使用ImageMagick压缩PNG图像 [英] Compress a PNG image with ImageMagick
问题描述
要压缩JPEG图片,我可以:
$ thumb = new Imagick
$ thumb-> readImage(url);
$ thumb-> setImageCompression(Imagick :: COMPRESSION_JPEG);
$ thumb-> setImageCompressionQuality(80);但是,我还需要压缩PNG图像(保留Alpha透明度)以保持大小不变。
<有没有办法用ImageMagick做?
pngquant
量化或减少图像中的颜色数量,直到恰好在可辨别的质量下降之前。你可以在ImageMagick中尝试类似这样的东西...
首先,使用内置的 rose:
,检查图片中的颜色数 - 是3,019:
convert rose:-format%k info:
3019
并制作 PNG
并检查大小 - 是6,975字节
convert rose:rose.png
ls -l rose.png
-rw-r - r - @ 1 mark staff 6975 5 Sep 20:57 rose.png
现在将玫瑰转换为255种颜色,并检查大小 - 最小为3,691字节:
convert rose:-colors 255 rose255.png
ls -l rose255.png
-rw- r - r-- 1 mark staff 3691 5 Sep 21:02 rose255.png
现在将玫瑰色转换为64种颜色,并检查尺寸 - 最小为2,361字节
convert rose:-colors 64 rose64.png
ls -l rose64.png
-rw-r - r-- 1 mark staff 2361 5 Sep 21:04 rose64.png
另一种优化或减少PNG文件大小的方法是使用 -strip
删除图片中的任何元数据,例如拍摄照片的日期和时间,相机和镜头型号,创建图像的程序名称以及版权和颜色配置文件。
此外,注意...通常,透明像素的颜色是不相关的,因为你看不到它们,但统一的东西通常压缩得更好。因此,通过使用 -alpha background
,保存PNG文件时,使所有透明像素的颜色相同是个好主意。
#创建随机噪声的图像
-rw-r - r - @ 1 mark staff 1576107 6 Sep 11:37 a.png#157kB
convert -size 512x512 xc:gray + noise random -alpha transparent a.png#recreate but make transparent
-rw-r - r - @ 1 mark staff 1793567 6 Sep 11:38 a.png#179kB,extra transparency channel
convert -size 512x512 xc:gray + noise random -alpha transparent -alpha background a.png#make all transparent pixels black
-rw-r - r - @ 1 mark staff 1812 6 Sep 11 :38 a.png#Presto!
To compress a JPEG image, I can do:
$thumb = new Imagick();
$thumb->readImage("url");
$thumb->setImageCompression(Imagick::COMPRESSION_JPEG);
$thumb->setImageCompressionQuality(80);
However, I need to also compress PNG images (preserving alpha transparency) to keep sizes down. Is there a way to do it with ImageMagick?
pngquant
effectively quantizes, or reduces the number of colours in an image till just before there is a discernible drop in quality. You can try something similar in ImageMagick like this...
First, using the built-in rose:
image, check the number of colours in the image - it is 3,019:
convert rose: -format %k info:
3019
and make a PNG
of it and check the size - it is 6,975 bytes
convert rose: rose.png
ls -l rose.png
-rw-r--r--@ 1 mark staff 6975 5 Sep 20:57 rose.png
Now convert the rose to 255 colours and check the size - it is down to 3,691 bytes:
convert rose: -colors 255 rose255.png
ls -l rose255.png
-rw-r--r-- 1 mark staff 3691 5 Sep 21:02 rose255.png
Now convert the rose to 64 colours and check the size - down to 2,361 bytes
convert rose: -colors 64 rose64.png
ls -l rose64.png
-rw-r--r-- 1 mark staff 2361 5 Sep 21:04 rose64.png
Another way of optimising or reducing PNG filesizes is to use -strip
to strip out any metadata from images - such as the date and time the picture was taken, the camera and lens model, the name of the program that created the image and the copyright and colour profiles.
Also, worth bearing in mind... normally, the colour of transparent pixels is irrelevant because you can't see them, but uniform things generally compress better. So, it may be a good idea to make all transparent pixels the same colour when saving PNG files, by using -alpha background
.
Example
convert -size 512x512 xc:gray +noise random a.png # create an image of random noise
-rw-r--r--@ 1 mark staff 1576107 6 Sep 11:37 a.png # 157kB
convert -size 512x512 xc:gray +noise random -alpha transparent a.png # recreate but make transparent
-rw-r--r--@ 1 mark staff 1793567 6 Sep 11:38 a.png # 179kB, extra transparency channel
convert -size 512x512 xc:gray +noise random -alpha transparent -alpha background a.png # make all transparent pixels black
-rw-r--r--@ 1 mark staff 1812 6 Sep 11:38 a.png # Presto!
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