Pro OpenGL ES for iOS

182 CHAPTER 6: Will It Blend?^

//glEnable(GL_BLEND);

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

//SQUARE 1

//glEnableClientState(GL_COLOR_ARRAY);

glColorPointer(4, GL_FLOAT, 0, squareColorsYMCA);

glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

//SQUARE 2

glLoadIdentity();

glTranslatef( (GLfloat)(sinf(transY)/2.0),0.0, -3.0);

glColorPointer(4, GL_FLOAT, 0, squareColorsRGBA);

glBindTexture(GL_TEXTURE_2D,m_Texture1.name);

glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

transY += 0.075f;

}

In addition to this, make sure to add loadTexture() from the Chapter 5 examples, and

initialize it in the usual place. Because we need two different textures, initialize the first

as m_Texture0 and the second as m_Texture1. You will likely notice that while I have both

blending and color stuff, I commented out some lines just for this first run-through to

ensure that the basic stuff is working. If it’s working, you should see something like

Figure 6-8 (left). And if that works, unleash the vertex colors by uncommenting

glEnableClientState(GL_COLOR_ARRAY) and glEnable(GL_BLEND), which should yield

Figure 6-8 (center). And for Figure 6-8 (right), the Golden Gate Bridge is colored with a

solid red. I shall let you, dear reader, figure out how to do this.

Using a single bitmap and colorizing it is a common practice to save memory. If you are

doing some UI components in the OpenGL layer, consider using a single image, and

colorize it using these techniques. You might ask why is it a solid red as opposed to

merely being tinted red, allowing for some variation in colors. What is happening here is

that the vertex’s colors are being multiplied by the colors of each fragment. For the red,

I’ve used the RGB triplet of 1.0,0.0,0.0. So when each fragment is being calculated in a

channel-wise multiplication, the green and blue channels are going to be multiplied by 0,

so they are completely filtered out, leaving just the red. If you wanted to let some of the

other colors leak through, you would specify the vertices to lean toward a more neutral

tone, with the desired tint color being a little higher than the others, such as 1.0,0.7,0.7.