feat(文件拆分): 拆分字体图像基础能力

internal/base/image/bmp.go

@@ -0,0 +1,125 @@
+// 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 (
+	"bufio"
+	"encoding/binary"
+	"fmt"
+	"image"
+	"io"
+	"math/bits"
+)
+
+// bmpColorMask 保存一个BMP颜色分量的位域位置和取值范围
+type bmpColorMask struct {
+	mask  uint32
+	shift uint
+	Max   uint32
+}
+
+// DecodeConfig 读取图片尺寸，BMP和TIFF只读取布局信息，不解码像素
+// 入参: input 图片输入流
+// 返回: image.Config 图片配置, string 图片格式, error 头部错误
+func DecodeConfig(input io.Reader) (image.Config, string, error) {
+	buffer := bufio.NewReader(input)
+	prefix, _ := buffer.Peek(34)
+	if IsTIFF(prefix) {
+		layout, err := readTIFFLayout(buffer)
+		if err != nil {
+			return image.Config{}, "tiff", err
+		}
+		size := layout.DisplaySize()
+		return image.Config{Width: size.X, Height: size.Y}, "tiff", nil
+	}
+	if len(prefix) == 34 && string(prefix[:2]) == "BM" {
+		header := binary.LittleEndian.Uint32(prefix[14:18])
+		bpp := binary.LittleEndian.Uint16(prefix[28:30])
+		compression := binary.LittleEndian.Uint32(prefix[30:34])
+		if (bpp == 16 || bpp == 32 && compression == 3) && (header == 40 || header == 52 || header == 56 || header == 108 || header == 124) {
+			size := int(header) + 14
+			if header == 40 && compression == 3 {
+				size += 12
+			}
+			data := make([]byte, size)
+			if _, err := io.ReadFull(buffer, data); err != nil {
+				return image.Config{}, "bmp", err
+			}
+			config, err := decodeBMPConfig(data)
+			return config, "bmp", err
+		}
+	}
+	return image.DecodeConfig(buffer)
+}
+
+// sample 将位域采样值映射到八位通道，缺省透明度为不透明
+// 入参: value 编码像素
+// 返回: uint8 通道值
+func (m bmpColorMask) sample(value uint32) uint8 {
+	if m.mask == 0 {
+		return 255
+	}
+	return uint8(uint64((value&m.mask)>>m.shift) * 255 / uint64(m.Max))
+}
+
+// BMPColorMasks 读取BMP真彩色通道布局，校验位域连续性、重叠及像素范围
+// 入参: data 已校验头部的BMP数据, offset 像素偏移, bpp 像素位数, compression 编码方式
+// 返回: [4]bmpColorMask 红绿蓝和透明度通道, error 格式错误
+func BMPColorMasks(data []byte, offset int, bpp uint16, compression uint32) ([4]bmpColorMask, error) {
+	var result [4]bmpColorMask
+	if bpp != 16 && bpp != 24 && bpp != 32 {
+		return result, fmt.Errorf("unsupported bmp depth")
+	}
+	masks := [4]uint32{0xff0000, 0xff00, 0xff}
+	if bpp == 16 {
+		masks = [4]uint32{0x7c00, 0x3e0, 0x1f}
+	}
+	switch compression {
+	case 0:
+	case 3:
+		if bpp == 24 {
+			return result, fmt.Errorf("invalid bmp bitfield depth")
+		}
+		header := binary.LittleEndian.Uint32(data[14:18])
+		if offset < 66 || len(data) < 66 || header != 40 && header < 52 {
+			return result, fmt.Errorf("truncated bmp color masks")
+		}
+		for i := range 3 {
+			masks[i] = binary.LittleEndian.Uint32(data[54+i*4:])
+		}
+		if header >= 56 {
+			masks[3] = binary.LittleEndian.Uint32(data[66:70])
+		}
+	default:
+		return result, fmt.Errorf("unsupported bmp compression")
+	}
+	var used uint32
+	for i, mask := range masks {
+		if mask == 0 {
+			if i < 3 {
+				return result, fmt.Errorf("empty bmp color mask")
+			}
+			continue
+		}
+		shift := uint(bits.TrailingZeros32(mask))
+		maximum := mask >> shift
+		if mask&used != 0 || maximum&(maximum+1) != 0 || uint64(mask)>>bpp != 0 {
+			return result, fmt.Errorf("invalid bmp color mask")
+		}
+		result[i] = bmpColorMask{mask: mask, shift: shift, Max: maximum}
+		used |= mask
+	}
+	return result, nil
+}