feat(后端架构): 完善GG后端，修复渐变与选区异常

ofdgo_render_shading.go

@@ -0,0 +1,108 @@
+// Copyright 2025-2026 肖其顿 (XIAO QI DUN)
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ofdgo
+
+import "math"
+
+// reflectShdPosition 获取镜像周期内的位置
+// 入参: position 渐变位置
+// 返回: float64 周期内的位置
+func reflectShdPosition(position float64) float64 {
+	index := math.Floor(position)
+	position -= index
+	if math.Mod(index, 2) != 0 {
+		return 1 - position
+	}
+	return position
+}
+
+// shadingClip 按渐变延伸规则裁剪绘制与选区，避免只在采样时隐藏区间外像素
+// 入参: path 页面路径, paint 渐变画刷, matrix 渐变父矩阵
+// 返回: GeometryPath 可见路径, error 几何错误
+func (c *semanticCompiler) shadingClip(path GeometryPath, paint Paint, matrix Matrix) (GeometryPath, error) {
+	gradient, view := semanticGradient(paint, matrix)
+	spread := gradient.Spread
+	if spread == nil || spread.Extend == 3 {
+		return path, nil
+	}
+	inverse, ok := view.Invert()
+	if !ok {
+		return nil, nil
+	}
+	local, err := c.geometry.Transform(path, inverse)
+	if err != nil {
+		return nil, err
+	}
+	if gradient.Kind == RasterLinear {
+		dx, dy := gradient.End.X-gradient.Start.X, gradient.End.Y-gradient.Start.Y
+		axis := Matrix{a: dx, b: dy, c: -dy, d: dx, e: gradient.Start.X, f: gradient.Start.Y}
+		inv, _ := axis.Invert()
+		box, err := c.geometry.Bounds(local)
+		if err != nil {
+			return nil, err
+		}
+		box = inv.TransformBox(box)
+		end := box.X + box.W
+		if spread.Extend&1 == 0 {
+			box.X = math.Max(box.X, 0)
+		}
+		if spread.Extend&2 == 0 {
+			end = math.Min(end, 1)
+		}
+		box.W = end - box.X
+		if box.W <= 0 {
+			return nil, nil
+		}
+		clip, err := c.geometry.Transform(geometryRectangle(box), axis)
+		if err != nil {
+			return nil, err
+		}
+		local, err = c.geometry.Combine(local, clip, GeometryIntersect)
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		inner, outer, r0, r1, extend := gradient.Start, gradient.End, gradient.R0, gradient.R1, spread.Extend
+		if r1 < r0 {
+			inner, outer, r0, r1 = outer, inner, r1, r0
+			extend = (extend&1)<<1 | (extend&2)>>1
+		}
+		if extend&2 == 0 {
+			local, err = c.geometry.Combine(local, geometryCircle(outer, r1), GeometryIntersect)
+			if err != nil {
+				return nil, err
+			}
+		}
+		if extend&1 == 0 && r0 > 0 {
+			local, err = c.geometry.Combine(local, geometryCircle(inner, r0), GeometrySubtract)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+	return c.geometry.Transform(local, view)
+}
+
+// geometryCircle 构建标准圆弧，不提前折线化
+// 入参: center 圆心, radius 半径
+// 返回: GeometryPath 圆路径
+func geometryCircle(center RasterPoint, radius float64) GeometryPath {
+	return GeometryPath{
+		{Verb: GeometryMove, End: Point{X: center.X + radius, Y: center.Y}},
+		{Verb: GeometryArc, RadiusX: radius, RadiusY: radius, Sweep: true, End: Point{X: center.X - radius, Y: center.Y}},
+		{Verb: GeometryArc, RadiusX: radius, RadiusY: radius, Sweep: true, End: Point{X: center.X + radius, Y: center.Y}},
+		{Verb: GeometryClose},
+	}
+}