JPEG文件量化表定义
当我使用 Photoshop 另存为功能并选择 jpeg 文件格式时，我会看到以下窗口：
如您所见，我选择基线（“标准”）格式和最高图像质量。当我在十六进制编辑器中打开这张图片时，我看到几个 FF DB 标记（这是量化表的开始）。还没有问题，但让我们看下一张图片： 
正如您在上面的表格图片中看到的那样，在地址行 BDA 处启动 FFDB 标记。前两个字节是 00 84，这意味着该标记保存 132 字节的数据。做一些数学计算，我们可以得出结论，该标记定义了两个量化表。第一个表的值为：0C 08 08 08 09 等...
在同一个文件中，还有另一个 FFDB 标记，从 2885 地址行开始，如图所示：
同样，前两个字节的值为 00 84，这意味着 132 字节的数据。但这一次，第一个量化表值是：01 01 01 等...
如何知道应该使用哪个 FF DB 标记，以及为什么文件中有多个 FFDB 标记？< /强>
When I use Photoshop Save As function, and pick jpeg file format I get following window:
As you can see, I select Baseline ("Standard") format, and maximum picture quality. When I open this picture in Hex editor, I see several FF DB markers (which is start of Quantization table/s). No problem yet, but lets look next picture:
As you can see form picture above, at address row BDA starts FFDB marker. First two bytes are 00 84 which means that this marker holds 132 bytes of data. Doing some math, we can conclude that two Quantization tables are defined by this marker. Values of first table are: 0C 08 08 08 09 etc...
In the same file, there is another FFDB marker, starting from 2885 address row, as you can see in picture:
Again, value of first two bytes are 00 84 which means 132 bytes of data. But this time, first Quantization table values are: 01 01 01 etc...
How to know which of FF DB markers should I use, and why there is several FFDB markers in file?
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如果没有看到整个文件，很难确定，但看起来您的第一个量化表是用于以较低质量压缩的嵌入缩略图。第二个量化表用于主图像，其值为 01,01,01,...，因为您选择了最高质量，因此系数值被量化为尽可能少的量。
Without seeing the entire file, it's hard to say with certainty, but it looks like your first quantization table is for an embedded thumbnail which is compressed with a lower quality. The second quantization table is for the main image and has values of 01,01,01,... because you chose the highest quality and therefore the coefficient values are quantized the least possible amount.