How to change fragment shader color between renders?

Question

I am struggling with coding this simple boat scene. It is a homework question but I am truly stumped. I have two models which I upload. One is a cube which has a fragment shader to color itself blue. The second is a boat which has a fragment shader to color itself white. When I use two different shaders the boat model is invisible. With much struggling and searching

I am stumped so I am trying plan B which is find a way to change the colors of a fragment shader between rendering the two objects, but I can't find out how to do so.

void render() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    //BOAT START
    //Rocks the ship, simulating ocean rocking
    if(shiprocker < 5000){
        shiprocker++;
        theta += 0.00002f;
    }
    if(shiprocker ==15000){
        shiprocker = -5000;
    }
    if(shiprocker > 4999){
        shiprocker++;
        theta -= 0.00002f;
    }
    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);  

    boat_model->mR = glm::rotate(            
        glm::mat4( 1.0f ),
        glm::degrees( theta ),
        glm::vec3(0.0f, 0.0f, 1.0f)
        );

    glm::mat4 mtx_trans = glm::translate(
        glm::mat4(1.0f),
        glm::vec3( 0.0f, 0.0f, -15.0f )
        );

    boat_model->mM =  mtx_trans * boat_model->mR;        
    boat_model->render();
    //BOAT END


    //OCEAN CUBE START
    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);  

    glm::mat4 mtx_trans_cube = glm::translate(
        glm::mat4( 1.0f ),
        glm::vec3( -20.0f, -50.0f, -50.0f ) 
        );

    plane_model->mM = glm::scale(  
        mtx_trans_cube,              
        glm::vec3( 10.0f, 10.0f, 10.0f )
        );

    //gl_FragColor = (0.0f, 0.0f, 1.0f , 1.0f); wont compile
    //glColor3f(0.0f, 0.0f, 1.0f); changes nothing

    camera->set_uniform_view_perspective( plane_model->shader->program_ID );
    plane_model->render();
    //OCEAN CUBE END


    glutSwapBuffers();
}



int main (int argc, char** argv) {

    // Initialize GLUT
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGBA|GLUT_DEPTH|GLUT_MULTISAMPLE | GLUT_RGB);


    //WINDOW SETTINGS
    glutInitWindowSize(1000, 1000);
    glutCreateWindow("SailBoat");

    //PASSIVE FUNCS
    glutReshapeFunc(change_viewport);
    glutDisplayFunc(render);
    glutIdleFunc( animate );

    //ERROR CHECK
    if (GLEW_OK != glewInit()) {
        exit(1);
    }

    //Clear color used for sky tones.
    glClearColor(0.529f, 0.807f, .98f, 1.0f);
    glEnable (GL_DEPTH_TEST);
    glEnable (GL_MULTISAMPLE);

    Shader* boatShader = new Shader( VERTEX_SHADER, BOAT_FRAGMENT_SHADER );
    Shader* cubeShader = new Shader( VERTEX_SHADER, CUBE_FRAGMENT_SHADER );

    //BOAT
    boat_model = new Model(boatShader); 
    load_model(boat_model ,MODEL_BOAT_FILENAME);
    boat_model->upload_2_server();


    //OCEAN CUBE
    plane_model = new Model( boatShader );
    load_model(plane_model ,MODEL_PLANE_FILENAME);
    plane_model->upload_2_server();

    // Set up the camera 
    theta = 0.0f;
    shiprocker = 0;
    camera = new FPSCamera(60.0f, 1.0f, 1.0f, 10000.0f, true);
    glutMainLoop();

    return 0;
}

FRAGMENT SHADER

#version 150

in vec4 color;
out vec4 fColor;

void main () {
    fColor = vec4(1.0, 1.0, 1.0, 0.5);

}

Answer 1

You can use uniform variables in GLSL to pass in values in between render calls. Set up your fragment shader like so:

#version 150

in vec4 color;
out vec4 fColor;
uniform vec3 boatColor;

void main () {
    fColor = vec4(boatColor, 0.5);
}

And in your C++ code:

glUseProgram(program); // Where program is your shader program
GLint uniform = glGetUniformLocation(program, "boatColor"); // since boatColor is what we called the uniform variable in our fragment shader
glUniform3f(uniform, 1.0f, 0.0f, 0.0f); // Set the boatColor variable to be a solid red
// Render your first boat
glUniform3f(uniform, 0.0f, 0.0f, 1.0f); // Set the second boat to be a solid blue
// Render your second boat
glUseProgram(0);

And that should work nicely. If anyone else here has a better way, please let me know; I'm relatively new to GLSL.