c++ - 使用 Stencil-buffer 绘制 3D 模型的轮廓

Question

首先，我想提一下，我在这里发现了我认为是完全相同的问题，不幸的是没有答案: Java Using OpenGL Stencil to create Outline

我将在下面发布我的代码，但首先是问题所在:从这个捕获**中，您可以看到整个框架结构都在显示，而不是围绕球体的单条线。我想摆脱里面的所有那些线!

** 显然我无法添加图片:请参阅此链接 - 想象一个球体，其四边形的所有边缘都以 3 像素大线可见。
http://srbwks36224-03.engin.umich.edu/kdi/images/gs_sphere_with_frame.jpg


这是给出该结果的代码:

// First render the sphere:
// inside "show" is all the code to display a textured sphere
// looking like earth
sphe->show();

// Now get ready for stencil buffer drawing pass:
// 1. Clear and initialize it
// 2. Activate stencil buffer
// 3. On the first rendering pass, we want to "SUCCEED ALWAYS"
//    and write a "1" into the stencil buffer accordingly
// 4. We don't need to actually render the object, hence disabling RGB mask
glClearStencil(0);   //Edit: swapped this line and below
glClear(GL_STENCIL_BUFFER_BIT);                 
glEnable(GL_STENCIL_TEST);                  
glStencilFunc(GL_NEVER, 0x1, 0x1);          //Edit: GL_ALWAYS
glStencilOp(GL_REPLACE, GL_KEEP, GL_KEEP);  //Edit: GL_KEEP, GL_KEEP, GL_REPLACE                    
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
glDepthMask(GL_FALSE);   // As per Andon's comment
sphe->show();

// At this point, I expect to have "1" on the entire
// area covered by the sphere, so...
// 1. Stencil test should fail for anything, but 0 value
// RM: commented is another option that should work too I believe                       
// 2. The stencil op instruction at the point is somewhat irrelevant 
//    (if my understanding is correct), because we won't do anything 
//    else with the stencil buffer after that.
// 3. Re-enable RGB mask, because we want to draw this time  
// 4. Switch to LINE drawing instead of FILL and 
// 5. set a bigger line width, so it will exceed the model boundaries. 
//    We do want this, otherwise the line would not show 
// 6. Don't mind the "uniform" setting instruction, this is so
//    that my shader knows it should draw in plain color
// 7. Draw the sphere's frame
// 8. The principle, as I understand it is that all the lines should
//    find themselves matched to a "1" in the stencil buffer and therefore
//    be ignored for rendering. Only lines on the edges of the model should
//    have half their width not failing the stencil test.
glStencilFunc(GL_EQUAL, 0x0, 0x1);
//glStencilFunc(GL_NOTEQUAL, 0x1, 0x1);
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
glDepthMask(GL_TRUE);
glLineWidth(3);
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);  
psa::shader::setUniform("outlining", 1);
sphe->show();
psa::shader::setUniform("outlining", 0);

现在只是为了证明一点，我厌倦了使用模板缓冲区做一些不同的事情——我只是想确保我的代码中的所有内容都已到位，以使其正常工作。

** 再次不幸的是，我无法显示我得到的结果的屏幕截图:场景是这样的
http://mathworld.wolfram.com/images/eps-gif/SphereSphereInterGraphic_700.gif
但是较小的球体是不可见的(RGB mask 已停用)并且可以通过孔看到世界背景(而不是较大球体的内部 - 面部剔除已停用)。

这就是代码...有趣的是，我可以更改很多东西，例如激活/停用 STENCIL_TEST，将操作更改为 GL_KEEP 无处不在，甚至将第二个 stencilFunc 更改为“不等于 0”...结果始终是相同的!我想我在这里缺少一些基本的东西。

void testStencil()
{
    // 1. Write a 1 in the Stencil buffer for 
    // every pixels of the first sphere:
    // All colors disabled, we don't need to see that sphere
    glEnable(GL_STENCIL_TEST);
    glStencilFunc(GL_ALWAYS, 0x1, 0x1);
    glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
    glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
    glDepthMask(GL_FALSE);   // Edit: added this
    {
        sphe->W = mat4::trans(psa::vec4(1.0, 1.0, 1.0)) * mat4::scale(0.9);
        sphe->show();
    }

