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The following command will resize an image to a height of 100: convert example.png -resize x100 example.png Rotating an Image The following command will resize an image to a width of 200: convert example.png -resize 200 example.png You can also specify a specific width or height and ImageMagick will resize the image to that width or height while preserving the aspect ratio. Compression filtering is a pre-compression step that reorganizes the image’s data so that the actual compression is more efficient I got the best results using adaptive. If you want to force the image to become a specific size - even if it messes up the aspect ratio - add an exclamation point to the dimensions: convert example.png -resize 200x100! example.png PNG compression in ImageMagick can be configured with three settings, -define png:compression-filter, -define png:compression-level and -define png:compression-strategy. It will alter the image to fit within a 200×100 area, but the image may not be exactly 200×100.
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ImageMagick will try to preserve the aspect ratio if you use this command. So, it may be a good idea to make all transparent pixels the same colour when saving PNG files, by using -alpha background.Įxample convert -size 512x512 xc:gray +noise random a.png # create an image of random 1 mark staff 1576107 6 Sep 11:37 a.png # 157kBĬonvert -size 512x512 xc:gray +noise random -alpha transparent a.png # recreate but make 1 mark staff 1793567 6 Sep 11:38 a.png # 179kB, extra transparency channelĬonvert -size 512x512 xc:gray +noise random -alpha transparent -alpha background a.png # make all transparent pixels 1 mark staff 1812 6 Sep 11:38 a.We’ve used the same file name here, so ImageMagick will overwrite the original file. normally, the colour of transparent pixels is irrelevant because you can't see them, but uniform things generally compress better. ) the input jpeg compression level and so if you don't add -quality NN at all, the output should use the same level as input. compressionZip the type of algorithm used to compress the image data. rw-r-r- 1 mark staff 2361 5 Sep 21:04 rose64.pngĪnother 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.Īlso, worth bearing in mind. First of all, ImageMagick reads (or better 'guess'. MIFF is a part of the ImageMagick toolkit of image manipulation utilities for the. Now convert the rose to 64 colours and check the size - down to 2,361 bytes convert rose: -colors 64 rose64.png convolve coefficients, apply a convolution kernel to the image. Now convert the rose to 255 colours and check the size - it is down to 3,691 bytes: convert rose: -colors 255 rose255.png -contrast, enhance or reduce the image contrast. Ls -l 1 mark staff 6975 5 Sep 20:57 rose.png You can try something similar in ImageMagick like this.įirst, using the built-in rose: image, check the number of colours in the image - it is 3,019: convert rose: -format %k info:Īnd make a PNG of it and check the size - it is 6,975 bytes convert rose: rose.png Pngquant effectively quantizes, or reduces the number of colours in an image till just before there is a discernible drop in quality. If the image in question contains text, or is mostly relevant because of the text stored within it, then strongly consider using a lossless compression.