Cesium近地天空盒，解决图片旋转问题

前言

当我们使用官网的例子设置天空盒后，会发现天空云彩是斜的，比如下边这张图：

通过查阅网上资料，需要修改cesium的源码，以下是修改后skybox的源码，在自己的项目中引入即可：

实现代码

const BoxGeometry = Cesium.BoxGeometry;
const Cartesian3 = Cesium.Cartesian3;
const defaultValue = Cesium.defaultValue;
const defined = Cesium.defined;
const destroyObject = Cesium.destroyObject;
const DeveloperError = Cesium.DeveloperError;
const GeometryPipeline = Cesium.GeometryPipeline;
const Matrix3 = Cesium.Matrix3;
const Matrix4 = Cesium.Matrix4;
const Transforms = Cesium.Transforms;
const VertexFormat = Cesium.VertexFormat;
const BufferUsage = Cesium.BufferUsage;
const CubeMap = Cesium.CubeMap;
const DrawCommand = Cesium.DrawCommand;
const loadCubeMap = Cesium.loadCubeMap;
const RenderState = Cesium.RenderState;
const VertexArray = Cesium.VertexArray;
const BlendingState = Cesium.BlendingState;
const SceneMode = Cesium.SceneMode;
const ShaderProgram = Cesium.ShaderProgram;
const ShaderSource = Cesium.ShaderSource;
//片元着色器，直接从源码复制
const SkyBoxFS = "uniform samplerCube u_cubeMap;\n\
varying vec3 v_texCoord;\n\
void main()\n\
{\n\
vec4 color = textureCube(u_cubeMap, normalize(v_texCoord));\n\
gl_FragColor = vec4(czm_gammaCorrect(color).rgb, czm_morphTime);\n\
}\n\
";
//顶点着色器有修改，主要是乘了一个旋转矩阵
const SkyBoxVS = "attribute vec3 position;\n\
varying vec3 v_texCoord;\n\
uniform mat3 u_rotateMatrix;\n\
void main()\n\
{\n\
vec3 p = czm_viewRotation * u_rotateMatrix * (czm_temeToPseudoFixed * (czm_entireFrustum.y * position));\n\
gl_Position = czm_projection * vec4(p, 1.0);\n\
v_texCoord = position.xyz;\n\
}\n\
";
/**

/**
 * A sky box around the scene to draw stars.  The sky box is defined using the True Equator Mean Equinox (TEME) axes.
 * <p>
 * This is only supported in 3D.  The sky box is faded out when morphing to 2D or Columbus view.  The size of
 * the sky box must not exceed {@link Scene#maximumCubeMapSize}.
 * </p>
 *
 * @alias SkyBox
 * @constructor
 *
 * @param {Object} options Object with the following properties:
 * @param {Object} [options.sources] The source URL or <code>Image</code> object for each of the six cube map faces.  See the example below.
 * @param {Boolean} [options.show=true] Determines if this primitive will be shown.
 *
 *
 * @example
 * scene.skyBox = new Cesium.SkyBox({
 *   sources : {
 *     positiveX : 'skybox_px.png',
 *     negativeX : 'skybox_nx.png',
 *     positiveY : 'skybox_py.png',
 *     negativeY : 'skybox_ny.png',
 *     positiveZ : 'skybox_pz.png',
 *     negativeZ : 'skybox_nz.png'
 *   }
 * });
 *
 * @see Scene#skyBox
 * @see Transforms.computeTemeToPseudoFixedMatrix
 */
function SkyBox(options) {
  /**
   * The sources used to create the cube map faces: an object
   * with <code>positiveX</code>, <code>negativeX</code>, <code>positiveY</code>,
   * <code>negativeY</code>, <code>positiveZ</code>, and <code>negativeZ</code> properties.
   * These can be either URLs or <code>Image</code> objects.
   *
   * @type Object
   * @default undefined
   */
  this.sources = options.sources;
  this._sources = undefined;
  /**
      * 是否开启近地模式
      */
  this.nearGround = options.nearGround;
  /**
   * Determines if the sky box will be shown.
   *
   * @type {Boolean}
   * @default true
   */
  this.show = defaultValue(options.show, true);

