iPad Opengl ES program works fine on simulator but not device

Question

For the device, all of my shaders load fine except one. For this shader program I get "Fragment program failed to compile with current context state" error, followed by a similar error for the vertex shader when I make a call to glGetProgramInfoLog(...);

Vertex shader:

#version 100

uniform mat4 Projection;
uniform mat4 Modelview;
uniform mat4 Rotation;
uniform vec3 Translation;

uniform vec4 LightDirection;
uniform vec4 MaterialDiffuse;
uniform float MaterialShininess;

attribute vec3 position;
attribute vec3 normal;

varying vec4 color;
varying float specularCoefficient;

void main() {
    vec3 _normal = normalize(mat3(Modelview[0].xyz, Modelview[1].xyz, Modelview[2].xyz)*normal); 
    // There is an easier way to do the above using typecast, but is apparently broken

    float NdotL = dot(-_normal, normalize(vec3(LightDirection)));
    if(NdotL < 0.0){
        NdotL = 0.0;
    }
    color = NdotL * MaterialDiffuse;
    float NdotO = dot(-_normal, vec3(0.0, 0.0, -1.0));
    if(NdotO < 0.0){
        NdotO = 0.0;
    }
    specularCoefficient = pow(NdotO, MaterialShininess);

    vec3 p = position + Translation;
    gl_Position = Projection*Modelview*vec4(p, 1.0);
}

Fragment shader:

#version 100
precision mediump float;

varying vec4 color;
varying float specularCoefficient;
uniform vec4 MaterialSpecular;

void main(){
    gl_FragColor = vec4((color + specularCoefficient*MaterialSpecular).rgb, 1.0);
}

I am not sure what is going on, especially since I have a similar program that is exactly as above with the addition of texture coordinates. Also, I checked the compile status of each shader when I linked the programs using glGetShaderiv(theShader, GL_COMPILE_STATUS, &result) and they all checked out fine. Any ideas?

Answer 1

Changing the line

gl_FragColor = vec4((color + specularCoefficient*MaterialSpecular).rgb, 1.0);

in the fragment shader to

gl_FragColor = vec4((1.0*color + specularCoefficient*MaterialSpecular).rgb, 1.0);

fixes the problem. I suspect it has something to do with precision related to the varying variable color, for a reordering of the line to

gl_FragColor = vec4((MaterialSpecular + specularCoefficient*color).rgb, 1.0);

works as well.