linux convert thumbnail but move position - linux

Question

I have used linux convert command to successfully create thumbnails as follows:
$disposition='200x200';
$str="convert -size $disposition \"$fullPath\" -resize $disposition +profile '*' \"$fullPathThumb\"";
$result=`$str`;
However what I want to also do is "slide the sized down image some so that the upper left corner of the image is moved around, usually negative value up and negative value left, to center and crop. How would I do that? Thanks.

Answer 1

I am not too sure I understand your "sliding around" idea, but hopefully the following will explain how you can crop with offsets and resize. If not, please ask further questions.
Let's start with an image made of 8 blocks, each 100x100 pixels and off-square so we know which is side is which.
convert -size 100x100 \
\( xc:red xc:blue xc:green xc:black +append \) \
\( xc:cyan xc:magenta xc:yellow xc:white +append \) -append out.png
Now, we check its size is indeed 400x200:
identify out.png
out.png PNG 400x200 400x200+0+0 8-bit sRGB 8c 467B 0.000u 0:00.000
So, first we will do a simple resize, which will preserve the 2:1 aspect ratio:
convert out.png -resize 300x300 simple_resize.png
identify simple_resize.png
simple_resize.png PNG 300x150 300x150+0+0 8-bit sRGB 47c 672B 0.000u 0:00.000
Now we use the bang operator (!) to tell ImageMagick rather forcefully "Do what I said"
convert out.png -resize 300x300! simple_resize_changed_aspect.png
identify simple_resize_changed_aspect.png
simple_resize_changed_aspect.png PNG 300x300 300x300+0+0 8-bit sRGB 52c 894B 0.000u 0:00.000
And finally, we come to what is hopefully the bit you want, which is crop and resize. So let's crop an area 150 pixels wide by 100 pixels high starting 150 pixels across to the right from the top left corner and 50 pixels down from the top left corner:
convert out.png -crop 150x100+150+50 -resize 300x300 x.png
That looks right, but if we use identify we will see that ImageMagick has remembered too much about where our image came from and is considering it a part of the bigger original image:
identify x.png
x.png PNG 300x200 800x400+300+100 8-bit sRGB 30c 732B 0.000u 0:00.000
So, actually we better tell ImageMagick to reset the dimensions of the image so it is sitting on a canvas of "just the right size"... by using the +repage option on the previous command like this:
convert out.png -crop 150x100+150+50 -resize 300x300 +repage x.png
and check again
identify x.png
x.png PNG 300x200 300x200+0+0 8-bit sRGB 30c 690B 0.000u 0:00.000
That's better - probably best to use +repage whenever you crop. I hope that addresses your question, as I said, please ask further if not.