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kroneml

OpenGL Transform Feedback / VBO moves vertex position?

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Hello, I am currently trying to implement a mulit-pass triangle subdivision algorithm using the geometry shader and transform_feedback, similar to the "Transform Feedback Fractal"-Demo from the nvidia OpenGL-SDK 10. That far I managed to render the vertices to the vertex buffer object, but when I render that VBO, the triangles don't appear at the correct position! I'm kind of stuck here since I have no idea what I am doing wrong - perhaps one of you can spot my mistake? I read through everything about transform_feedback that I found using Google but I am still not any the wiser... First I create my VBOs:
glGenBuffers( 2, subdivisionVBO);
glBindBuffer( GL_ARRAY_BUFFER, subdivisionVBO[0]);
glBufferData( GL_ARRAY_BUFFER, max_buffer_verts*27*sizeof(GLfloat), 0, GL_STATIC_DRAW);

glBindBuffer( GL_ARRAY_BUFFER, subdivisionVBO[1]);
glBufferData( GL_ARRAY_BUFFER, max_buffer_verts*27*sizeof(GLfloat), 0, GL_STATIC_DRAW);

GLint attribs[] = { GL_PRIMARY_COLOR, 3, 0, GL_POSITION, 3, 0 };
glTransformFeedbackAttribsNV( 2, attribs, GL_INTERLEAVED_ATTRIBS_NV);

Then I render my data to the first VBO using transform_feedback:
glBindBufferBaseNV( GL_TRANSFORM_FEEDBACK_BUFFER_NV, 0, subdivisionVBO[0]);
glBeginTransformFeedbackNV( GL_TRIANGLES);

glBeginQuery( GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN_NV, query);

// draw data
glEnableClientState( GL_VERTEX_ARRAY);
glEnableClientState( GL_COLOR_ARRAY);
glVertexPointer( 3, GL_FLOAT, 0, vertexArry);
glColorPointer( 3, GL_FLOAT, 0, colorArray);
glDrawArrays( GL_TRIANGLES, 0, numVerts);
glDisableClientState( GL_COLOR_ARRAY);
glDisableClientState( GL_VERTEX_ARRAY);

glEndQuery( GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN_NV);
glDisable( GL_RASTERIZER_DISCARD_NV);
glEndTransformFeedbackNV();

glGetQueryObjectuiv( query, GL_QUERY_RESULT, &primitives_written);
glBindBufferBaseNV( GL_TRANSFORM_FEEDBACK_BUFFER_NV, 0, 0);

This is my code to draw the VBO:
glBindBuffer( GL_ARRAY_BUFFER, subdivisionVBO[0]);
glInterleavedArrays( GL_C3F_V3F, 6*sizeof( float), 0);
glDrawArrays( GL_TRIANGLES, 0, primitives_written);
glBindBuffer( GL_ARRAY_BUFFER, 0);

As said, the triangles get rendered and the colors are correct, but the positions of the triangles are wrong! I'm pretty sure my original data is correct - when I render the vertex and color pointers without the VBO-/transform_feedback-stuff, everything is fine. When I give the vertex data directly to the VBO using glBufferData( GL_ARRAY_BUFFER, numVerts*sizeof(GLfloat), vertexArray, GL_STATIC_DRAW);, the triangles are also rendered correctly. Many thanks in advance! Greetings, Michael

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I think I spotted the mistake myself, seems to be a real newbie mistake! ;-)

I applied vertex transformation in the geometry shader: gl_Position = gl_ModelViewProjectionMatrix * gl_PositionIn[0];. This should be applied only in the rendering pass, not in the transform-feedback pass, is this correct?
I wrote some dummy shaders to test my assumption, but the vertex positions are still weird (but different from the first results).

My shaders for the transform feedback pass are:

Fragment shader:

void main()
{
gl_FragColor = gl_Color;
}



Vertex shader:

void main()
{
gl_FrontColor = gl_Color;
gl_Position = gl_Vertex;
}



Geometry shader:

#extension GL_EXT_geometry_shader4 : enable
#extension GL_EXT_gpu_shader4 : enable

