iPhone opengl es alpha-blending. I have black color in edge

Question

I am developing iPhone game. I have the below Source image to draw it to the background. The Source image has alpha 0x00 and gradation around edge and the background's alpha is 0xff. When I draw the source image to the background, I have black color like you could see the Result image. I am using OpenGL ES glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) method. I have changed every possible arguments, but every time it is not moved.

Why do I have the black color in the edge of the source image? What is the problem? Does not every games in iphone use gradation?

Do I need to make the source image with out gradation?

Does anyone know the solution?

Source image :

Result image :

Answer 1

Does your image have pre-multiplied alpha? Then you should be using

glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

Here is some explanations of what it is.

Note: I rewrote the answer after doing some research. The article rotoglup mentioned has some insight on what's going on and it makes more sense then my original formula. I also tried to see what other people are using in their code and it looks like this formula is the most used one. It's amazing how such a basic thing in 3D graphics as alpha blending can be so controversial and everyone seems to reinvent the wheel and come up with their own solutions.