Question

i currently trying to get my model loader to work and got a error when binding new shader attributes.

At this point i would post the error message, but MonoTouch don’t let me do this. The only thing i got is “WARNING: Could n”, seems to be a issue with MonoTouch, because i used a template of the framework.

Here is the shader code:

<Shader>
  <Uniforms>
    <Uniform type="mat4" name="modelViewMatrix"/>
  </Uniforms>

  <Vertex>
    <Attributes>
      <Attribute type="vec3" name="position" binding="Position"/>
      <Attribute type="vec3" name="normal" binding="Normals"/>
      <!--<Attribute type="vec4" name="color" binding="Color"/>-->
    </Attributes>

    <Code><![CDATA[

attribute vec3 position;
attribute vec3 normal;          

varying vec4 colorVarying;

uniform mat4 modelViewMatrix;           

void main()
{
  gl_Position = modelViewMatrix * vec4(position.xyz, 1.0);
  float z = gl_Position.z / 100.0;
  colorVarying = vec4(z, z, z, 1.0) ;   
}

    ]]></Code>
  </Vertex>
  <Pixel>
    <Code><![CDATA[

varying lowp vec4 colorVarying;

void main()
{
  gl_FragColor = colorVarying;
}

    ]]></Code>
  </Pixel>
</Shader>

The shader works perfectly when removing the “normal” attribute. When adding it i got the following error from mono ( Not from OpenGL ) after calling GL.LinkProgram:

App(553,0xacb752c0) malloc: *** error for object 0x1025a3c4: incorrect checksum for freed object - object was probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug

I noticed that it has something to do with the attribute binding. Here is the code:

// Bind attribute locations
for (int i = 0; i < _VertexAttributeList.Length; i++)
{
    ShaderAttribute attribute = _VertexAttributeList[i];
    GL.BindAttribLocation(_Program, i, attribute.Name);
}

When i replace “_VertexAttributeList.Length” with the constant 1 it works without any errors.

Cheers
Felix

Answer 1

I found the error after using the following lines of code:

for (int i = 0; i < _VertexAttributeList.Length) {
    ShaderAttribute attribute = _VertexAttributeList[i];
    Console.WriteLine("Attribute {0} @ {1}", attribute.Name,
                      GL.GetAttribLocation(_Program, attribute.Name));
}

It turns out that GLSL Ignores unused attributes and the attribute “normal” has a location of -1.

It has been a very annoying error, because of the fact that i know now what the error message means which i could not read because of the MonoTouch error: “Variable not used and ignored”.