void main()
{
gl_FrontColor = gl_FrontColorIn[0];
gl_Position = vec4( gl_PositionIn[0].xyz / gl_PositionIn[0].w, 1.0);
EmitVertex();
gl_FrontColor = gl_FrontColorIn[1];
gl_Position = vec4( gl_PositionIn[1].xyz / gl_PositionIn[1].w, 1.0);
EmitVertex();
gl_FrontColor = gl_FrontColorIn[2];
gl_Position = vec4( gl_PositionIn[2].xyz / gl_PositionIn[2].w, 1.0);
EmitVertex();
EndPrimitive();
gl_FrontColor = gl_FrontColorIn[0];
gl_Position = vec4( gl_PositionIn[0].xyz / gl_PositionIn[0].w, 1.0) + vec4( 0.0, 0.1, 0.0, 0.1);
EmitVertex();
gl_FrontColor = gl_FrontColorIn[1];
gl_Position = vec4( gl_PositionIn[1].xyz / gl_PositionIn[1].w, 1.0) + vec4( 0.0, 0.1, 0.0, 0.1);
EmitVertex();
gl_FrontColor = gl_FrontColorIn[2];
gl_Position = vec4( gl_PositionIn[2].xyz / gl_PositionIn[2].w, 1.0) + vec4( 0.0, 0.1, 0.0, 0.1);
EmitVertex();
EndPrimitive();
}



As you can see, the geometry shader simply draws the original triangle and generates a second triangle. The number of GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN_NV is correct (twice the input).

Using shaders now I have to read back varyings:


GLint attribs[2];
attribs[0] = glGetVaryingLocationNV( this->subdivisionProgramPT, "gl_FrontColor");
attribs[1] = glGetVaryingLocationNV( this->subdivisionProgramPT, "gl_Position");
glTransformFeedbackVaryingsNV( this->subdivisionProgramPT, 2, attribs, GL_INTERLEAVED_ATTRIBS_NV);



The code for drawing the VBO has not really changed, apart from the shader that is now called. The fragment and vertex shaders are exactly the same af for the transform-feedback-pass, the geometry shader only transforms the vertex positions:


#extension GL_EXT_geometry_shader4 : enable
#extension GL_EXT_gpu_shader4 : enable

void main()
{
gl_FrontColor = gl_FrontColorIn[0];
gl_Position = gl_ModelViewProjectionMatrix * gl_PositionIn[0];
EmitVertex();
gl_FrontColor = gl_FrontColorIn[1];
gl_Position = gl_ModelViewProjectionMatrix * gl_PositionIn[1];
EmitVertex();
gl_FrontColor = gl_FrontColorIn[2];
gl_Position = gl_ModelViewProjectionMatrix * gl_PositionIn[2];
EmitVertex();
EndPrimitive();
}



I thought I got everything correct by now, but still my triangles are not in the right place. Any idea why that is so?

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Okay, I got it working! :)

I didn't realize that after changing to glGetVaryingLocationNV, my output were not longer vec3 but vec4! After telling the draw routine to use vec4, everything went fine. As always, the most obvious mistakes are the most hard to find (at least for me ).
Now I am using GL_SEPARATE_ATTRIBS_NV and two VBOs (one for the vertices, one for the colors). I'm still not sure what's the best way to draw that, my current (working) solution looks like that:

	glUseProgram( this->subdivisionProgramVT);

// map the data of the two buffers
glBindBuffer( GL_ARRAY_BUFFER, subdivisionColorVBO[1 - this->current_buffer]);
float *col = (float*)glMapBuffer( GL_ARRAY_BUFFER, GL_READ_ONLY);
glBindBuffer( GL_ARRAY_BUFFER, subdivisionVBO[1 - this->current_buffer]);
float *pos = (float*)glMapBuffer( GL_ARRAY_BUFFER, GL_READ_ONLY);
glBindBuffer( GL_ARRAY_BUFFER, 0);

glPushAttrib( GL_POLYGON_BIT);
glPolygonMode( GL_FRONT_AND_BACK, GL_LINE);
// draw the buffers
glEnableClientState( GL_VERTEX_ARRAY);
glEnableClientState( GL_COLOR_ARRAY);
glVertexPointer( 4, GL_FLOAT, 0, pos);
glColorPointer( 4, GL_FLOAT, 0, col);
glDrawArrays( GL_TRIANGLES, 0, this->primitives_written*3);
glDisableClientState( GL_COLOR_ARRAY);
glDisableClientState( GL_VERTEX_ARRAY);
glPopAttrib();

// unmap the data of the two buffers
glBindBuffer( GL_ARRAY_BUFFER, subdivisionColorVBO[1 - this->current_buffer]);
glUnmapBuffer( GL_ARRAY_BUFFER);
glBindBuffer( GL_ARRAY_BUFFER, subdivisionVBO[1 - this->current_buffer]);
glUnmapBuffer( GL_ARRAY_BUFFER);
glBindBuffer( GL_ARRAY_BUFFER, 0);

glUseProgram( 0);


If anyone has a suggestion how to do this better, please tell me!

My next problem is, that I use one of the color channels to pass data to the shader, but after the first shader pass, the values are clamped to [0.0, 1.0], which is pretty unconvenient for me! I think I'll have to use a user-defined varying instead of the gl_FrontColor, what do you think?

Greetings, Michael

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