Wednesday, March 20, 2013

OpenGL ES Tutorial for Android – Part III – Adding colors

Last tutorial was about transformations. This tutorial will be a short one. I'm going to talk about adding color to your mesh. I will continue with the source code from tutorial II.

Adding color

3D models with no colors are pretty boring so let's add some color to it. In general colors need no explanation. OpenGL ES uses a color model called RGBA ( Red, Green, Blue and Alpha ). The first three are self explained. The fourth is transparency, how solid the color should be. If you like to read more about colors go to: RGB color model - Wikipedia, the free encyclopedia
You might be familiar with defining colors with hex (#FF00FF) or with decimals (255, 0, 255) we will use 0..1 where 0 map to 0 (#00) and 1 map against 255 (#FF).
The easiest way of coloring meshes is called vertex coloring and I am going to show you two different ways of doing that. Flat coloring that gives one solid color and smooth coloring that will blend colors specified for each vertex. Texturing is also a way of giving your mesh colors but it is not vertex coloring so I will show you how to do that in a later tutorial.

Flat coloring

Flat ColoringFlat coloring is really easy just tell OpenGL ES what color to use when it is going to render. One thing to remember is that when you set the color OpenGL ES uses this color until you change the color. This means that if you have two different squares and you tell OpenGL ES to change the color right before the second square the first frame the two squares will have different color but the next rendered frame both squares will have the same color.
To tell OpenGL ES what color to work with you use this command:
public abstract void glColor4f(float red, float green, float blue, float alpha)
The default values are: red = 1, green = 1, blue = 1 and alpha = 1. Those values are white, and that's why all the squares we previous made has a white color.
Create a new class called FlatColoredSquare it should be identical to the Square class. Then in the FlatColoredSquare function draw, add this line:
gl.glColor4f(0.5f, 0.5f, 1.0f, 1.0f); // 0x8080FFFF
I usually add a comment like the one above ( // 0x8080FFFF ) because I am used to read that. It makes it easier for me when reviewing the code.
It should now look like this:
public void draw(GL10 gl) {
        gl.glColor4f(0.5f, 0.5f, 1.0f, 1.0f);
        ...
Then change in the renderer so it uses the FlatColoredSquare instead of the Square.
public class OpenGLRenderer implements Renderer {
 private FlatColoredSquare flatSquare; // CHANGED

 public OpenGLRenderer() {
  // Initialize our square.
  flatSquare = new FlatColoredSquare(); // CHANGED
 }

        public void onDrawFrame(GL10 gl) {
                ...
  flatSquare.draw(gl); // Don't forget to change this one.
                ...
 }
Remember that anything rendered after you set a color uses the same color and that this spans over frames and will not be reset in-between.
If you compile and run the application you will see one big flat colored blue square.
Just to give place to the smooth colored square coming up we move the flat square up.
public void onDrawFrame(GL10 gl) {
 gl.glLoadIdentity();
 // Translates 7 units into the screen and 1.5 units up.
 gl.glTranslatef(0, 1.5f, -7);
        // Draw our flat square.
 flatSquare.draw(gl);
}
Notice that with flat coloring you don't need to tell OpenGL ES to turn it on or off. OpenGL ES uses flat coloring as a default way of coloring the meshes.

Smooth coloring

Smooth ColoringSmooth coloring is gained when you give each vertex its own color. OpenGL ES will interpolate the colors between the vertices and you will gain a smooth coloring effect. Just as with the flat coloring you tell OpenGL ES what to work with and it will be used as long as you don't say anything else.
Create a new class called SmoothColoredSquare it should be identical to the Square class just as you did with the FlatColoredSquare. Modify the new class with this:
Define the colors you like your vertices to have.
public class SmoothColoredSquare {
        ...
        // The colors mapped to the vertices.
        float[] colors = {
                1f, 0f, 0f, 1f, // vertex 0 red
                0f, 1f, 0f, 1f, // vertex 1 green
                0f, 0f, 1f, 1f, // vertex 2 blue
                1f, 0f, 1f, 1f, // vertex 3 magenta
        };
        ...
The order of defining the colors are important since they map against the vertices so in this example above the first color (1f, 0f, 0f, 1f ) map against the top left vertex ( -1.0f, 1.0f, 0.0f ) the green against the bottom left vertex and the rest you can figure out. Hint: Look at the image above.
And put them in a buffer just as we did with the vertices and indices.
public SmoothColoredSquare() {
 ...

 // float has 4 bytes, colors (RGBA) * 4 bytes
 ByteBuffer cbb = ByteBuffer.allocateDirect(colors.length * 4);
 cbb.order(ByteOrder.nativeOrder());
 colorBuffer = cbb.asFloatBuffer();
 colorBuffer.put(colors);
 colorBuffer.position(0);
 }
Don't forget to add colorBuffer as a variable to the class as well.
        // Our color buffer.
 private FloatBuffer colorBuffer;
We also need to enable the color buffer and tell openGL where it is.
public void draw(GL10 gl) {
        ...
 gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexBuffer);

 // Enable the color array buffer to be used during rendering.
 gl.glEnableClientState(GL10.GL_COLOR_ARRAY); // NEW LINE ADDED.
 // Point out the where the color buffer is.
 gl.glColorPointer(4, GL10.GL_FLOAT, 0, colorBuffer); // NEW LINE ADDED.

 gl.glDrawElements(GL10.GL_TRIANGLES, indices.length,
    GL10.GL_UNSIGNED_SHORT, indexBuffer);
 ...
        // Disable the color buffer.
 gl.glDisableClientState(GL10.GL_COLOR_ARRAY);
        ...
 }
Don't forget to disable the use of the color array. If you don't disable the color array both squares will be smooth colored. Try it.
Let's use this new smooth square as well. Start by adding it to your renderer.
public class OpenGLRenderer implements Renderer {
 private FlatColoredSquare flatSquare;
 private SmoothColoredSquare smoothSquare; // NEW LINE ADDED.

 public OpenGLRenderer() {
  // Initialize our squares.
  flatSquare = new FlatColoredSquare();
  smoothSquare = new SmoothColoredSquare(); // NEW LINE ADDED.
 }
We need to move the square down a bit so they don't collide.
public void onDrawFrame(GL10 gl) {
 ...
        // Translate to end up under the flat square.
 gl.glTranslatef(0, -3f, 0);
 // Draw our smooth square.
 smoothSquare.draw(gl);
}