Changing the alpha/opacity channel on a texture using GLKit - opengl-es-2.0

Question

I am new to opengl es, and I can't seem to figure out how you would change the alpha / opacity
on a texture loaded with GLKTextureLoader.
Right now I just draw the texture with the following code.
self.texture.effect.texture2d0.enabled = YES;
self.texture.effect.texture2d0.name = self.texture.textureInfo.name;
self.texture.effect.transform.modelviewMatrix = [self modelMatrix];
[self.texture.effect prepareToDraw];
glEnableVertexAttribArray(GLKVertexAttribPosition);
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
NKTexturedQuad _quad = self.texture.quad;
long offset = (long)&_quad;
glVertexAttribPointer(GLKVertexAttribPosition,
2,
GL_FLOAT,
GL_FALSE,
sizeof(NKTexturedVertex),
(void *)(offset + offsetof(NKTexturedVertex, geometryVertex)));
glVertexAttribPointer(GLKVertexAttribTexCoord0,
2,
GL_FLOAT, GL_FALSE,
sizeof(NKTexturedVertex),
(void *)(offset + offsetof(NKTexturedVertex, textureVertex)));
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
Any advice would be very helpful :)

Answer 1

i am no gl expert but to draw with a changed alpha value does not seem to work as described by rickster.
as far as i understand, the values passed to glBlendColor are only used when using glBlendFunc constants like: GL_CONSTANT_…
this will overwrite the textures alpha values and use a defined value to draw with:
glEnable(GL_BLEND);
glBlendFunc(GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA);
glBlendColor(1.0, 1.0, 1.0, yourAlphaValue);
glDraw... // the draw operations
further reference can be found here http://www.opengl.org/wiki/Blending#Blend_Color

Answer 2

As long as you're in the OpenGL ES 1.1 world (or the emulated-1.1 world of GLKBaseEffect), alpha is a property either of the (per-pixel) bitmap data in the texture or of the (complete) OpenGL ES state you're drawing with. You can't set an opacity level for a texture as a whole, on its own. So, you have three options:
Change the alpha of the texture. This means changing the texture bitmap data itself -- use the 2D image context of your choice to draw the image at half (or whatever) alpha, and read the resulting image into an OpenGL ES texture. Probably not a great idea unless the alpha you want will be constant for the life of your app. In which case you might as well just go back to Photoshop (or whatever you're using to create your image assets) and set the alpha there.
Change the alpha you're drawing with. After you prepareToDraw, set up blending in GL:
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA);
glBlendColor(1.0, 1.0, 1.0, myDesiredAlphaValue);
glDraw... // whatever you're drawing
Don't forget to A) draw your partially transparent content after any content you want it blended on top of and B) disable blending before rendering opaque content again on the next frame.
Ditch GLKBaseEffect and write your own shaders. Shaders that work like the 1.1 fixed-function pipeline are a dime a dozen -- you can even get started by using the shaders packaged with the Xcode "OpenGL Game" project template or looking at the shaders GLKit writes in the Xcode Frame Capture tool. Once you have such shaders, changing the alpha of a color you got out of a texel lookup is a simple operation:
vec4 color = texture2D(texUnit, texCoord);
color.a = myDesiredAlphaValue;
gl_FragColor = color;