Question

I’m tweaking the shader examples in OpenGL Superbible 2010 to learn about shader usage.

I wanna change the red component of a directional light that’s passed as a uniform to a diffuse light shader. The vector

GLfloat vDiffuseColor [] = {redDirectionalLight, greenDirectionalLight, blueDirectionalLight, 1.0f};

is defined globally.

I can tweak the light color if I specify the variables before compiling but not when the code is running. The glut keyboard function that I’m using does increment the color values at runtime, but the modified uniform isn’t being reloaded. In the SetupRC function I locate the position of the diffuseColor uniform

locColor = glGetUniformLocation(DirectionalLightSimpleDiffuseShader, "diffuseColor");

What I’m missing here?

vertex program:
#version 130

// Incoming per vertex... position and normal
in vec4 vVertex;
in vec3 vNormal;

// Set per batch
uniform vec4    diffuseColor;   
uniform vec3    vLightPosition;
uniform mat4    mvpMatrix;
uniform mat4    mvMatrix;
uniform mat3    normalMatrix;

// Color to fragment program
smooth out vec4 vVaryingColor;

void main(void) 
    { 
    // Get surface normal in eye coordinates
    vec3 vEyeNormal = normalMatrix * vNormal;

    // Get vertex position in eye coordinates
    vec4 vPosition4 = mvMatrix * vVertex;
    vec3 vPosition3 = vPosition4.xyz / vPosition4.w;

    // Get vector to light source
    vec3 vLightDir = normalize(vLightPosition - vPosition3);

    // Dot product gives us diffuse intensity
    float diff = max(0.0, dot(vEyeNormal, vLightDir));

    // Multiply intensity by diffuse color
    vVaryingColor.rgb = diff * diffuseColor.rgb;
    vVaryingColor.a = diffuseColor.a;

    // Let's not forget to transform the geometry
    gl_Position = mvpMatrix * vVertex;
    }

fragment program:

#version 130

out vec4 vFragColor;
smooth in vec4 vVaryingColor;

void main(void)
   { 
   vFragColor = vVaryingColor;
   }

Here’s the RenderScene code, which is called via glutDisplayFunc:

void RenderScene (void)
{

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);



    modelViewMatrix.PushMatrix(); 

    M3DMatrix44f mCamera; // CAMERA MATRIX

       cameraFrame.GetCameraMatrix(mCamera);
    modelViewMatrix.PushMatrix(mCamera);  


    M3DMatrix44f vLightPos= {0.0f, 10.0f, 5.0f, 1.0f};
    M3DVector4f  vLightEyePos;
    m3dTransformVector4 (vLightEyePos, vLightPos, mCamera);


    shaderManager.UseStockShader (GLT_SHADER_FLAT,transformPipeline.GetModelViewProjectionMatrix(),
                                  vFloorColor);


    floorBatch.Draw();

    for (int i=0; i<NUM_SPHERES; i++){

         modelViewMatrix.PushMatrix(); 
         modelViewMatrix.MultMatrix(spheres[i]); 


            glUseProgram (DirectionalLightSimpleDiffuseShader);
            glUniform4fv (locColor, 1, vDiffuseColor); 
            glUniform3fv (locLight, 1, vEyeLight);
            glUniformMatrix4fv (locMVP, 1, GL_FALSE, transformPipeline.GetModelViewProjectionMatrix());



            glUniformMatrix4fv (locMV, 1, GL_FALSE, transformPipeline.GetModelViewMatrix());
            glUniformMatrix3fv (locNM, 1, GL_FALSE, transformPipeline.GetNormalMatrix());


            sphereBatch.Draw();
        //  glUseProgram(0);
            modelViewMatrix.PopMatrix(); 
    }

Answer 1

Please tell me you’re changing vDiffuseColor and not redDirectionalLight, greenDirectionalLight, blueDirectionalLight.

GLfloat vDiffuseColor [] = {redDirectionalLight, greenDirectionalLight, blueDirectionalLight, 1.0f};

makes an array by copying the initializer, there is no connection to the original three variables.

If you want to address the three component by name, instead use (assuming C++):

GLfloat vDiffuseColor[4] = {0.0f, 0.0f, 0.0f, 1.0f};
GLfloat&   redDirectionalLight = vDiffuseColor[0];
GLfloat& greenDirectionalLight = vDiffuseColor[1];
GLfloat&  blueDirectionalLight = vDiffuseColor[2];