2 questions...

• Do you check that the shaders are compiled correctly? (ie. if you add an error, you DO get a warning)
• Are you using glGetAttribLocationARB() for uniforms? If so, try using glGetUniformLocationARB()...

Yes, I get errors and warnings when there are ones and yes, I use Attrib and Uniform in the correct way, both when getting the locations and when passing values (I just double checked this). Any other ideas?

If you haven't done so already, update to Tao.OpenGl 2.1.0.12+ (check the development forum).

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OpenTK

I updated the files as suggested and moved the glGetLocation() calls to my render method. The latter resulted in that the locations get valid numbers. In fact, all values now seem to be set and passed correctly (I checked this with glslDevil), but somehow they still have the wrong values in the shader.

This is the code that I am running. I am well aware of that there are far quicker ways of doing the same thing, but speed is not the issue here. Getting it to work at all is.

//======================================================
C# program
//======================================================

void Render()
{
// Irrelevant stuff
/* ... */

//Pick a shader program
Gl.glUseProgram(_myShader);

_timeLocation = Gl.glGetUniformLocationARB(_myShader, "time");
_numberOfVerticesLocation = Gl.glGetUniformLocationARB(_myShader, "noOfVertices");
_numberOfObjectsLocation = Gl.glGetUniformLocationARB(_myShader, "noOfObjects");
_vertexIDLocation = Gl.glGetAttribLocationARB(_myShader, "vertexID");
_objectIDLocation = Gl.glGetAttribLocationARB(_myShader, "objectID");

// Set shader variables
Gl.glUniform1fARB(_timeLocation, (float)(currentTime - _startTime) * 0.001f);
Gl.glUniform1fARB(_numberOfVerticesLocation, (float)_myTexture.Height);
Gl.glUniform1fARB(_numberOfObjectsLocation, (float)_myTexture.Width);
Gl.glBindTexture(Gl.GL_TEXTURE_2D, _myTextureID);

DrawSquares();

openGLControl.SwapBuffers();
}

private void DrawSquares()
{
Gl.glColor4f(0.5f, 0.5f, 0.5f, 1.0f);

Gl.glBegin(Gl.GL_QUADS);

for (float i = 0; i < _grassParameterTexture.Width; i++)
{
Gl.glVertexAttrib1fARB(_objectIDLocation, i);
DrawSquare();
}

Gl.glEnd();
}

private void DrawSquare()
{
Gl.glNormal3f(-1, 0, 0);
Gl.glVertexAttrib1fARB(_vertexIDLocation, 0f);
Gl.glVertex3f(0, -.5f, -.5f);
Gl.glVertexAttrib1fARB(_vertexIDLocation, 1f);
Gl.glVertex3f(0, .5f, -.5f);
Gl.glVertexAttrib1fARB(_vertexIDLocation, 2f);
Gl.glVertex3f(0, .5f, .5f);
Gl.glVertexAttrib1fARB(_vertexIDLocation, 3f);
Gl.glVertex3f(0, -.5f, .5f);
}

//======================================================

The vertex shader used is listed below. The first three lines that are commented away results in eight squares positioned in a rotating circle and colored according to their objectID. The last three lines are what I try to do which doesn't work. As soon as I use the variable vertexID in any way, the previously working code stops working and results in all the squares being rendered in the same position.

//======================================================
Vertex shader
//======================================================

uniform float time;
uniform sampler2D paramTexture;
attribute float vertexID;
attribute float objectID;
uniform float noOfVertices;
uniform float noOfObjects;
varying vec3 v_color;

void main( void )
{
//float angle = 6.283 * (objectID + time) / noOfObjects;
//gl_Position = gl_ModelViewProjectionMatrix * (gl_Vertex + vec4(0.0, sin(angle), cos(angle), 0.0));
//v_color = clamp(vec3(objectID*0.1, 0.0, 1.0 / noOfVertices), 0.0, 1.0);

float angle = 6.283 * (objectID + time) / noOfObjects;
gl_Position = gl_ModelViewProjectionMatrix * (gl_Vertex + vec4(0.0, sin(angle), cos(angle), 0.0));
v_color = clamp(vec3(objectID*0.1, 0.0, vertexID / noOfVertices), 0.0, 1.0);
}

//======================================================