OpenGL ES: A Gentle Introduction - Beginning Android Games - page 323

For vertex colors, we always have to specify an alpha component, either by using glColor4f() ,
where the last argument is the alpha value, or by specifying the four components per vertex,
where again, the last component is the alpha value.
Let's put this into practice with a brief example. We want to draw Bob twice: once by using
the image bobrgb888.png , which does not have an alpha channel per pixel, and a second
time by using the image bobargb8888.png , which has alpha information. Note that the PNG
image actually stores the pixels in ARGB8888 format instead of RGBA8888. Luckily, the
GLUtils.texImage2D() method we use to upload the image data for a texture will do the
conversion for us automatically. Listing 7-11 shows the code of our little experiment using the
Texture and Vertices classes.
Listing 7-11. Excerpt from BlendingTest.java; Blending in Action
class BlendingScreen extends Screen {
GLGraphics glGraphics;
Vertices vertices;
Texture textureRgb;
Texture textureRgba;
public BlendingScreen(Game game) {
super (game);
glGraphics = ((GLGame)game).getGLGraphics();
textureRgb = new Texture((GLGame)game, "bobrgb888.png");
textureRgba = new Texture((GLGame)game, "bobargb8888.png");
vertices = new Vertices(glGraphics, 8, 12, true , true );
float [] rects = new float [] {
100, 100, 1, 1, 1, 0.5f, 0, 1,
228, 100, 1, 1, 1, 0.5f, 1, 1,
228, 228, 1, 1, 1, 0.5f, 1, 0,
100, 228, 1, 1, 1, 0.5f, 0, 0,
100, 300, 1, 1, 1, 1, 0, 1,
228, 300, 1, 1, 1, 1, 1, 1,
228, 428, 1, 1, 1, 1, 1, 0,
100, 428, 1, 1, 1, 1, 0, 0
};
vertices.setVertices(rects, 0, rects.length);
vertices.setIndices( new short [] {0, 1, 2, 2, 3, 0,
4, 5, 6, 6, 7, 4 }, 0, 12);
}
Our little BlendingScreen implementation holds a single Vertices instance where we'll store the two
rectangles, as well as two Texture instances—one holding the RGBA8888 image of Bob and the
other one storing the RGB888 version of Bob. In the constructor, we load both textures from the
files bobrgb888.png and bobargb8888.png and rely on the Texture class and GLUtils.texImag2D()
to convert the ARGB8888 PNG to RGBA8888, as needed by OpenGL ES. Next up, we define our
vertices and indices. The first rectangle, consisting of four vertices, maps to the RGB888 texture
of Bob. The second rectangle maps to the RGBA8888 version of Bob and is rendered 200 units