OpenGL：如何修复矩形（线或线环）中缺失的角像素

Question

OpenGL：如何修复矩形（线或线环）中缺失的角像素

7

请看中间绿色矩形的左下角：

他们在左下角缺少一个像素。

我是这样画的：

class Rect: public StaticModel {
  public:
    Rect() {  
        constexpr glm::vec2 vertices[] {
                {-0.5,0.5},  // top left
                {0.5,0.5},  // top right
                {0.5,-0.5},  // bottom right
                {-0.5,-0.5},  // bottom left
        };
        _buf.bufferData<glm::vec2>(vertices,BufferUsage::StaticDraw);
        _idxBuf.bufferData<GLuint>({0,1,3,2,0,3,1,2},BufferUsage::StaticDraw);
    }

    void bind() const override {
        _buf.bindVertex(); 
        _idxBuf.bind();
    }

    void draw() const override {
        gl::drawElements(8,DrawMode::Lines);
    }

  private:
    VertexBuffer _buf{sizeof(glm::vec2)};
    ElementArrayBuffer _idxBuf{};
};

那段代码使用了我的一些辅助方法/类，但你应该能够理解它的作用。我尝试使用简单的GL_LINE_LOOP来绘制矩形，但是出现了同样的问题，所以现在我正在尝试使用GL_LINES，并按相同方向绘制所有线条：从上到下和从左到右，但仍然缺少一个像素。

这些坐标正在通过正交投影进行处理：

gl_Position = projection * model * vec4(inPos, 0.0, 1.0);

因此，着色器将那些0.5的坐标缩放到像素坐标，但我认为这不是一个舍入误差。

还有什么其他方法可以尝试使角落对齐吗？

- mpen

1

@Rabbid76 这里我不是在寻找粗线，那是另一个问题（事实上，在第一张截图中我正在处理的就是那个长对角矩形），只是这个角落像素让我很烦恼。 - mpen

@Rabbid76 看起来实际上是一样的：https://i.imgur.com/h7Wt56M.png - mpen

你可能需要一些保守光栅化，但我不确定。 - Rabbid76

1个回答

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- Yakov Galka · Accepted Answer

OpenGL 给了很多自由，可以决定如何对线条进行光栅化。它要求某些理想的属性，但这些并不能防止在混合x-major（“水平”）和y-major（“垂直”）线条时出现间隙。

First thing, the "spirit of the spec" is to rasterize half-open lines; i.e. include the first vertex and exclude the final one. For that reason you should ensure that each vertex appears exactly once as a source and once as destination:
```
  _idxBuf.bufferData<GLuint>({0,1,1,2,2,3,3,0},BufferUsage::StaticDraw);
```
This is contrary to your attempt of drawing "top to bottom and left to right".

GL_LINE_LOOP already does that though, and you say that it doesn't solve the problem. That is indeed not guaranteed to solve the problem because you mix x-major and y-major lines here, but you still should follow the rule in order for the next point to work.
Next, I bet, some of your vertices fall right between the pixels; i.e. the window coordinates fractional part is exactly zero. When rasterizing such primitives the differences between different implementations (or in our case between x-major and y-major lines on the same implementation) become prominent.

To solve that you can snap your vertices to the pixel grid:
```
  // xy - vertex in window coordinates, i.e. same as gl_FragCoord
  xy = floor(xy) + 0.5
```
You can do this either in C++, or in the vertex shader. In either case you'll need to apply the projection and viewport transformations, and then undo them so that OpenGL can re-apply them afterwards. It's ugly, I know.

然而，实现像素完美的线条的唯一可靠方法是渲染三角形以覆盖形状（可以是每条线单独渲染或整个矩形），并在片段着色器中从gl_FragCoord.xy计算覆盖率。