, 0, BYTES_IN_BUFFER, @vbo_buffer)
				when :mapAfterAllocate
					nullPtr = Pointer.new("^I")
					# This call will allocate the vertex buffer object but not initialise it.
					glBufferData(GL_ARRAY_BUFFER, BYTES_IN_BUFFER, nullPtr, GL_STATIC_DRAW)
					# Get a pointer to the allocated buffer and copy the data over manually.
					buffer = glMapBuffer(GL_ARRAY_BUFFER, GL_READ_WRITE).cast!('f')
					(BYTES_IN_BUFFER / 4).times {|i| buffer[i] = @vbo_buffer[i]}	# copy as floats
					glUnmapBuffer(GL_ARRAY_BUFFER)
			end

Finally to draw the vertex buffer object

		glBindBuffer(GL_ARRAY_BUFFER, @cubeVBO)
		
		glEnableClientState(GL_VERTEX_ARRAY)
		
		v = Pointer.new("^c")		# gives us a null pointer - need to pass in 0 in next call
		glVertexPointer(3, GL_FLOAT, v)

		glColor3f(1.0, 0.0, 0.0)		
		glDrawArrays(GL_QUADS, 0, 24)	# 0 = start offset, 24 = number of vertices


None of the three methods to load the vertex buffer object make a difference. The other problem I will have is the colour data is interleaved in with the vertex data so ideally I need to call:
		glVertexPointer(3, GL_FLOAT, 12)
but this function needs to take a void* pointer so how do I create a pointer with a value?

Dave.