mapbox源码解析（mvp矩阵）
由于gl-matrix 矩阵的计算规则（因此js代码中看计算顺序是倒序，我们看的话从右看到左，先模型矩阵，再视图矩阵，再投影矩阵）
pvm *position（范围：0-EXTENT）
源码路径：src /geo/transform.js
1.坐标范围
原始单位切片的坐标范围为0-8192,主要看瓦片生产时候的规范吧
2.模型矩阵，转世界坐标
2.1计算转当前切片内的真正相对坐标（本地坐标）
mat4.scale(posMatrix, posMatrix, [scale / EXTENT, scale / EXTENT, 1]);
其中EXTENT=8192，
其中
scale = this.worldSize / this.zoomScale(canonical.z)
=this.tileSize * Math.pow(2, zoom) / Math.pow(2, Math.floor(zoom))
//一般计算结果为512-1024，主要是在 Math.floor（zoom）-zoom切换的时候，瓦片大小从512变化到1024
//注意此处zoom可以是小数；
2.2计算转当前切片内各点平移后的真正绝对坐标（世界坐标），得到模型矩阵ModelMatrix
mat4.translate(posMatrix, posMatrix, [unwrappedX * scale, canonical.y * scale, 0]);
//unwrappedX 表示切片行号
//canonical.y表示切片列号
这里计算得到的瓦片的xy坐标已经是世界坐标下的绝对坐标（世界宽度当然是动态改变的，this.worldSize = Math.pow(2,zoom(可能带小数)* this.tileSize）
3.视图转换，转到相机空间坐标下ViewMatrix
mat4.scale(m, m, [1, 1, mercatorZfromAltitude(1, this.center.lat) * this.worldSize, 1]); //因为上面xy已经转为世界坐标单位了，这里对z拉伸，也将其转为世界坐标单位，这里也可以归类为模型变化其实
mat4.translate(m, m, [-x, -y, 0]);//从左上角移动到屏幕中心点的世界坐标
mat4.rotateZ(m, m, this.angle);
mat4.rotateX(m, m, this._pitch);
mat4.translate(m, m, [0, 0, -this.cameraToCenterDistance]); //相机到世界中心点的距离，平移过去
mat4.scale(m, m, [1, -1, 1]);//y轴转方向
备注：
mapbox customerLayer里面的u_marirx是src/geo/transform.js 中的this.mercatorMatrix = mat4.scale([], m, [this.worldSize, this.worldSize, this.worldSize]);矩阵，
4.投影转换
mat4.perspective(m, this._fov, this.width / this.height, nearZ, farZ);
5.src/geo/transform.js相关矩阵计算源码备注
5.1 视图矩阵和投影矩阵相关计算（CustomerLayer比较特殊）
_calcMatrices() {
if (!this.height) return;
const halfFov = this._fov / 2;
const offset = this.centerOffset;
this.cameraToCenterDistance = 0.5 / Math.tan(halfFov) * this.height;
// Find the distance from the center point [width/2 + offset.x, height/2 + offset.y] to the
// center top point [width/2 + offset.x, 0] in Z units, using the law of sines.
// 1 Z unit is equivalent to 1 horizontal px at the center of the map
// (the distance between[width/2, height/2] and [width/2 + 1, height/2])
const groundAngle = Math.PI / 2 + this._pitch;
const fovAboveCenter = this._fov * (0.5 + offset.y / this.height);
const topHalfSurfaceDistance = Math.sin(fovAboveCenter) * this.cameraToCenterDistance / Math.sin(clamp(Math.PI - groundAngle - fovAboveCenter, 0.01, Math.PI - 0.01));
const point = this.point;
const x = point.x, y = point.y;
// Calculate z distance of the farthest fragment that should be rendered.
const furthestDistance = Math.cos(Math.PI / 2 - this._pitch) * topHalfSurfaceDistance + this.cameraToCenterDistance;
// Add a bit extra to avoid precision problems when a fragment's distance is exactly `furthestDistance`
const farZ = furthestDistance * 1.01;
// The larger the value of nearZ is
// - the more depth precision is available for features (good)
// - clipping starts appearing sooner when the camera is close to 3d features (bad)
//
// Smaller values worked well for mapbox-gl-js but deckgl was encountering precision issues
// when rendering it's layers using custom layers. This value was experimentally chosen and
// seems to solve z-fighting issues in deckgl while not clipping buildings too close to the camera.
