Graphicsmagick在压缩jpg时提供更高的文件大小 [英] Graphicsmagick gives higher filesize when compressing jpg
问题描述
首先,我将 lena512color.tiff 转换为jpg然后我以90%的质量运行它.在降低质量文件大小后感到惊讶.我在这里做错了什么?
First i took lena512color.tiff amd converted it to jpg and then i ran mogrify on it with a quality of 90%.Surprising after i decrease the quality filesize is higher.What am I doing wrong here ?
gm convert lena512color.tiff lena512color.jpg # filesize = 37,043
gm mogrify -quality 90 lena512color.jpg # filesize = 58,132
推荐答案
默认情况下,输出质量设置为 GraphicsMagick 认为输入文件的质量.因此,我认为它认为TIFF的质量低于90,而您实际上是在增加设置.
By default, the output quality is set to whatever GraphicsMagick thinks the quality of the input file is. So I guess it thinks the TIFF's quality is lower than 90 and you are actually increasing the setting.
您可以使用以下命令查看 GraphicsMagick 对图像质量的感知:
You can use the following command to see what GraphicsMagick perceives the quality of an image to be:
identify -format %Q someImage.jpg
90
如果您的目标是限制生成的JPEG的大小,则可以使用以下-define
设置目标大小:
If your aim is to limit the size of the resulting JPEG, you can use the following -define
to set the target size:
convert in.jpg -define jpeg:extent=50KB out.jpg
结果
-rw-r--r-- 1 mark staff 49802 30 Oct 12:38 out.jpg
或
convert a.jpg -define jpeg:extent=30KB out.jpg
结果
-rw-r--r-- 1 mark staff 28996 30 Oct 12:38 out.jpg
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