Changing shader-code in xcode on iphone opengl es 2.0 project results in an error.

Question

Im new to shaders and I'm trying to learn the basics. But everytime I change some code in my vertex shader it results in an error.

"Validation Failed: Program is not successfully linked."

Im using the standard openGL project for the iphone as a starter in Xcode ( with a 2d cube moving in y-position).

From the beginning the shader looks like this:

attribute vec4 position;  
attribute vec4 color;  

varying vec4 colorVarying;  

void main()  
{  
     gl_Position = position;  
     gl_Position.y += sin(translate) / 2.0;  
     colorVarying = color;  
}  

and I want to change it to :

uniform mat4 gl_ProjectionMatrix;  
uniform mat4 gl_ModelViewMatrix;  

attribute vec4 gl_Vertex;  

void main()  
{  
     gl_Position = gl_ProjectionMatrix * gl_ModelViewMatrix * gl_Vertex;  
}  

I all get is a white screen and the error mentioned above.

What's the problem?

/Niclas

Answer 1

Notice that you are using a "varying" parameter. This needs to be defined for both the vertex and the fragment shader. Since you appear to have only changed the vertex shader I'm guessing that this is causing your link problem as the colorVarying is probably still in the fragment shader. Try putting the logic of "colorVarying" back into your vertex shader. Something like:

uniform mat4 gl_ProjectionMatrix;   
uniform mat4 gl_ModelViewMatrix;   

attribute vec4 gl_Vertex;   

attribute vec4 color;
varying vec4 colorVarying;

void main()   
{   
    gl_Position = gl_ProjectionMatrix * gl_ModelViewMatrix * gl_Vertex;  
    colorVarying = color; 
}