Earth with Clouds AlphaBlending Shader

cloud-alpha-shader.cpp

//testApp.cpp

#include "ofMain.h"

class testApp : public ofBaseApp {
public:
    ofShader shader;
    ofImage earth,clouds;
    ofEasyCam cam;
    GLUquadricObj *quadric; //for gluSphere
    
    void setup() {

        //setup window
        ofSetWindowShape(500,500);
        ofSetWindowPosition(0,0);
        ofBackground(0);
        ofSetFrameRate(20);
        
        //load textures with normalized texcoords (0..1)
        ofDisableArbTex();
        earth.loadImage("earth2k.jpg");
        clouds.loadImage("clouds.jpg");
        
        //load the vertex and fragment shaders
        shader.load("shaders/blend");
        
        //initialize sphere
        quadric = gluNewQuadric();
        gluQuadricTexture(quadric, GL_TRUE);
        gluQuadricNormals(quadric, GLU_SMOOTH);
    }
    
    void draw() {
        
        //start camera and flip y-axis
        cam.begin();
        ofScale(1,-1); 
        
        //start shader en set uniform variables
        shader.begin();
        shader.setUniformTexture("earth", earth, 1); 
        shader.setUniformTexture("clouds", clouds, 2);
        shader.setUniform1f("blendAlpha", sin(ofGetElapsedTimef())/2+.5); //fade clouds
        
        //draw sphere
        glEnable(GL_DEPTH_TEST);
        gluSphere(quadric, 150, 400, 400);
        
        //that's all
        shader.end();
        cam.end();
    }
};

//////////////////////////////////////////////////////////////
//blend.frag

uniform sampler2D earth;  
uniform sampler2D clouds;  
uniform float blendAlpha;

void main(void) {  
    vec2 texcoord = gl_TexCoord[0].xy;
    vec4 background = texture2D(earth, texcoord);  
    vec4 foreground = texture2D(clouds, texcoord);  
    
    gl_FragColor = foreground * blendAlpha + background;
}

//////////////////////////////////////////////////////////////
//blend.vert

void main() {  
    gl_TexCoord[0] = gl_MultiTexCoord0;
    gl_Position = ftransform();     
}