[MacRuby-devel] Vertex buffer objects in OpenGL

[Matt Aimonetti]

Thanks Dave for the update, I believe Laurent just added support for that in
trunk:

ptr = Pointer.magic_cookie(12)
glVertexPointer(3, GL_FLOAT, ptr)

It looks like you are having fun with OpenGL and MacRuby, would you mind
writing a small intro article for the MacRuby website? That would be really
appreciated.

Thanks,

- Matt


On Wed, Jan 12, 2011 at 1:29 PM, Dave Baldwin <dave.baldwin at dsl.pipex.com>wrote:

> 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.
>
>
>
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>
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