feat(编辑创建): 支持图片建页

ofdgo_image_metadata.go

@@ -0,0 +1,189 @@
+// 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 (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"hash/crc32"
+)
+
+// imagePageInfo 图片方向与像素密度，不改变原始编码
+type imagePageInfo struct {
+	orientation int
+	xdpi, ydpi  float64
+}
+
+// imagePageMetadata 读取PNG物理尺寸与JPEG的JFIF、EXIF信息，EXIF密度优先于JFIF
+func imagePageMetadata(data []byte, format string, dpi float64) (imagePageInfo, error) {
+	info := imagePageInfo{orientation: 1, xdpi: dpi, ydpi: dpi}
+	var exif []byte
+	if format == "png" {
+		for offset := 8; offset+12 <= len(data); {
+			n := uint64(binary.BigEndian.Uint32(data[offset:]))
+			if n > uint64(len(data)-offset-12) {
+				return info, fmt.Errorf("truncated PNG chunk")
+			}
+			end := offset + 8 + int(n)
+			kind, payload := string(data[offset+4:offset+8]), data[offset+8:end]
+			if kind == "pHYs" || kind == "eXIf" {
+				if crc32.ChecksumIEEE(data[offset+4:end]) != binary.BigEndian.Uint32(data[end:]) {
+					return info, fmt.Errorf("invalid PNG %s checksum", kind)
+				}
+			}
+			if kind == "pHYs" && len(payload) == 9 {
+				x, y := float64(binary.BigEndian.Uint32(payload)), float64(binary.BigEndian.Uint32(payload[4:]))
+				if x > 0 && y > 0 {
+					if payload[8] == 1 {
+						info.xdpi, info.ydpi = x*0.0254, y*0.0254
+					} else if payload[8] == 0 {
+						info.ydpi = dpi * y / x
+					}
+				}
+			} else if kind == "eXIf" {
+				exif = payload
+			}
+			offset = end + 4
+			if kind == "IEND" {
+				break
+			}
+		}
+	} else if format == "jpeg" {
+		for offset := 2; offset < len(data); {
+			if data[offset] != 0xff {
+				return info, fmt.Errorf("invalid JPEG marker")
+			}
+			for offset < len(data) && data[offset] == 0xff {
+				offset++
+			}
+			if offset >= len(data) {
+				break
+			}
+			marker := data[offset]
+			offset++
+			if marker == 0xda || marker == 0xd9 {
+				break
+			}
+			if marker == 0x01 || marker >= 0xd0 && marker <= 0xd7 {
+				continue
+			}
+			if offset+2 > len(data) {
+				return info, fmt.Errorf("truncated JPEG marker")
+			}
+			n := int(binary.BigEndian.Uint16(data[offset:]))
+			if n < 2 || n > len(data)-offset {
+				return info, fmt.Errorf("truncated JPEG segment")
+			}
+			payload := data[offset+2 : offset+n]
+			if marker == 0xe0 && len(payload) >= 14 && bytes.Equal(payload[:5], []byte("JFIF\x00")) {
+				x, y := float64(binary.BigEndian.Uint16(payload[8:])), float64(binary.BigEndian.Uint16(payload[10:]))
+				if x > 0 && y > 0 {
+					switch payload[7] {
+					case 0:
+						info.ydpi = dpi * y / x
+					case 1:
+						info.xdpi, info.ydpi = x, y
+					case 2:
+						info.xdpi, info.ydpi = x*2.54, y*2.54
+					}
+				}
+			} else if marker == 0xe1 && bytes.HasPrefix(payload, []byte("Exif\x00\x00")) {
+				exif = payload[6:]
+			}
+			offset += n
+		}
+	}
+	if len(exif) != 0 {
+		orientation, x, y, err := imageExifMetadata(exif)
+		if err != nil {
+			return info, err
+		}
+		info.orientation = orientation
+		// PNG的物理尺寸由pHYs定义，EXIF只用于方向。
+		if format == "jpeg" && x > 0 && y > 0 {
+			info.xdpi, info.ydpi = x, y
+		}
+	}
+	return info, nil
+}
+
+// imageExifMetadata 只读取主图IFD中的方向和密度，不跟随缩略图与其他目录
+func imageExifMetadata(data []byte) (int, float64, float64, error) {
+	invalid := fmt.Errorf("invalid EXIF image metadata")
+	if len(data) < 8 {
+		return 0, 0, 0, invalid
+	}
+	var order binary.ByteOrder
+	switch string(data[:2]) {
+	case "II":
+		order = binary.LittleEndian
+	case "MM":
+		order = binary.BigEndian
+	default:
+		return 0, 0, 0, invalid
+	}
+	if order.Uint16(data[2:]) != 42 {
+		return 0, 0, 0, invalid
+	}
+	offset := uint64(order.Uint32(data[4:]))
+	if offset < 8 || offset+2 > uint64(len(data)) {
+		return 0, 0, 0, invalid
+	}
+	count := uint64(order.Uint16(data[offset:]))
+	if count*12+offset+6 > uint64(len(data)) {
+		return 0, 0, 0, invalid
+	}
+	orientation, unit := 1, 2
+	var x, y float64
+	for i := uint64(0); i < count; i++ {
+		entry := data[offset+2+i*12 : offset+14+i*12]
+		tag, kind, count := order.Uint16(entry), order.Uint16(entry[2:]), order.Uint32(entry[4:])
+		switch tag {
+		case 0x112, 0x128:
+			if kind != 3 || count != 1 {
+				return 0, 0, 0, invalid
+			}
+			value := int(order.Uint16(entry[8:]))
+			if tag == 0x112 {
+				if value < 1 || value > 8 {
+					return 0, 0, 0, invalid
+				}
+				orientation = value
+			} else {
+				unit = value
+			}
+		case 0x11a, 0x11b:
+			at := uint64(order.Uint32(entry[8:]))
+			if kind != 5 || count != 1 || at+8 > uint64(len(data)) {
+				return 0, 0, 0, invalid
+			}
+			n, d := order.Uint32(data[at:]), order.Uint32(data[at+4:])
+			if n > 0 && d > 0 {
+				if tag == 0x11a {
+					x = float64(n) / float64(d)
+				} else {
+					y = float64(n) / float64(d)
+				}
+			}
+		}
+	}
+	if unit == 3 {
+		x, y = x*2.54, y*2.54
+	} else if unit != 2 {
+		x, y = 0, 0
+	}
+	return orientation, x, y, nil
+}