iPhone opengl es alpha-blending。 我的边缘有黑色

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.

Answer 2

One way this can occur is if you haven't correctly set your glTexParameteri for minification and magnification filters correctly, along with your clamp method. See Adam Redwood's comment here . Note issues of power-of-two textures, texture boundaries etc. may also come into play.

glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE );

Answer 3

Just check the view on which you are drawing is opaque or not. set opaque property to false