What am I doing that's unnecessary? (iPhone, OpenGL ES)

Question

I'm using OpenGL ES to draw things in my iPhone game. Sometimes I like to change the alpha of the textures I'm drawing. Here is the (working) code I use. What, if anything, in this code sample is unnecessary? Thanks!

    // draw texture with alpha "myAlpha"
 glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE,GL_COMBINE );
 glTexEnvf(GL_TEXTURE_ENV, GL_COMBINE_RGB,GL_MODULATE);
 glTexEnvf(GL_TEXTURE_ENV, GL_SRC0_RGB,GL_PRIMARY_COLOR);
 glTexEnvf(GL_TEXTURE_ENV, GL_OPERAND0_RGB,GL_SRC_COLOR);
 glTexEnvf(GL_TEXTURE_ENV, GL_SRC1_RGB,GL_TEXTURE);
 glTexEnvf(GL_TEXTURE_ENV, GL_OPERAND1_RGB,GL_SRC_COLOR);
 glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

 glEnable(GL_BLEND);
 glTexEnvf(GL_TEXTURE_ENV, GL_OPERAND1_RGB, GL_SRC_COLOR);
 glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
 glColor4f(myAlpha, myAlpha, myAlpha, myAlpha);

 glPushMatrix();
 glTranslatef(xLoc, yLoc, 0);
 [MyTexture drawAtPoint:CGPointZero];
 glPopMatrix();

 glColor4f(1.0, 1.0, 1.0, 1.0);

Edit:

The above code sample is for drawing w/ a modified alpha value (so I can fade things in and out). When I just want to draw w/o modifying the alpha value I'd use the last 5 lines of the above sample just without the last call to glColor4f.

Answer 1

I'm not a expert but I was reading the Optimizing OpenGL ES for iPhone OS and it had a section on "Optimizing Texturing" which may help you out.

Answer 2

You have calls to configure both the Texture Environment and Framebuffer Blending, which are entirely different features. You are also calling glTexEnvf(GL_TEXTURE_ENV, GL_OPERAND1_RGB, x) twice before drawing, and so only the second call matters.

If all you really want to do is multiply the primary color by the texture color, then it is as simple as glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE). The rest of the calls to glTexEnv are unnecessary.

Answer 3

My drawing looks like:

glBindTexture(GL_TEXTURE_2D, tex->name); // bind to the name
glVertexPointer(3, GL_FLOAT, 0, vertices); // verices is the alignment in triangles
glTexCoordPointer(2, GL_FLOAT, 0, coordinates); // coordinates describes the rectangle
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4); //draw

It's hard to know what you can remove without knowing exactly what you're trying to do.

edit:

Unless you are disabling blend at some point, you can get rid of all of the blend calls:

glEnable(GL_BLEND);
glBlendFunc...

Put them in the initialization of your GL state if you need to set it once. If you need to enable and disable it is better to set the state once for everything you need to draw blended and then set the state again (once) for the unblended.

OpenGL is a state machine so this general idea applies anywhere you need to change the GL state (like setting texture environment with glTexEnvf).

About state machines:

A current state is determined by past states of the system. As such, it can be said to record information about the past, i.e., it reflects the input changes from the system start to the present moment. A transition indicates a state change and is described by a condition that would need to be fulfilled to enable the transition.