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Comparison of JPEG Markers in Photoshop
Save As Save For Web Description
SOI SOI Start of Image marker
APP0 APP0 Identifier, Version, resolution, JFIF extensions
APP1 EXIF data, Makernotes removed in both. (长度 5KB)
APP13 Photoshop IRB (Photoshop-specific data, including quality)
(长度5KB)
APP1 XML metadata (EXIF, IPTC, etc.) (长度17KB)
APP2* APP2* ICC color profile (* optional) (长度3KB)
APP12 Ducky tag - Save for Web Quality - stores the quality level
(length 17B)
APP14 APP14 Encoding info: color transform (e.g. YCbCr, YCCK, RGB, CMYK)
and whether or not the samples were blended (length 14B)
DQT DQT Quantization Tables
SOF0 SOF0 Baseline DCT, chroma subsampling
DRI Restart Intervals - Adds a level of recovery in case of errors
(length is variable, depends on frequency)
DHT DHT Huffman compression tables
SOS SOS Start of Scan: Image Data stream
EOI EOI End of Image marker
As can be seen in the above table, the additional data stored within a Save As JPEG versus as Save for Web JPEG is typically in the order of ~ 40 KB. In most high-resolution photos, this extra is insignificant, but when trying to compress small images for web use, this overhead could be significant.
Photoshop两种保存方式输出文件的jpeg标记对比
另存为 另存为网页格式 说明
SOI SOI 图像开始标记
APP0 APP0 识别标记、版本、分辨率、jfif扩展
APP1 EXIF数据, 两种保存方式都移除了制作者注释(长度5KB)
APP13 Photoshop IRB (Photoshop特有数据,包括质量) (长度5KB)
APP1 XML数据(EXIF, IPTC等) (长度17KB)
APP2* APP2* ICC颜色配置(*可选) (长度3KB)
APP12 Ducky标签-保存为网络格式质量-储存质量等级(长度17B)
APP14 APP14 编码信息: 颜色转换 (例如YCbCr, YCCK, RGB, CMYK)
以及样本是否混合(长度14B)
DQT DQT 量化表
SOF0 SOF0 基线DCT, 色度抽样
DRI 重新开始间隔-加入出错时的复原标准(长度不定)
DHT DHT Huffman压缩表
SOS SOS 扫描开始: 图像数据流
EOI EOI 图像结束标记
如上表所示，另存为输出文件所包含的附加信息使文件比另存为网页格式多40kb。对于大部分高分辨率的图像文件来说，这部分数据并不明显，但如果想要压缩图像文件大小以供网页使用，那么这部分就很明显了。
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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.
（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
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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。
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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。
保存方法 JPEG质量 预设 色度抽样
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
