Extraction of Quality Setting in Save As

In files generated using Save As, you can derive the quality setting from the Photoshop IRB resource (0x0406) in the JPEG APP13 (0xFFED) marker. The following hex view is of an image saved with quality setting 2.
（图见下）

Value	Quality
0xFFFD	Quality 1 (Low)
0xFFFE	Quality 2
0xFFFF	Quality 3
0x0000	Quality 4
0x0001	Quality 5 (Medium)
0x0002	Quality 6
0x0003	Quality 7
0x0004	Quality 8 (High)
0x0005	Quality 9
0x0006	Quality 10 (Maximum)
0x0007	Quality 11
0x0008	Quality 12

另存为的图像质量设置

在用另存为方式输出的文件中，你能根据JPEG APP13 (0xFFED)标记中的photoshop IRB资源(0x0406)数据推得图片质量的设置值。下面是一副保存质量为2的图片文件的16进制视图。

数值	质量
0xFFFD	质量1 (低)
0xFFFE	质量2
0xFFFF	质量3
0x0000	质量4
0x0001	质量5 (中)
0x0002	质量6
0x0003	质量7
0x0004	质量8 (高)
0x0005	质量9
0x0006	质量10 (最佳)
0x0007	质量11
0x0008	质量12

Extraction of Quality Setting in Save For Web

In files generated using Save For Web, you can derive the quality setting (1..100) from the Ducky resource in the JPEG APP12 (0xFFEC) marker (starts with null-terminated ASCII string Ducky). The percent value is stored in hex. The following hex view shows a file saved with quality setting 51 = 0x31:（图见下）

另存为网页格式的质量设置

在另存为网页格式的图片文件中，你能根据JPEG APP12 (0xFFEC)标记(以空数据开头、ASCII编码Ducky结束)中的Ducky资源推得图像质量设置（1－100％）数据是以16进制保存的。图中在0x31处，16进制数值33表示图片质量为51。

Comparison of Save As vs Save For Web Quality

Many people are confused about why there is a difference in the definition of quality settings within Photoshop and furthermore, how they compare. The following table shows the two methods and the relative ordering of quality between the settings, from best to worst quality. See description below for the limitations of such an ordered comparison.
For more information: read about JPEG chroma subsampling.

Method	JPEG Quality	Preset	Chroma Subsampling
Save For Web	100	Maximum	1x1
Save As	12	Maximum	1x1
Save For Web	90		1x1
Save As	11		1x1
Save For Web	80	Very High	1x1
Save As	10		1x1
Save For Web	70		1x1
Save As	9		1x1
Save For Web	60	High	1x1
Save As	8	High	1x1
Save For Web	51		1x1
Save For Web	50		2x2
Save As	6		2x2
Save As	7		1x1
Save As	5	Medium	2x2
Save For Web	40		2x2
Save As	4		2x2
Save As	3	Low	2x2
Save For Web	30	Medium	2x2
Save As	2		2x2
Save For Web	20		2x2
Save As	1		2x2
Save For Web	10	Low	2x2

In graphical form, here is a chart that shows a comparison of the JPEG luminance and chrominance across the default presets, each with the equivalent JPEG standard "quality factor". Of course, it is important to note that the quality factor shown is not an accurate measure of image quality since they are only truly comparable quantities if both Save for Web and Save As were based on linear scalings of the quantization tables included in the JPEG Standard Annex. Since neither is based on the sample tables, the quality factor should only be taken as a very rough comparison measure. The graph is ordered by decreasing luminance quality factors.
（图见下）
Ordered Graph
（图见下）
Comparison Graph

另存为和另存为网页格式的比较

很多人对为何photoshop中的两种保存方式质量设定的定义有区别而不解，还有怎么比较他们。下面的表格是两种保存方法设置质量后的图像质量排序，从最佳质量到最低质量。这种排序比较的局限性请看后面的说明。
更多信息请查阅jpeg色度抽样。

Method	JPEG Quality	Preset	Chroma Subsampling
另存为网页格式	100	最佳	1x1
另存为	12	最佳	1x1
另存为网页格式	90		1x1
另存为	11		1x1
另存为网页格式	80	较高	1x1
另存为	10		1x1
另存为网页格式	70		1x1
另存为	9		1x1
另存为网页格式	60	高	1x1
另存为	8	高	1x1
另存为网页格式	51		1x1
另存为网页格式	50		2x2
另存为	6		2x2
另存为	7		1x1
另存为	5	中	2x2
另存为网页格式	40		2x2
另存为	4		2x2
另存为	3	低	2x2
另存为网页格式	30	中	2x2
另存为	2		2x2
另存为网页格式	20		2x2
另存为	1		2x2
另存为网页格式	10	低	2x2

下面是默认设置下的jpeg亮度和色度的对比图表，每个都有对应的jpeg标准质量系数。当然，有必要指出，质量系数不是测量图片质量的精确标准，只是如果另存为网页格式和另存为都是基于jpeg标准附录中的量化表的线性测量，那么质量系数是唯一能用来比较图片质量的参数。质量系数只能作为一个粗糙的标准来比较图片。图表按亮度质量系数降序排列。

排序图表

对比图表

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本文链接：http://www.blueidea.com/tech/graph/2008/5839.asp　

出处：蓝色理想
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