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    GLuint positionFramebuffer, positionRenderbuffer, positionRenderTexture;
    glEnable(GL_TEXTURE_2D);
        glDisable(GL_DEPTH_TEST);

        // Offscreen position framebuffer object
        glGenFramebuffers(1, &positionFramebuffer);
    glBindFramebuffer(GL_FRAMEBUFFER, positionFramebuffer);

    glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8_OES, 2, 2);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, positionRenderbuffer);


        // Offscreen position framebuffer texture target
        glGenTextures(1, &positionRenderTexture);
    glBindTexture(GL_TEXTURE_2D, positionRenderTexture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
        glHint(GL_GENERATE_MIPMAP_HINT, GL_NICEST);

    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 2, 2, 0, GL_RGBA, GL_FLOAT, X);

        glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, positionRenderTexture, 0);

    // and read back
        // Update uniform values
    static const GLfloat squareVertices[] = {
        -1.0f, -1.0f,
        1.0f, -1.0f,
        -1.0f,  1.0f,
        1.0f,  1.0f,
    };

        static const GLfloat textureVertices[] = {
        1.0f, 1.0f,
        1.0f, 0.0f,
        0.0f,  1.0f,
        0.0f,  0.0f,
    };

    GLfloat thresholdColor[3];
    thresholdColor[0] = 0.89f;
    thresholdColor[1] = 0.78f;
    thresholdColor[2] = 0.0f;
    GLfloat thresholdSensitivity = 0.7f;

        glUniform1i(uniforms[UNIFORM_VIDEOFRAME], 0);
        glUniform4f(uniforms[UNIFORM_INPUTCOLOR], thresholdColor[0], thresholdColor[1], thresholdColor[2], 1.0f);
        glUniform1f(uniforms[UNIFORM_THRESHOLD], thresholdSensitivity);

        // Update attribute values.
        glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, 0, 0, squareVertices);
        glEnableVertexAttribArray(ATTRIB_VERTEX);
        glVertexAttribPointer(ATTRIB_TEXTUREPOSITON, 2, GL_FLOAT, 0, 0, textureVertices);
        glEnableVertexAttribArray(ATTRIB_TEXTUREPOSITON);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);


    glReadPixels(0, 0, rows, cols,GL_RGBA,GL_FLOAT,output);

    for (int i=0; i<rows*cols; i++) {
        NSLog(@"O[%d]=%f", i, output[i]);
    }