  this._command = new DrawCommand({
    modelMatrix: Matrix4.clone(Matrix4.IDENTITY),
    owner: this,
  });
  this._cubeMap = undefined;

  this._attributeLocations = undefined;
  this._useHdr = undefined;
}

/**
 * Called when {@link Viewer} or {@link CesiumWidget} render the scene to
 * get the draw commands needed to render this primitive.
 * <p>
 * Do not call this function directly.  This is documented just to
 * list the exceptions that may be propagated when the scene is rendered:
 * </p>
 *
 * @exception {DeveloperError} this.sources is required and must have positiveX, negativeX, positiveY, negativeY, positiveZ, and negativeZ properties.
 * @exception {DeveloperError} this.sources properties must all be the same type.
 */
SkyBox.prototype.update = function (frameState, useHdr) {
  const that = this;

  if (!this.show) {
    return undefined;
  }

  if (
    frameState.mode !== SceneMode.SCENE3D &&
    frameState.mode !== SceneMode.MORPHING
  ) {
    return undefined;
  }

  // The sky box is only rendered during the render pass; it is not pickable, it doesn't cast shadows, etc.
  if (!frameState.passes.render) {
    return undefined;
  }

  const context = frameState.context;

  if (this._sources !== this.sources) {
    this._sources = this.sources;
    const sources = this.sources;

    //>>includeStart('debug', pragmas.debug);
    if (
      !defined(sources.positiveX) ||
      !defined(sources.negativeX) ||
      !defined(sources.positiveY) ||
      !defined(sources.negativeY) ||
      !defined(sources.positiveZ) ||
      !defined(sources.negativeZ)
    ) {
      throw new DeveloperError(
        "this.sources is required and must have positiveX, negativeX, positiveY, negativeY, positiveZ, and negativeZ properties."
      );
    }

    if (
      typeof sources.positiveX !== typeof sources.negativeX ||
      typeof sources.positiveX !== typeof sources.positiveY ||
      typeof sources.positiveX !== typeof sources.negativeY ||
      typeof sources.positiveX !== typeof sources.positiveZ ||
      typeof sources.positiveX !== typeof sources.negativeZ
    ) {
      throw new DeveloperError(
        "this.sources properties must all be the same type."
      );
    }
    //>>includeEnd('debug');

    if (typeof sources.positiveX === "string") {
      // Given urls for cube-map images.  Load them.
      loadCubeMap(context, this._sources).then(function (cubeMap) {
        that._cubeMap = that._cubeMap && that._cubeMap.destroy();
        that._cubeMap = cubeMap;
      });
    } else {
      this._cubeMap = this._cubeMap && this._cubeMap.destroy();
      this._cubeMap = new CubeMap({
        context: context,
        source: sources,
      });
    }
  }

  const command = this._command;

  if (!defined(command.vertexArray)) {
    command.uniformMap = {
      u_cubeMap: function () {
        return that._cubeMap;
      },
      u_rotateMatrix: function () {
        return that.nearGround ? (command.modelMatrix = Transforms.eastNorthUpToFixedFrame(frameState.camera._positionWC),
          Matrix4.getMatrix3(command.modelMatrix, new Matrix3)) : Matrix3.IDENTITY;
      }
    };

    const geometry = BoxGeometry.createGeometry(
      BoxGeometry.fromDimensions({
        dimensions: new Cartesian3(2.0, 2.0, 2.0),
        vertexFormat: VertexFormat.POSITION_ONLY,
      })
    );
    const attributeLocations = (this._attributeLocations = GeometryPipeline.createAttributeLocations(
      geometry
    ));

    command.vertexArray = VertexArray.fromGeometry({
      context: context,
      geometry: geometry,
      attributeLocations: attributeLocations,
      bufferUsage: BufferUsage.STATIC_DRAW,
    });

    command.renderState = RenderState.fromCache({
      blending: BlendingState.ALPHA_BLEND,
    });
  }

  if (!defined(command.shaderProgram) || this._useHdr !== useHdr) {
    const fs = new ShaderSource({
      defines: [useHdr ? "HDR" : ""],
      sources: [SkyBoxFS],
    });
    command.shaderProgram = ShaderProgram.fromCache({
      context: context,
      vertexShaderSource: SkyBoxVS,
      fragmentShaderSource: fs,
      attributeLocations: this._attributeLocations,
    });
    this._useHdr = useHdr;
  }

  if (!defined(this._cubeMap)) {
    return undefined;
  }

  return command;
};

