Hi,
I want to draw a certain range of a Array Buffer A into a different range of a Transform Feedback Buffer B.
The draw size is the same, but the draw offset varies.
I PingPong those buffers with an increasing offset, but the example shows only one step.
Draw A1,A2 and Record into B2,B3:
A : A0 A1 A2 A3 A4 A5
B : B0 B1 A1 A2 B4 B5
I set up all my Buffers, and at draw time I use:
// Transform Feedback
glBindBuffer( GL_TRANSFORM_FEEDBACK_BUFFER , buffer_B ) ;
glBindBufferRange( GL_TRANSFORM_FEEDBACK_BUFFER , 0 , bufferFB , offsetBytes , sizeBytes ) ;
// Draw into Feddback Buffer
glUseProgram( transFeedbackProg ) ;
glBeginTransformFeedback( GL_POINTS ) ;
glBindBuffer( GL_ARRAY_BUFFER , buffer_A ) ;
glDrawArrays( GL_POINTS , offset - 1 , size ) ;
glEndTransformFeedback() ;
// Draw Result
glUseProgram( drawProg ) ;
glBindBuffer( GL_ARRAY_BUFFER , buffer_B ) ;
glDrawArrays( GL_POINTS , 0 , completeBuffer ) ;
offset and size are per Vertex and offsetBytes and sizeBytes are per Vertex per Byte.
In the transFeedbackProg I update Vertex Positions.
When I don’t use ranges but allways render the whole buffers I get expected results.
But the setup above does not update my Vertices properly.
One problem might be, that render Range does not re-base the gl_VertexID, when I render a
range from 4-8 the gl_VertexIDs are still from 4-8 and not from 0-4. Now I fear that I cannot
read from and write to different ranges, as in both cases the absolute gl_VertexID is used,
and as the ranges are disjunct no data gets updated at all.
Could somebody confirm, clarify or object ?
Thank you,
Cheers, ParticlePeter