    // 2. Draw the second sphere with the following rule:
    // fail the stencil test for every pixels with a 1.
    // This means that  any pixel from first sphere will
    // not be draw as part of the second sphere.
    glStencilFunc(GL_EQUAL, 0x0, 0x1);
    glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
    glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
    glDepthMask(GL_TRUE);    // Edit: added this
    {
        sphe->W = mat4::trans(psa::vec4(1.2, 1.2, 1.2)) * mat4::scale(1.1);
        sphe->show();
    }
}

等等!如果有人能指出我正确的方向，我将不胜感激。我还会确保将您的答案引用我找到的另一篇文章。

Answer 1

此问题中发布的 OpenGL 代码有效。问题的原因在于窗口初始化/创建:

下面分别是有效代码的 SDL1.2 和 SDL2 版本。请注意，在这两种情况下，SetAttribute 语句都放置在窗口创建之前。主要问题是错放的语句不一定会在运行时失败，但也不会起作用。

SDL1.2:

if(SDL_Init(SDL_INIT_EVERYTHING) < 0) 
{
    throw "Video initialization failed";
}

SDL_GL_SetAttribute(SDL_GL_RED_SIZE,     5);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE,   5);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE,    5);
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE,  24);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES,16);  
SDL_GL_SetAttribute(SDL_GL_ACCELERATED_VISUAL, 1);

const SDL_VideoInfo * i;
if((i = SDL_GetVideoInfo()) == NULL)  
{
    throw "Video query failed";
}

int flag = (fs ? SDL_OPENGL | SDL_FULLSCREEN : SDL_OPENGL);
if(SDL_SetVideoMode(w, h, i->vfmt->BitsPerPixel, flag) == 0)
{
    throw "Video mode set failed";
}

glewExperimental = GL_TRUE;
if(glewInit() != GLEW_OK)
{
    throw "Could not initialize GLEW";
}

if(!glewIsSupported("GL_VERSION_3_3"))
{
    throw "OpenGL 3.3 not supported";
}

SDL2(本质上是相同的代码，只是窗口创建函数发生了变化):

if(SDL_Init(SDL_INIT_EVERYTHING) < 0) 
{
    throw "Video initialization failed";
}

SDL_GL_SetAttribute(SDL_GL_RED_SIZE,     5);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE,   5);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE,    5);
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE,  24);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES,16);  
SDL_GL_SetAttribute(SDL_GL_ACCELERATED_VISUAL, 1);

int flag = SDL_WINDOW_OPENGL;
if((win = SDL_CreateWindow("engine", 100, 100, w, h, flag)) == NULL)
{
    throw "Create SDL Window failed";
}

context = SDL_GL_CreateContext(win);

glewExperimental = GL_TRUE;
if(glewInit() != GLEW_OK)
{
    throw "Could not initialize GLEW";
}

if(!glewIsSupported("GL_VERSION_3_3"))
{
    throw "OpenGL 3.3 not supported";
}

有几点值得一提:
1.尽管我知道不建议这样做，但 SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 1) 也有效
2.在 SDL2 中，SDL_GL_SetAttribute SDL_GL_MULTISAMPLESAMPLES 对我来说上升到 16，在 glewInit() 失败之后，但是，在窗口创建之后移动多重采样设置，然后 glewInit 突然停止提示:我的猜测是它只是被忽略了。
3.在 SDL1.2 中，多重采样的任何值“似乎”都有效
4.考虑到模板缓冲区功能，只有下面的代码也可以工作，但我发布它主要是为了提出一个问题:有多少属性设置在实际工作？以及如何知道，因为代码编译和运行没有明显问题？

// PROBABLY WRONG:
// ----
SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8);

const SDL_VideoInfo * i;
if((i = SDL_GetVideoInfo()) == NULL)  
{
        throw "Video query failed";
}

int flag = (fs ? SDL_OPENGL | SDL_FULLSCREEN : SDL_OPENGL);
if(SDL_SetVideoMode(w, h, i->vfmt->BitsPerPixel, flag) == 0)
{
    throw "Video mode set failed";
}

// No idea if the below is actually applied!
SDL_GL_SetAttribute(SDL_GL_RED_SIZE,     5);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE,   5);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE,    5);
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE,  24);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 16); 
SDL_GL_SetAttribute(SDL_GL_ACCELERATED_VISUAL, 1);