[magick-users] RE: Thumbnails with center auto crop

[Fred Weinhaus]

Dave,

Sorry, I got started with an idea and then found a counter situation. 
Using 180x130^ is not quite right.

The problem is that if you do what I said below and your image is 
square (say 200x200), then what you get is an image that is 130x130 
(I think - I have not tried it). If so, then you have no image to 
make the width 180 and would have to pad it with some background.

So say, if your image is 270 (or bigger) x 200, then you would get an 
image that was 180 (or bigger) x 130, if you scaled to 130. Then you 
would have enough room to get the center 180x130.

But if it is less than 270 in width, then you have too little width 
after resizing to 130 to get 180x130 without padding.

To put it another way, if the aspect ratio w/h is greater than 
180/130=1.385, this works. But if it is less, then it does not work.

When the size is less than 270 x 200, or more properly the aspect 
ratio, w/h is less than 1.385, then you want to scale to the larger 
dimension, which is 180. Then you get something that is 180 x 130 (or 
bigger). So for example if you have your 200x200 and resize to 180 
for the width, then you get a height and width of 180 and can then 
get a 180x130 center area from it.

So the key is to check the aspect ratio. If w/h > 180/130, then 
resize to the minimum dimension (130) and crop out the center 
180x130; otherwise resize to the maximum dimension (180) and crop out 
the center 180x130.

I think this means:

if aspect=w/h > 180/130, then

convert 1.jpg -filter Lanczos -resize 130x130 -gravity center 1_.jpg

else

convert 1.jpg -filter Lanczos -resize 180x180 -gravity center 1_.jpg

(similarly if you use -thumbnail in stead of -resize)

Again, I have not tested this. Perhaps tomorrow I can check it out.


Fred




>Hehe, that was a little confusing, sorry. So you're saying do 
>something like this:
>
>convert 1.jpg -filter Lanczos -resize 180x130^ -gravity center 
>1_.jpg but it wont work for square images.
>
>Dave
>