Question

I building a simple Framework for OpenGL UI’s for MonoTouch. I set up everything and also succeeded rendering 3D Models, but a simple 2D texture object fails. The texture has a size of 256×256 so it’s not to large and its power of two.

Here is some rendering code( Note: I did remove the existing, and working code ):

// Render the gui objects ( flat )
Projection = Matrix4x4.Orthographic(0, WindowProperties.Width, WindowProperties.Height, 0);
View = new Matrix4x4();
GL.Disable(All.CullFace);
GL.Disable(All.DepthTest);
_Stage.RenderGui();

Stage:

public void RenderGui ()
{
    Draw(this);
    // Renders every child control, all of them call "DrawImage" when rendering something
}


public void DrawImage (Control caller, ITexture2D texture, PointF position, SizeF size)
{
    PointF gposition = caller.GlobalPosition; // Resulting position is 0,0 in my tests
    gposition.X += position.X;
    gposition.Y += position.Y;

    // Renders the ui model, this is done by using a existing ( and working vertex buffer )
    // The shader gets some parameters ( this works too in 3d space )   
    _UIModel.Render(new RenderParameters() {
        Model = Matrix4x4.Scale(size.Width, size.Height, 1) * Matrix4x4.Translation(gposition.X, gposition.Y, 0),
       TextureParameters = new TextureParameter[] {
           new TextureParameter("texture", texture)
       }
    });
}

The model is using a vector2 for positions, no other attributes are given to the shader.

The shader below should render the texture.

Vertex:

attribute vec2 position;        
uniform mat4 modelViewMatrix;           
varying mediump vec2 textureCoordinates;

void main()
{
    gl_Position = modelViewMatrix * vec4(position.xy, -3.0, 1.0);
    textureCoordinates = position;
}

Fragment:

varying mediump vec2 textureCoordinates;
uniform sampler2D texture;          

void main()
{
    gl_FragColor = texture2D(texture, textureCoordinates) + vec4(0.5, 0.5, 0.5, 0.5);
}

I found out that the drawing issue is caused by the shader. This line produces a GL_INVALID_OPERATION( It works with other shaders ):

GL.UniformMatrix4(uni.Location, 1, false, (parameters.Model * _Device.View * _Device.Projection).ToArray());

EDIT:

It turns out that the shader uniform locations changed( Yes i’m wondering about this too, because the initialization happens when the shader is completly initialized. I changed it, and now everything works.

As mentioned in the other thread the texture is wrong, but this is another issue ( OpenGL ES 2.0 / MonoTouch: Texture is colorized red )

The shader initialization with the GL.GetUniformLocation problem mentioned above:

[... Compile shaders ...]
// Attach vertex shader to program.
GL.AttachShader (_Program, vertexShader);

// Attach fragment shader to program.
GL.AttachShader (_Program, pixelShader);        

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

// Link program
if (!LinkProgram (_Program)) {
    GL.DeleteShader (vertexShader);
    GL.DeleteShader (pixelShader);
    GL.DeleteProgram (_Program);

    throw new Exception ("Shader could not be linked");
}

// Get uniform locations
for (int i = 0; i < _UniformList.Length; i++) {
    ShaderUniform uniform = _UniformList [i];
    uniform.Location = GL.GetUniformLocation (_Program, uniform.Name);
    Console.WriteLine ("Uniform: {0} Location: {1}", uniform.Name, uniform.Location);
}

// Detach shaders
GL.DetachShader (_Program, vertexShader);
GL.DetachShader (_Program, pixelShader);

GL.DeleteShader (vertexShader);
GL.DeleteShader (pixelShader);

// Shader is initialized add it to the device
_Device.AddResource (this);

Answer 1

It turns out that the shader uniforms change at a unknown time. Everything is created and initialized when i ask OpenGL ES for the uniform location, so it must be a bug in OpenGL.

Calling GL.GetUniformLocation(..) each time i set the shader uniforms solves the problem.