Notes - ImageMagick

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Resize an image to a fixed size while maintaining the aspect ratio

convert file.jpg -resize 800x -background white -gravity center -extent 800x600 file-aspect.jpg

This will pad above/below or to the right/left side of the image with the colour specified as -background, resize the image to the width or height as supplied to -resize and force the image to be the size supplied to -extent.