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POWhy does alpha blending gradient textures yield unexpected results in OpenGL ES 2.0?
 

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  1. POWhy does alpha blending gradient textures yield unexpected results in OpenGL ES 2.0?
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    <p>I have a radial gradient texture (RGBA) that goes from a black fully opaque color in the center to fully transparent one at the edges:</p> <p><img src="https://i.stack.imgur.com/94jHH.png" alt="ball.png"></p> <p>I've set up an iOS project using the recent OpenGL template, which uses GLKit. I added texture capability and have the following code set in the setupGL method:</p> <pre><code>glEnable(GL_DEPTH_TEST); glEnable(GL_BLEND); glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); </code></pre> <p>When I overlap two of these textures (each has a separate z) in an ortho projection on a white background, I expect something like this:</p> <p><img src="https://i.stack.imgur.com/NrkCI.png" alt="right.png"></p> <p>But instead I get this:</p> <p><img src="https://i.stack.imgur.com/TgQp5.png" alt="wrong.png"></p> <p>My question is simply: why? Can OpenGL ES 2.0's blending not achieve this effect? (I've gotten it to work before in OpenGL ES 1.1) Is there a common thing I'm missing? Is it maybe a premultiplied alpha problem with my texture? If so could it be something strange with my PNG or the way I'm loading it? I'm loading textures using GLKTextureLoader.</p> <p>It's very possible I'm wrong, but I understand that it couldn't be a shader problem as alpha blending happens before it gets to the fragment shader. My shaders do little more than pass the information directly through. (Maybe that's a problem?)</p> <p>Vertex Shader:</p> <pre><code>attribute vec4 position; attribute vec2 texCoordIn; varying vec2 texCoordOut; uniform mat4 modelViewProjectionMatrix; void main() { gl_Position = modelViewProjectionMatrix * position; texCoordOut = texCoordIn; } </code></pre> <p>Fragment Shader:</p> <pre><code>varying lowp vec2 texCoordOut; uniform sampler2D texture; void main() { gl_FragColor = texture2D(texture, texCoordOut); } </code></pre> <p>I've also tried setting the glBendFunc as follows, but I get the same results:</p> <pre><code>glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA); </code></pre> <p>Any tips or common hangups are welcome. Thanks!</p>
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