feat(文件拆分): 拆分字体图像基础能力

internal/base/image/tiff_fax.go

@@ -0,0 +1,185 @@
+// 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 image
+
+import (
+	"context"
+	"encoding/binary"
+	"fmt"
+	"image"
+	"image/color"
+	"math/bits"
+
+	"github.com/xiaoqidun/pdfgo"
+)
+
+// tiffFaxImage 组合独立解码的传真条带或瓦片，保留紧凑的一位像素存储
+type tiffFaxImage struct {
+	blocks []image.Image
+	bounds image.Rectangle
+	width  int
+	height int
+	across int
+}
+
+// decodeTIFFFax 按TIFF传真选项解码各图像块，复用PDFGo的T.4与T.6解码器
+// 入参: ctx 取消上下文，可为nil, data TIFF编码, layout 已校验的主图布局
+// 返回: image.Image 图像, error 字段、解码或取消错误
+func decodeTIFFFax(ctx context.Context, data []byte, layout TIFFLayout) (image.Image, error) {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+	if layout.Samples != 1 || layout.compression == 3 && layout.t4 & ^uint32(7) != 0 || layout.compression == 4 && layout.t6 & ^uint32(2) != 0 {
+		return nil, fmt.Errorf("invalid TIFF fax options")
+	}
+	var order binary.ByteOrder = binary.LittleEndian
+	if data[0] == 'M' {
+		order = binary.BigEndian
+	}
+	offset := int(layout.Offset)
+	count := int(order.Uint16(data[offset:]))
+	entries := make(map[uint16][]byte, count)
+	for i := range count {
+		entry := data[offset+2+i*12 : offset+14+i*12]
+		entries[order.Uint16(entry)] = entry
+	}
+	value := func(tag uint16, fallback uint32) (uint32, error) {
+		entry := entries[tag]
+		if entry == nil {
+			return fallback, nil
+		}
+		if order.Uint32(entry[4:]) != 1 {
+			return 0, fmt.Errorf("invalid TIFF scalar field %d", tag)
+		}
+		return tiffPlaneValue(data, order, entry, 0)
+	}
+	for _, tag := range []uint16{258, 317, 339} {
+		v, err := value(tag, 1)
+		if err != nil || v != 1 || entries[tag] != nil && order.Uint16(entries[tag][2:]) != 3 {
+			return nil, fmt.Errorf("invalid TIFF fax sample field %d", tag)
+		}
+	}
+	photo, err := value(262, 2)
+	if err != nil || photo > 1 && photo != 4 || entries[338] != nil {
+		return nil, fmt.Errorf("invalid TIFF fax color model")
+	}
+	width := uint32(layout.Width)
+	height, err := value(278, ^uint32(0))
+	if err != nil || height == 0 {
+		return nil, fmt.Errorf("invalid TIFF rows per strip")
+	}
+	height = min(height, uint32(layout.Height))
+	locations, lengths := entries[273], entries[279]
+	tiled := entries[322] != nil
+	if tiled {
+		width, err = value(322, 0)
+		if err != nil || width == 0 || width%16 != 0 {
+			return nil, fmt.Errorf("invalid TIFF tile width")
+		}
+		height, err = value(323, 0)
+		if err != nil || height == 0 || height%16 != 0 {
+			return nil, fmt.Errorf("invalid TIFF tile height")
+		}
+		locations, lengths = entries[324], entries[325]
+		if locations != nil && order.Uint16(locations[2:]) != 4 {
+			return nil, fmt.Errorf("invalid TIFF tile offsets")
+		}
+	}
+	if uint64(width) > uint64(int(^uint(0)>>1)) || uint64(height) > uint64(int(^uint(0)>>1)) || locations == nil || lengths == nil {
+		return nil, fmt.Errorf("invalid TIFF fax blocks")
+	}
+	across := (uint64(layout.Width)-1)/uint64(width) + 1
+	down := (uint64(layout.Height)-1)/uint64(height) + 1
+	blocks := across * down
+	if blocks != uint64(order.Uint32(locations[4:])) || blocks != uint64(order.Uint32(lengths[4:])) {
+		return nil, fmt.Errorf("invalid TIFF fax block count")
+	}
+	result := &tiffFaxImage{bounds: image.Rect(0, 0, layout.Width, layout.Height), width: int(width), height: int(height), across: int(across)}
+	for i := uint64(0); i < blocks; i++ {
+		if err := ctx.Err(); err != nil {
+			return nil, err
+		}
+		at, err := tiffPlaneValue(data, order, locations, uint32(i))
+		if err != nil {
+			return nil, err
+		}
+		size, err := tiffPlaneValue(data, order, lengths, uint32(i))
+		if err != nil || size == 0 || uint64(at)+uint64(size) > uint64(len(data)) {
+			return nil, fmt.Errorf("truncated TIFF fax block")
+		}
+		encoded := data[uint64(at) : uint64(at)+uint64(size)]
+		if layout.fillOrder == 2 {
+			reversed := make([]byte, len(encoded))
+			for start := 0; start < len(encoded); {
+				if err := ctx.Err(); err != nil {
+					return nil, err
+				}
+				end := start + min(32768, len(encoded)-start)
+				for j := start; j < end; j++ {
+					reversed[j] = bits.Reverse8(encoded[j])
+				}
+				start = end
+			}
+			encoded = reversed
+		}
+		rows := height
+		if !tiled {
+			rows = min(height, uint32(uint64(layout.Height)-i*uint64(height)))
+		}
+		k := pdfgo.Integer(0)
+		if layout.compression == 4 {
+			k = -1
+		} else if layout.compression == 3 && layout.t4&1 != 0 {
+			k = 1
+		}
+		params := pdfgo.Dictionary{
+			"K": k, "Columns": pdfgo.Integer(width), "Rows": pdfgo.Integer(rows),
+			"EndOfBlock": pdfgo.Boolean(false), "EndOfLine": pdfgo.Boolean(layout.compression == 3),
+			"EncodedByteAlign": pdfgo.Boolean(layout.compression == 2), "BlackIs1": pdfgo.Boolean(photo == 1 || photo == 4),
+		}
+		object := &pdfgo.Image{Width: int(width), Height: int(rows), BitsPerComponent: 1, ColorSpace: pdfgo.Name("DeviceGray"), Stream: &pdfgo.Stream{Dictionary: pdfgo.Dictionary{"Filter": pdfgo.Name("CCITTFaxDecode"), "DecodeParms": params}, Data: encoded}}
+		pixels, err := object.DecodeSamplesContext(ctx)
+		if err != nil {
+			return nil, fmt.Errorf("TIFF fax block %d: %w", i, err)
+		}
+		result.blocks = append(result.blocks, pixels)
+	}
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+	return result, nil
+}
+
+// Bounds 返回主图显示区域，不包含边缘瓦片的填充像素
+// 返回: image.Rectangle 图像区域
+func (s *tiffFaxImage) Bounds() image.Rectangle { return s.bounds }
+
+// ColorModel 返回二值图像使用的灰度颜色模型
+// 返回: color.Model 灰度模型
+func (s *tiffFaxImage) ColorModel() color.Model { return color.GrayModel }
+
+// At 读取所属条带或瓦片中的像素，区域外返回黑色
+// 入参: x 横向像素坐标, y 纵向像素坐标
+// 返回: color.Color 像素颜色
+func (s *tiffFaxImage) At(x, y int) color.Color {
+	if !image.Pt(x, y).In(s.bounds) {
+		return color.Gray{}
+	}
+	block := s.blocks[(y/s.height)*s.across+x/s.width]
+	return block.At(x%s.width, y%s.height)
+}