Cesium Model 中的剪裁平面 (ClippingPlane)

参考: https://www.cnblogs.com/webgl-angela/p/9197672.html

Cesium Model 中的剪裁平面 (ClippingPlane)

// 相关类:
class ClippingPlaneCollection {}
class ClippingPlane {}

// 剪裁的整体流程:
Model.prototype.update = () => {
  //...

  // 1.剪裁纹理的构建
  updateClippingPlanes()

  // 2.Shader 的构建
  buildDrawCommands()

  // 3.剪裁矩阵的构建
  updateReferenceMatrices();

  //...
}

1、剪裁纹理的构建

ClippingPlaneCollection.prototype.update = () => {
  //...

  // 1. 计算纹理的分辨率, { x: 剪裁平面的数量, y: 2 }
  computeTextureResolution()

  // 2. 创建纹理
  clippingPlanesTexture = new Texture()

  // 3. 将剪裁平面数据打包成浮点数的数组 _float32View
  packPlanesAsFloats()

  // 4.拷贝图片数据 _float32View 到纹理中
  clippingPlanesTexture.copyFrom()

  //...
}

2、Shader 的构建

ModelClippingPlanesPipelineStage.process = () => {
  const uniformMap = {
    model_clippingPlanes: function () {
      return clippingPlanes.texture;
    },
    model_clippingPlanesEdgeStyle: function () {
      const style = Color.clone(clippingPlanes.edgeColor);
      style.alpha = clippingPlanes.edgeWidth;
      return style;
    },
    model_clippingPlanesMatrix: function () {
      return model._clippingPlanesMatrix;
    },
  };
};

3、剪裁矩阵的构建

functin updateReferenceMatrices() {
  // ...

  // 模型空间 转换到 世界空间
  const referenceMatrix = model.modelMatrix;

  // 模型空间 转换到 裁剪平面空间
  const clippingPlanesModelMatrix = model._clippingPlanes.modelMatrix;

  // 世界空间 转换到 视图空间
  clippingPlanesMatrix = Matrix4.multiply(
    context.uniformState.view3D,
    referenceMatrix,
    clippingPlanesMatrix
  );

  // 视图空间 转换到 裁剪平面空间, 在视图空间中调整模型的裁剪平面
  // 由于裁剪平面的定义与模型空间无关，因此即使在视图空间中调整，裁剪平面的功能仍然相同：定义哪些模型部分应该被渲染、哪些部分应该被剔除。
  clippingPlanesMatrix = Matrix4.multiply(
    clippingPlanesMatrix,
    clippingPlanesModelMatrix,
    clippingPlanesMatrix
  );

 // 裁剪平面空间 转换到 视图空间
  model._clippingPlanesMatrix = Matrix4.inverseTranspose(
    clippingPlanesMatrix,
    model._clippingPlanesMatrix
  );

  // ...
}

4、Shader 赏析

/**
 * Transforms a plane.
 *
 * @name czm_transformPlane
 * @glslFunction
 *
 * @param {vec4} plane The plane in Hessian Normal Form.
 * @param {mat4} transform The inverse-transpose of a transformation matrix.
 */
vec4 czm_transformPlane(vec4 plane, mat4 transform) {
    vec4 transformedPlane = transform * plane;
    // Convert the transformed plane to Hessian Normal Form
    float normalMagnitude = length(transformedPlane.xyz);
    return transformedPlane / normalMagnitude;
}

vec4 getClippingPlane(
    highp sampler2D packedClippingPlanes,
    int clippingPlaneNumber,
    mat4 transform
) {
    int pixY = clippingPlaneNumber / CLIPPING_PLANES_TEXTURE_WIDTH;
    int pixX = clippingPlaneNumber - (pixY * CLIPPING_PLANES_TEXTURE_WIDTH);
    float pixelWidth = 1.0 / float(CLIPPING_PLANES_TEXTURE_WIDTH);
    float pixelHeight = 1.0 / float(CLIPPING_PLANES_TEXTURE_HEIGHT);
    float u = (float(pixX) + 0.5) * pixelWidth; // sample from center of pixel
    float v = (float(pixY) + 0.5) * pixelHeight;
    vec4 plane = texture(packedClippingPlanes, vec2(u, v));
    return czm_transformPlane(plane, transform);
}

float clip(vec4 fragCoord, sampler2D clippingPlanes, mat4 clippingPlanesMatrix) {
  // 视图空间的位置
    vec4 position = czm_windowToEyeCoordinates(fragCoord);
    vec3 clipNormal = vec3(0.0);

    // 视图空间的剪裁平面位置
    vec3 clipPosition = vec3(0.0);
    float pixelWidth = czm_metersPerPixel(position);

    float clipAmount = 0.0;
    bool clipped = true;

    for (int i = 0; i < CLIPPING_PLANES_LENGTH; ++i) {
        vec4 clippingPlane = getClippingPlane(clippingPlanes, i, clippingPlanesMatrix);
        clipNormal = clippingPlane.xyz;
        clipPosition = -clippingPlane.w * clipNormal;

        // 计算当前像素位置到裁剪平面的投影距离。这个距离是沿着裁剪平面法线的距离。如果这个距离小于等于0,则意味着像素位于裁剪平面的内部。
        float amount = dot(clipNormal, (position.xyz - clipPosition)) / pixelWidth;

        clipAmount = max(amount, clipAmount);
        clipped = clipped && (amount <= 0.0);
    }

    if (clipped) {
        discard;
    }
    return clipAmount;
}

void modelClippingPlanesStage(inout vec4 color)
{
    float clipDistance = clip(gl_FragCoord, model_clippingPlanes, model_clippingPlanesMatrix);
    vec4 clippingPlanesEdgeColor = vec4(1.0);
    clippingPlanesEdgeColor.rgb = model_clippingPlanesEdgeStyle.rgb;
    float clippingPlanesEdgeWidth = model_clippingPlanesEdgeStyle.a;

    if (clipDistance > 0.0 && clipDistance < clippingPlanesEdgeWidth) {
        color = clippingPlanesEdgeColor;
    }
}