暂存服务端渲染功能，材质渲染计划迁移至前端

internal/service/skin_renderer/polygon.go

@@ -0,0 +1,200 @@
+package skin_renderer
+
+import (
+	"image"
+	"image/color"
+)
+
+// Polygon 表示一个四边形面片
+type Polygon struct {
+	dots        [4]*Point
+	color       color.RGBA
+	isProjected bool
+	face        string  // 面的方向: "x", "y", "z"
+	faceDepth   float64 // 面的深度
+}
+
+// NewPolygon 创建一个新的多边形
+func NewPolygon(dots [4]*Point, c color.RGBA) *Polygon {
+	p := &Polygon{
+		dots:  dots,
+		color: c,
+	}
+
+	// 确定面的方向
+	x0, y0, z0 := dots[0].GetOriginCoord()
+	x1, y1, z1 := dots[1].GetOriginCoord()
+	x2, y2, z2 := dots[2].GetOriginCoord()
+
+	if x0 == x1 && x1 == x2 {
+		p.face = "x"
+		p.faceDepth = x0
+	} else if y0 == y1 && y1 == y2 {
+		p.face = "y"
+		p.faceDepth = y0
+	} else if z0 == z1 && z1 == z2 {
+		p.face = "z"
+		p.faceDepth = z0
+	}
+
+	return p
+}
+
+// Project 投影多边形的所有顶点
+func (p *Polygon) Project(cosAlpha, sinAlpha, cosOmega, sinOmega float64, minX, maxX, minY, maxY *float64) {
+	for _, dot := range p.dots {
+		if !dot.IsProjected() {
+			dot.Project(cosAlpha, sinAlpha, cosOmega, sinOmega, minX, maxX, minY, maxY)
+		}
+	}
+	p.isProjected = true
+}
+
+// PreProject 预投影多边形的所有顶点
+func (p *Polygon) PreProject(dx, dy, dz, cosAlpha, sinAlpha, cosOmega, sinOmega float64) {
+	for _, dot := range p.dots {
+		dot.PreProject(dx, dy, dz, cosAlpha, sinAlpha, cosOmega, sinOmega)
+	}
+}
+
+// IsProjected 返回是否已投影
+func (p *Polygon) IsProjected() bool {
+	return p.isProjected
+}
+
+// AddToImage 将多边形绘制到图像上
+func (p *Polygon) AddToImage(img *image.RGBA, minX, minY, ratio float64) {
+	// 检查透明度，完全透明则跳过
+	if p.color.A == 0 {
+		return
+	}
+
+	// 获取投影后的 2D 坐标
+	points := make([][2]float64, 4)
+	var coordX, coordY *float64
+
+	samePlanX := true
+	samePlanY := true
+
+	for i, dot := range p.dots {
+		x, y, _ := dot.GetDestCoord()
+		points[i] = [2]float64{
+			(x - minX) * ratio,
+			(y - minY) * ratio,
+		}
+
+		if coordX == nil {
+			coordX = &x
+			coordY = &y
+		} else {
+			if *coordX != x {
+				samePlanX = false
+			}
+			if *coordY != y {
+				samePlanY = false
+			}
+		}
+	}
+
+	// 如果所有点在同一平面（退化面），跳过
+	if samePlanX || samePlanY {
+		return
+	}
+
+	// 使用扫描线算法填充多边形
+	fillPolygon(img, points, p.color)
+}
+
+// fillPolygon 使用扫描线算法填充四边形
+func fillPolygon(img *image.RGBA, points [][2]float64, c color.RGBA) {
+	// 找到 Y 的范围
+	minY := points[0][1]
+	maxY := points[0][1]
+	for _, pt := range points {
+		if pt[1] < minY {
+			minY = pt[1]
+		}
+		if pt[1] > maxY {
+			maxY = pt[1]
+		}
+	}
+
+	bounds := img.Bounds()
+
+	// 扫描每一行
+	for y := int(minY); y <= int(maxY); y++ {
+		if y < bounds.Min.Y || y >= bounds.Max.Y {
+			continue
+		}
+
+		// 找到这一行与多边形边的交点
+		var intersections []float64
+		n := len(points)
+		for i := 0; i < n; i++ {
+			j := (i + 1) % n
+			y1, y2 := points[i][1], points[j][1]
+			x1, x2 := points[i][0], points[j][0]
+
+			// 检查这条边是否与当前扫描线相交
+			if (y1 <= float64(y) && y2 > float64(y)) || (y2 <= float64(y) && y1 > float64(y)) {
+				// 计算交点的 X 坐标
+				t := (float64(y) - y1) / (y2 - y1)
+				x := x1 + t*(x2-x1)
+				intersections = append(intersections, x)
+			}
+		}
+
+		// 排序交点
+		for i := 0; i < len(intersections)-1; i++ {
+			for j := i + 1; j < len(intersections); j++ {
+				if intersections[i] > intersections[j] {
+					intersections[i], intersections[j] = intersections[j], intersections[i]
+				}
+			}
+		}
+
+		// 填充交点之间的像素
+		for i := 0; i+1 < len(intersections); i += 2 {
+			xStart := int(intersections[i])
+			xEnd := int(intersections[i+1])
+
+			for x := xStart; x <= xEnd; x++ {
+				if x >= bounds.Min.X && x < bounds.Max.X {
+					// Alpha 混合
+					if c.A == 255 {
+						img.SetRGBA(x, y, c)
+					} else {
+						existing := img.RGBAAt(x, y)
+						blended := alphaBlend(existing, c)
+						img.SetRGBA(x, y, blended)
+					}
+				}
+			}
+		}
+	}
+}
+
+// alphaBlend 执行 Alpha 混合
+func alphaBlend(dst, src color.RGBA) color.RGBA {
+	if src.A == 0 {
+		return dst
+	}
+	if src.A == 255 {
+		return src
+	}
+
+	srcA := float64(src.A) / 255.0
+	dstA := float64(dst.A) / 255.0
+	outA := srcA + dstA*(1-srcA)
+
+	if outA == 0 {
+		return color.RGBA{}
+	}
+
+	return color.RGBA{
+		R: uint8((float64(src.R)*srcA + float64(dst.R)*dstA*(1-srcA)) / outA),
+		G: uint8((float64(src.G)*srcA + float64(dst.G)*dstA*(1-srcA)) / outA),
+		B: uint8((float64(src.B)*srcA + float64(dst.B)*dstA*(1-srcA)) / outA),
+		A: uint8(outA * 255),
+	}
+}