feat(编辑转换): 增强PDF转换与编辑体验

ofdgo_pdf_shading.go

@@ -0,0 +1,129 @@
+// 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"
+	"slices"
+
+	"github.com/xiaoqidun/pdfgo"
+)
+
+// pdfGradientSegments 将线性源颜色及设备四色的截断点表达为OFD分段
+// 入参: stops 源颜色分段, space 源颜色空间
+// 返回: []ShdSegment 等价RGB分段, error 不可线性表达的变换
+func pdfGradientSegments(stops []pdfgo.GradientStop, space *pdfgo.ColorSpace) ([]ShdSegment, error) {
+	if len(stops) == 0 {
+		return nil, &pdfgo.UnsupportedError{Feature: "nonlinear gradient conversion"}
+	}
+	calibrated := space != nil && space.Calibrated() && !space.SRGBEquivalent()
+	cmyk := space != nil && space.Model == "DeviceCMYK" && !space.Calibrated()
+	segments := make([]ShdSegment, 0, len(stops))
+	appendStop := func(position float64, rgb [3]float64) {
+		segments = append(segments, ShdSegment{Position: position, Color: ShdColor{Value: pdfNumbers(math.Round(rgb[0]*255), math.Round(rgb[1]*255), math.Round(rgb[2]*255))}})
+	}
+	for i, b := range stops {
+		if i > 0 {
+			a := stops[i-1]
+			if a.Position != b.Position && a.Values != b.Values {
+				if calibrated {
+					return nil, &pdfgo.UnsupportedError{Feature: "nonlinear ICC gradient conversion"}
+				}
+				if cmyk {
+					var crossings []float64
+					for c := 0; c < 3; c++ {
+						start, end := a.Values[c]+a.Values[3], b.Values[c]+b.Values[3]
+						if start != end {
+							if t := (1 - start) / (end - start); t > 0 && t < 1 {
+								crossings = append(crossings, t)
+							}
+						}
+					}
+					slices.Sort(crossings)
+					for _, t := range slices.Compact(crossings) {
+						var values [4]float64
+						for c := range values {
+							values[c] = a.Values[c]*(1-t) + b.Values[c]*t
+						}
+						rgb, err := space.RGB(values[:], "RelativeColorimetric")
+						if err != nil {
+							return nil, err
+						}
+						appendStop(a.Position+(b.Position-a.Position)*t, rgb)
+					}
+				}
+			}
+		}
+		appendStop(b.Position, b.RGB)
+	}
+	return segments, nil
+}
+
+// pdfGradientError 检查渐变能否精确映射为OFD线性分段
+// 入参: paint 画刷
+// 返回: error 不可等价表达的渐变
+func pdfGradientError(paint pdfgo.Paint) error {
+	if paint.Axial != nil {
+		_, err := pdfGradientSegments(paint.Axial.Stops, paint.Axial.Space)
+		return err
+	}
+	if paint.Radial != nil {
+		_, err := pdfGradientSegments(paint.Radial.Stops, paint.Radial.Space)
+		return err
+	}
+	return nil
+}
+
+// gradientPath 按原始函数合成渐变路径，保留填充描边的单一对象语义
+// 入参: mark 路径绘制信息
+// 返回: error 几何或着色错误
+func (p *pdfImporter) gradientPath(mark pdfgo.PathMark) error {
+	return p.compositeRegion(nil, pdfCompositeNode{path: &mark}, &pdfgo.ColorSpace{Model: "DeviceRGB"}, false)
+}
+
+// pdfShadingPosition 计算渐变参数，未延伸区域及不存在非负圆半径的位置不着色
+// 入参: paint 渐变画刷, point 页面坐标
+// 返回: float64 归一化参数, bool 是否着色
+func pdfShadingPosition(paint pdfgo.Paint, point pdfgo.Point) (float64, bool) {
+	if g := paint.Axial; g != nil {
+		dx, dy := g.End.X-g.Start.X, g.End.Y-g.Start.Y
+		t := ((point.X-g.Start.X)*dx + (point.Y-g.Start.Y)*dy) / (dx*dx + dy*dy)
+		return t, finite(t) && (t >= 0 || g.Extend[0]) && (t <= 1 || g.Extend[1])
+	}
+	g := paint.Radial
+	dx, dy, dr := g.End.X-g.Start.X, g.End.Y-g.Start.Y, g.EndRadius-g.StartRadius
+	px, py := point.X-g.Start.X, point.Y-g.Start.Y
+	a, b, c := dx*dx+dy*dy-dr*dr, -2*(px*dx+py*dy+g.StartRadius*dr), px*px+py*py-g.StartRadius*g.StartRadius
+	roots := [2]float64{math.NaN(), math.NaN()}
+	if a == 0 {
+		if b != 0 {
+			roots[0] = -c / b
+		}
+	} else if discriminant := b*b - 4*a*c; discriminant >= 0 {
+		q := -.5 * (b + math.Copysign(math.Sqrt(discriminant), b))
+		if q == 0 {
+			roots[0] = -b / (2 * a)
+		} else {
+			roots[0], roots[1] = q/a, c/q
+		}
+	}
+	t := math.Inf(-1)
+	for _, root := range roots {
+		if finite(root) && g.StartRadius+root*dr >= 0 && (root >= 0 || g.Extend[0]) && (root <= 1 || g.Extend[1]) && root > t {
+			t = root
+		}
+	}
+	return t, finite(t)
+}