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我想使用可编程管道（使用 glsl）而不是固定管道进行体积渲染。我用 2 个着色器程序和 2 个通道来实现它，exitPointProg并且rayCastProg. 第一遍是获取立方体边界框的出口点（即背面），它将在下一次进行光线投射的过程中用作纹理。进行光线投射体积渲染的想法来自这里。我认为我已经做了大部分正确的事情，因为我已经使用固定管道和可编程管道实现了这一点。让我感到困惑的是，我在第一帧就达到了效果，并且随着闪光灯的闪烁，屏幕上的显示变成了全白（我设置了glClearColor(1.0f, 1.0f, 1.0f, 1.0f). 我认为着色器程序或 FBO 的切换可能存在一些问题。这是我在 render() 函数中所做的。

  1. 第一遍，使用着色器程序渲染到绑定了纹理的 fboexitPoints以获得立方体边界框的出口点exitPointProg。

  2. 2ed pass，将exitPoints纹理（绑定到）作为统一的 sampler2D 变量映射GL_TEXTURE1到着色器程序中，该变量将在着色器中使用。rayCastProg这里是setupFBOandrender和两个子例程：setupFBO()函数：

    void SimpleRayCasting::setupFBO()
{
    GLuint textureHandles[1];
    glGenTextures(1, textureHandles);
    entryPoints = textureHandles[0];


    glActiveTexture(GL_TEXTURE1);
    glBindTexture(GL_TEXTURE_2D, exitPoints);
    glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER);
    glTexImage2D(GL_TEXTURE_2D, 0,GL_RGBA16F, width, height, 0, GL_RGBA, GL_FLOAT, NULL);


    GLuint depthRenderBuffer;
    glGenRenderbuffers(1, &depthRenderBuffer);
    glBindRenderbuffer(GL_RENDERBUFFER, depthRenderBuffer);
    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, width, height);

    glGenFramebuffers(1, &frameBuffer);
    glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer);
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, 
                    exitPoints, 0/* level */);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, 
                    depthRenderBuffer);


    checkFramebufferState(__FILE__, __LINE__);
    GLenum drawbufs[] = {GL_COLOR_ATTACHMENT0};
    glDrawBuffers(1, drawbufs);
    glBindFramebuffer(GL_FRAMEBUFFER,0);
}

    render()功能：

    void SimpleRayCasting::render()
{
    getExitPoints();
    GLUtils::checkForOpenGLError(__FILE__,__LINE__);
    rayCasting();
    GLUtils::checkForOpenGLError(__FILE__,__LINE__);
}

    getExitPoints()功能：

    void SimpleRayCasting::getExitPoints() 
{
    // render to the texture
    glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer);
    glEnable(GL_DEPTH_TEST);
    glViewport(0, 0, width, height);
    // checkFramebufferState(__FILE__, __LINE__); 
    glClearColor(0.2f, 0.2f, 0.2f, 1.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    exitPointsProg.use();
    // note that the model will recalculated.
    model = mat4(1.0f);
    model *= glm::rotate(angle , vec3(0.0f,1.0f,0.0f));
    model *= glm::rotate(90.0f, vec3(1.0f, 0.0f, 0.0f));
    model *= glm::translate(vec3(-0.5f, -0.5f, -0.5f));
    view = glm::lookAt(vec3(0.0f,0.0f,2.0f), vec3(0.0f,0.0f,0.0f), vec3(0.0f,1.0f,0.0f));
    projection = mat4(1.0f);
    projection = glm::perspective(60.0f, (float)width/(float)height,0.01f, 400.0f);   
    setMatrices(exitPointsProg);
    glEnable(GL_CULL_FACE);
    glCullFace(GL_FRONT);
    drawBoundBox();
    glDisable(GL_CULL_FACE);
    glBindFramebuffer(GL_FRAMEBUFFER, 0);
}

    rayCasting功能：

    void SimpleRayCasting::rayCasting()
{
    // Directly render to the screen
    glBindFramebuffer(GL_FRAMEBUFFER, 0);
    glEnable(GL_DEPTH_TEST);
    glViewport(0, 0, width, height);
    glClearColor(1.0f, 1.0f, 1.0f, 1.0);
    glClear(GL_COLOR_BUFFER_BIT |  GL_DEPTH_BUFFER_BIT);
    rayCastProg.use();
    model = mat4(1.0f);
    model *= glm::rotate(angle, vec3(0.0f,1.0f,0.0f));
    model *= glm::rotate(90.0f, vec3(1.0f, 0.0f, 0.0f));
    model *= glm::translate(vec3(-0.5f, -0.5f, -0.5f));
    view = glm::lookAt(vec3(0.0f,0.0f,2.0f), vec3(0.0f,0.0f,0.0f), vec3(0.0f,1.0f,0.0f));
    projection = mat4(1.0f);
    projection = glm::perspective(60.0f, (float)width/(float)height,0.01f, 400.0f);   
    setMatrices(rayCastProg);
    rayCastProg.setUniform("StepSize", stepSize);
    glActiveTexture(GL_TEXTURE1);
    glBindTexture(GL_TEXTURE_2D, exitPoints);
    rayCastProg.setUniform("ExitPoints", 1);
    glActiveTexture(GL_TEXTURE4);
    glBindTexture(GL_TEXTURE_3D, volume_to);
    rayCastProg.setUniform("VolumeTex", 4);
    glActiveTexture(GL_TEXTURE5);
    glBindTexture(GL_TEXTURE_1D, transferFunc_to);
    rayCastProg.setUniform("TransferFunc", 5);
    glEnable(GL_CULL_FACE);
    glCullFace(GL_BACK);
    drawBoundBox();
    glDisable(GL_CULL_FACE);
    glBindFramebuffer(GL_FRAMEBUFFER, 0);
}

    我在 Qt 中使用从 QGLWidget 继承的类。正如我之前提到的，我只用第一帧就得到了正确的结果，然后它很快就消失了，变成了全白。不知道这是怎么回事，是shader程序的切换还是FBO的东西有问题？我一直在研究这个很长时间，但无法弄清楚，任何帮助将不胜感激！

编辑询问 是否有任何演示或教程演示如何将多个 glsl 着色器程序与 FBO 一起使用？

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