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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.
（圖見下）

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

另存為的圖像質(zhì)量設置

在用另存為方式輸出的文件中，你能根據(jù)JPEG APP13 (0xFFED)標記中的photoshop IRB資源(0x0406)數(shù)據(jù)推得圖片質(zhì)量的設置值。下面是一副保存質(zhì)量為2的圖片文件的16進制視圖。

數(shù)值	質(zhì)量
0xFFFD	質(zhì)量1 (低)
0xFFFE	質(zhì)量2
0xFFFF	質(zhì)量3
0x0000	質(zhì)量4
0x0001	質(zhì)量5 (中)
0x0002	質(zhì)量6
0x0003	質(zhì)量7
0x0004	質(zhì)量8 (高)
0x0005	質(zhì)量9
0x0006	質(zhì)量10 (最佳)
0x0007	質(zhì)量11
0x0008	質(zhì)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:（圖見下）

另存為網(wǎng)頁格式的質(zhì)量設置

在另存為網(wǎng)頁格式的圖片文件中，你能根據(jù)JPEG APP12 (0xFFEC)標記(以空數(shù)據(jù)開頭、ASCII編碼Ducky結束)中的Ducky資源推得圖像質(zhì)量設置（1－100％）數(shù)據(jù)是以16進制保存的。圖中在0x31處，16進制數(shù)值33表示圖片質(zhì)量為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.
（圖見下）
Ordered Graph
（圖見下）
Comparison Graph

另存為和另存為網(wǎng)頁格式的比較

很多人對為何photoshop中的兩種保存方式質(zhì)量設定的定義有區(qū)別而不解，還有怎么比較他們。下面的表格是兩種保存方法設置質(zhì)量后的圖像質(zhì)量排序，從最佳質(zhì)量到最低質(zhì)量。這種排序比較的局限性請看后面的說明。
更多信息請查閱jpeg色度抽樣。

Method	JPEG Quality	Preset	Chroma Subsampling
另存為網(wǎng)頁格式	100	最佳	1x1
另存為	12	最佳	1x1
另存為網(wǎng)頁格式	90		1x1
另存為	11		1x1
另存為網(wǎng)頁格式	80	較高	1x1
另存為	10		1x1
另存為網(wǎng)頁格式	70		1x1
另存為	9		1x1
另存為網(wǎng)頁格式	60	高	1x1
另存為	8	高	1x1
另存為網(wǎng)頁格式	51		1x1
另存為網(wǎng)頁格式	50		2x2
另存為	6		2x2
另存為	7		1x1
另存為	5	中	2x2
另存為網(wǎng)頁格式	40		2x2
另存為	4		2x2
另存為	3	低	2x2
另存為網(wǎng)頁格式	30	中	2x2
另存為	2		2x2
另存為網(wǎng)頁格式	20		2x2
另存為	1		2x2
另存為網(wǎng)頁格式	10	低	2x2

下面是默認設置下的jpeg亮度和色度的對比圖表，每個都有對應的jpeg標準質(zhì)量系數(shù)。當然，有必要指出，質(zhì)量系數(shù)不是測量圖片質(zhì)量的精確標準，只是如果另存為網(wǎng)頁格式和另存為都是基于jpeg標準附錄中的量化表的線性測量，那么質(zhì)量系數(shù)是唯一能用來比較圖片質(zhì)量的參數(shù)。質(zhì)量系數(shù)只能作為一個粗糙的標準來比較圖片。圖表按亮度質(zhì)量系數(shù)降序排列。

排序圖表

對比圖表

經(jīng)典論壇交流：
http://bbs.blueidea.com/thread-2860036-1-1.html

本文鏈接：http://m.95time.cn/tech/graph/2008/5839.asp　

出處：藍色理想
責任編輯：bluehearts

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