ImageMagick - Resize to max width

Question

I want to resize an image to a max width. So I don't care about the height of the image, but I just want to always resize it to a specific width.

I'm certain I've done this before I just can't remember how I did it now.

Answer 1

Seems this is the way it is done, noting that width is the first parameter.

convert -resize '100' image.png

For anyone else wondering about height, then you would do this:

convert -resize 'x100' image.png

Source: http://www.imagemagick.org/script/command-line-processing.php

Edit (Nov 2014): Note that in the latest versions of ImageMagick you can no longer use quotes around the values as per Kevin Labécot's comment.

Answer 2

Your question is ambiguous. Your titles asks to resize an image to a max width, but then you seem to say you want to resize the image to a specific width.

If you want to resize something to a max width of 600px (ie, any image with a width of less than 600px will be unaffected), use:

convert original_image.jpg -resize 600x\> result_image.jpg

Or, to directly modify the original image:

mogrify  -resize 600x\> original_image.jpg

If you'd like max height rather than max width:

convert original_image.jpg -resize x600\> result_image.jpg

Answer 3

Are you just chasing the math to work out the correct aspect ratio?

$new_width = 400; // config
$image_width = 480; // loaded from image
$image_height = 786; // loaded from image
$new_height = $new_width * ($image_height / $image_width);
echo "$image_width x $image_height becomes $new_width x $new_height";