/**
 * Returns true if this object was destroyed; otherwise, false.
 * <br /><br />
 * If this object was destroyed, it should not be used; calling any function other than
 * <code>isDestroyed</code> will result in a {@link DeveloperError} exception.
 *
 * @returns {Boolean} <code>true</code> if this object was destroyed; otherwise, <code>false</code>.
 *
 * @see SkyBox#destroy
 */
SkyBox.prototype.isDestroyed = function () {
  return false;
};

/**
 * Destroys the WebGL resources held by this object.  Destroying an object allows for deterministic
 * release of WebGL resources, instead of relying on the garbage collector to destroy this object.
 * <br /><br />
 * Once an object is destroyed, it should not be used; calling any function other than
 * <code>isDestroyed</code> will result in a {@link DeveloperError} exception.  Therefore,
 * assign the return value (<code>undefined</code>) to the object as done in the example.
 *
 * @exception {DeveloperError} This object was destroyed, i.e., destroy() was called.
 *
 *
 * @example
 * skyBox = skyBox && skyBox.destroy();
 *
 * @see SkyBox#isDestroyed
 */
SkyBox.prototype.destroy = function () {
  const command = this._command;
  command.vertexArray = command.vertexArray && command.vertexArray.destroy();
  command.shaderProgram =
    command.shaderProgram && command.shaderProgram.destroy();
  this._cubeMap = this._cubeMap && this._cubeMap.destroy();
  return destroyObject(this);
};
export default SkyBox;

修改代码的关键点：

1、添加近地属性，方便扩展

//......
function SkyBox(options) {
  this.sources = options.sources;
  this._sources = undefined;
  /**
      * 是否开启近地模式
      */
  this.nearGround = options.nearGround;
//......
}

2、修改顶点着色器

//顶点着色器有修改，主要是乘了一个旋转矩阵
const SkyBoxVS = "attribute vec3 position;\n\
varying vec3 v_texCoord;\n\
uniform mat3 u_rotateMatrix;\n\
void main()\n\
{\n\
vec3 p = czm_viewRotation * u_rotateMatrix * (czm_temeToPseudoFixed * (czm_entireFrustum.y * position));\n\
gl_Position = czm_projection * vec4(p, 1.0);\n\
v_texCoord = position.xyz;\n\
}\n\
";
/**

3、修改command.uniformMap

command.uniformMap = {
      u_cubeMap: function () {
        return that._cubeMap;
      },
      u_rotateMatrix: function () {
        return that.nearGround ? (command.modelMatrix = Transforms.eastNorthUpToFixedFrame(frameState.camera._positionWC),
          Matrix4.getMatrix3(command.modelMatrix, new Matrix3)) : Matrix3.IDENTITY;
      }
    };

使用

import { default as SkyBox } from "@/lib/cesium/sky-SkyBox"
// 定义
const blueSkySkybox = new SkyBox({
  sources: {
    positiveX: fileUtil.getAssetsFile("skybox/blueSky/Right.jpg"),
    negativeX: fileUtil.getAssetsFile("skybox/blueSky/Left.jpg"),
    positiveY: fileUtil.getAssetsFile("skybox/blueSky/Front.jpg"),
    negativeY: fileUtil.getAssetsFile("skybox/blueSky/Back.jpg"),
    positiveZ: fileUtil.getAssetsFile("skybox/blueSky/Up.jpg"),
    negativeZ: fileUtil.getAssetsFile("skybox/blueSky/Down.jpg")
  },
  nearGround: true
})
// 使用
viewer.scene.postRender.addEventListener(function () {
    const e = viewer.camera.position;
    if (Cesium.Cartographic.fromCartesian(e).height < 2000) {
      viewer.scene.skyBox.nearGround = true;
      viewer.scene.skyBox = blueSkySkybox;
      viewer.scene.skyAtmosphere.show = false;
    } else {
      viewer.scene.skyBox = defaultSkyBox;
      viewer.scene.skyAtmosphere.show = true;
    }
  });