OpenGL-GLKit：透明对象和纹理

Question

My project is an IOS app using OpenGL 2.0 with the GLKit. In a nutshell, i have a texture of star with a transparent background that i would like to apply on a square. The desired end result would be seeing a star; not a star on a square.

My current issue is that when i apply the texture on the square , i see a star with a colored square defined by its material color variables. What i would like is seeing only the star and having the rest of the square transparent.

The code is as follow:

    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); 


    [effectTmp prepareToDraw];

    glEnableVertexAttribArray(GLKVertexAttribPosition);
    glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
    glEnableVertexAttribArray(GLKVertexAttribNormal);

    glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(ColoredVertexData3D), &vertexDataTexture[0].vertex);
    glVertexAttribPointer(GLKVertexAttribNormal, 3, GL_FLOAT, GL_FALSE, sizeof(ColoredVertexData3D), &vertexDataTexture[0].normal);

    glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, sizeof(ColoredVertexData3D), &vertexDataTexture[0].color);

    glDrawArrays(GL_TRIANGLES, 0, [drawObjectTmp getSizeFromVertexIndicesArray]);
    glDisableVertexAttribArray(GLKVertexAttribPosition);
    glDisableVertexAttribArray(GLKVertexAttribTexCoord0);
    glDisableVertexAttribArray(GLKVertexAttribNormal);

    glDisable(GL_BLEND);

And the envMode is set to GLKTextureEnvModeDecal as below:

effect.texture2d0.envMode = GLKTextureEnvModeDecal;

Probably there is a big elephant in the corridor i am not seeing; and any help or pointer would be welcomed to help to see the star with its transparent square.

Cheers, Stéphane

Answer 1

From the docs - GLKTextureEnvModeDecal uses the texture's alpha component to blend the texture's color with the input color.

I think you want GLKTextureEnvModeReplace.