BMP file format
image file 1440x900.bmp 
1 | -rw-rw-r-- 1 lull lull 3888054 Mar 1 14:08 1440x900.bmp |
3888000 = 1440*900*3
.rgb 是裸rgb数据，在yuvview 指定 BGR8 1440*900 之后可以看到如.bmp一样的正常图像
开始分析 .bmp 文件 八进制编辑器打开bmp，我们只关注 前54个字节
| Offset hex | Offset dec | Size | Example | Purpose |
|---|---|---|---|---|
| 00 | 0 | 2 bytes | 0x42 0x4D | The header field used to identify the BMP and DIB file is 0x42 0x4D in hexadecimal, same as BM in ASCII. The following entries are possible:BMWindows 3.1x, 95, NT, … etc.BAOS/2 struct bitmap arrayCIOS/2 struct color iconCPOS/2 const color pointerICOS/2 struct iconPTOS/2 pointer |
| 02 | 2 | 4 bytes | 0xB6533B(小端序3888054) | The size of the BMP file in bytes |
| 06 | 6 | 2 bytes | Reserved; actual value depends on the application that creates the image, if created manually can be 0 | |
| 08 | 8 | 2 bytes | Reserved; actual value depends on the application that creates the image, if created manually can be 0 | |
| 0A | 10 | 4 bytes | 0x36 | The offset, i.e. starting address, of the byte where the bitmap image data (pixel array) can be found. |
00 处指出头类型为 Windows Bitmap Header
0A处指出的头长度 0x36=54， 这就是这个bmp文件头的长度
于是我们接着看 Windows Bitmap Header
| Offset (hex) | Offset (dec) | Size (bytes) | Windows BITMAPINFOHEADER | |
|---|---|---|---|---|
| 0E | 14 | 4 | 0x28=40 | the size of this header, in bytes (40) |
| 12 | 18 | 4 | 0xA0050000=1440 | the bitmap width in pixels (signed integer) |
| 16 | 22 | 4 | 0x84030000=900 | the bitmap height in pixels (signed integer) |
| 1A | 26 | 2 | 0x0100 | the number of color planes (must be 1) |
| 1C | 28 | 2 | 0x1800=24 | the number of bits per pixel, which is the color depth of the image. Typical values are 1, 4, 8, 16, 24 and 32. |
| 1E | 30 | 4 | the compression method being used. See the next table for a list of possible values | |
| 22 | 34 | 4 | 0x80533B00=1440*900=3888000 | the image size. This is the size of the raw bitmap data; a dummy 0 can be given for BI_RGB bitmaps. |
| 26 | 38 | 4 | the horizontal resolution of the image. (pixel per metre, signed integer) | |
| 2A | 42 | 4 | the vertical resolution of the image. (pixel per metre, signed integer) | |
| 2E | 46 | 4 | the number of colors in the color palette, or 0 to default to 2n | |
| 32 | 50 | 4 | the number of important colors used, or 0 when every color is important; generally ignored |
Bitmap的头分两部分：
- Bitmap file header: 12 字节
- Window Bitmap Header: 40 字节
- 第一部分的头长度是标准，其中的前两个字段指定接下来的第二段头的标准
- 第二段头中指定该段头的长度
两部分加起来就是.bmp文件（带有bitmap头）相对于 原始rgb文件的 增量大小