Creating a thumbnail of specified resolution

[hollymcr]

I need to create the best quality thumbnail I can, of a given size, from an original image. For the sake of simplicity assume a square thumbnail although that won't always be the case.

So let's say I want a 100x100 thumbnail from a 1024x768 image. What I want to do is crop the image to 768x768 (same aspect ratio as the desired thumbnail), using "center" gravity. Then I want to resize the result to 100x100.

Is there a general way to do this without making assumptions about the original image size or aspect ratio, and without going through multple steps?

[Bonzo]

Try:

Code: Select all

convert input.jpg -gravity center -crop 768x768+0+0 -resize 100x100 output.jpg

[fmw42]

add +repage after -crop and use -thumbnail rather than -resize so as to strip the headers and profiles, etc. to make the result smaller.

[hollymcr]

Almost perfect! (And a lot further than I got on my own, so huge thanks!)

Is it possible to handle the generalised case where I don't know the original image size? Ie I don't know that 768 is the smaller of the two dimensions?

For example, can I start by resizing so that the smallest dimension is 100, then crop to 100x100?

[fmw42]

see the ^ symbol for the geometry at http://www.imagemagick.org/script/comma ... p#geometry to resize to the smaller dimension. If on windows, see http://www.imagemagick.org/Usage/windows/

convert image -resize "100x100^" -gravity center -crop 100x100+0+0 +repage result

[hollymcr]

That's perfect, thanks!

I had got pretty close but not quite there on my own.

I'd say that for most purposes,
convert image -resize "100x100^" -gravity center -crop 100x100+0+0 +repage result
is the best general way to create a thumbnail, isn't it? (100x100 can be changed to whatever size wanted, of-course.)