const nearZ = this.height / 50;
// matrix for conversion from location to GL coordinates (-1 .. 1)
let m = new Float64Array(16);
mat4.perspective(m, this._fov, this.width / this.height, nearZ, farZ);
//Apply center of perspective offset
m[8] = -offset.x * 2 / this.width;
m[9] = offset.y * 2 / this.height;
mat4.scale(m, m, [1, -1, 1]);
mat4.translate(m, m, [0, 0, -this.cameraToCenterDistance]);
mat4.rotateX(m, m, this._pitch);
mat4.rotateZ(m, m, this.angle);
mat4.translate(m, m, [-x, -y, 0]);
// The mercatorMatrix can be used to transform points from mercator coordinates
// ([0, 0] nw, [1, 1] se) to GL coordinates.
this.mercatorMatrix = mat4.scale([], m, [this.worldSize, this.worldSize, this.worldSize]);
// scale vertically to meters per pixel (inverse of ground resolution):
mat4.scale(m, m, [1, 1, mercatorZfromAltitude(1, this.center.lat) * this.worldSize, 1]);
this.projMatrix = m;
this.invProjMatrix = mat4.invert([], this.projMatrix);
// Make a second projection matrix that is aligned to a pixel grid for rendering raster tiles.
// We're rounding the (floating point) x/y values to achieve to avoid rendering raster images to fractional
// coordinates. Additionally, we adjust by half a pixel in either direction in case that viewport dimension
// is an odd integer to preserve rendering to the pixel grid. We're rotating this shift based on the angle
// of the transformation so that 0°, 90°, 180°, and 270° rasters are crisp, and adjust the shift so that
// it is always <= 0.5 pixels.
const xShift = (this.width % 2) / 2, yShift = (this.height % 2) / 2,
angleCos = Math.cos(this.angle), angleSin = Math.sin(this.angle),
dx = x - Math.round(x) + angleCos * xShift + angleSin * yShift,
dy = y - Math.round(y) + angleCos * yShift + angleSin * xShift;
const alignedM = new Float64Array(m);
mat4.translate(alignedM, alignedM, [ dx > 0.5 ? dx - 1 : dx, dy > 0.5 ? dy - 1 : dy, 0 ]);
this.alignedProjMatrix = alignedM;
m = mat4.create();
mat4.scale(m, m, [this.width / 2, -this.height / 2, 1]);
mat4.translate(m, m, [1, -1, 0]);
this.labelPlaneMatrix = m;
m = mat4.create();
mat4.scale(m, m, [1, -1, 1]);
mat4.translate(m, m, [-1, -1, 0]);
mat4.scale(m, m, [2 / this.width, 2 / this.height, 1]);
this.glCoordMatrix = m;
// matrix for conversion from location to screen coordinates
this.pixelMatrix = mat4.multiply(new Float64Array(16), this.labelPlaneMatrix, this.projMatrix);
// inverse matrix for conversion from screen coordinaes to location
m = mat4.invert(new Float64Array(16), this.pixelMatrix);
if (!m) throw new Error("failed to invert matrix");
this.pixelMatrixInverse = m;
this._posMatrixCache = {};
this._alignedPosMatrixCache = {};
}
5.2 模型矩阵
calculatePosMatrix(unwrappedTileID: UnwrappedTileID, aligned: boolean = false): Float32Array {
const posMatrixKey = unwrappedTileID.key;
const cache = aligned ? this._alignedPosMatrixCache : this._posMatrixCache;
if (cache[posMatrixKey]) {
return cache[posMatrixKey];
}
const canonical = unwrappedTileID.canonical;
const scale = this.worldSize / this.zoomScale(canonical.z);
const unwrappedX = canonical.x + Math.pow(2, canonical.z) * unwrappedTileID.wrap;
const posMatrix = mat4.identity(new Float64Array(16));
mat4.translate(posMatrix, posMatrix, [unwrappedX * scale, canonical.y * scale, 0]);
mat4.scale(posMatrix, posMatrix, [scale / EXTENT, scale / EXTENT, 1]);
mat4.multiply(posMatrix, aligned ? this.alignedProjMatrix : this.projMatrix, posMatrix);
cache[posMatrixKey] = new Float32Array(posMatrix);
return cache[posMatrixKey];
}