Re: [MacRuby-devel] Vertex buffer objects in OpenGL

12 Jan 2011

      Just to give an answer for anyone finding this later.

On 21 Dec 2010, at 20:20, Dave Baldwin wrote:
...
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.
All the methods for loading the in vertex data work fine and the only problem (as alluded to) is how to pass an integer to a C function expecting a void * pointer.

There is no way to do this in MacRuby 0.8 and the only option is to write a small objective C stub to do this.  One such stub looks like:

- (void) glVBOVertexPointer: (GLint) size type: (GLenum) type stride: (GLsizei) stride offset:(GLint) offset
{
	glVertexPointer(size, type, stride, (void *) offset);
}

and once you have built this and loaded it (see http://www.macruby.org/recipes/create-an-objective-c-bundle.html) then calling it

	glVBOVertexPointer(3, type: GL_FLOAT, stride: strideInBytesBetweenVertices, offset: 0)

make everything work as expected.

Dave.