你必须传递2个纹理坐标到着色器和修改着色器

You have to pass 2 texture coordinates to shader and modify shader

添加到ObjectiveC

Add to ObjectiveC

glVertexAttribPointer(inputTextureCoordinate2, 2, GL_FLOAT, 0, 0, textureCoordinates2);

Vertex Shader

Vertex Shader

attribute vec4 position;
attribute vec4 inputTextureCoordinate;
attribute vec4 inputTextureCoordinate2;

varying vec2 textureCoordinate;
varying vec2 textureCoordinate2;

void main()
{
    gl_Position = position;
    textureCoordinate = inputTextureCoordinate.xy;
    textureCoordinate2 = inputTextureCoordinate2.xy;
}

Frag Shader

Frag Shader

varying highp vec2 textureCoordinate;
varying highp vec2 textureCoordinate2;

uniform sampler2D inputTextureTop;
uniform sampler2D inputTextureBot;

uniform highp float alphaTop;

void main()
{
    lowp vec4 pixelTop = texture2D(inputTextureTop, textureCoordinate);
    lowp vec4 pixelBot = texture2D(inputTextureBot, textureCoordinate2);

    gl_FragColor = someBlendOperation(pixelTop, pixelBot);
}

BTW inputTextureCoordinate不一定是vec4但它可能是vec2

BTW inputTextureCoordinate is not necessary to be vec4 but it could be vec2