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

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.