Search code examples
linuxshellgraphicsimagemagickpng

Resizing does not respect resolution specification

I've got a 16x512 image comprised of 16x16 images. I want to seperate them, individually scale each one down to 16x8, then put them all back into their full 16x512. I've got a basic idea, but I'm having trouble executing it.

Using the commands from unix stackexchange, I split by file by using convert -crop 16x16 my_image.png crop-%d.png, which yields 32 images (512 / 16 == 32). My next step was where trouble has started. From askubuntu, I found the command mogrify -resize 16x8 crop-*.png, however this does not yield 16x8 images, but rather 8x8, which I do not want. Furthermore, this post on stackoverflow gives me the command for merging these images, which is convert crop-*.png -append my_image_cropped.png, however it does not yield a 16x512 like I want, but rather 8x256 (the 8 is due to the previous bug, but I still want a height of 512, not 256).

What do I need to accomplish my goals? The image in question can be found on imgur.

Edit: Here are some images which will describe the basic idea

The full image:

example.png; the full image

Both 16x16 and 16x8 side by side

example_resize.png; the 16x16 and 16x8 side-by-side

The finalized image, basically the 16x8 will sit in the 16x16 area (right at the bottom part, that is essential), but won't fully fill it.

example_final.png; the finalized image

Solution

  • I am not sure I understand what you want to do. But if you resize 16x8, Imagemagick will keep aspect ratio. If you want to force it to be exactly 16x8 and can accept distortion, then use the ! flag. But you then say you want to put the 32 pieces back to form 16x512, but the resize will make it 16x256, since you have 32 image of height 8. So you have to resize again. Here is how to do that, if that is really what you want.

    Create a gradient image for testing:

    convert -size 16x512 gradient: grad.png


    enter image description here

    Do the processing:

    convert grad.png -crop 16x16 -resize 16x8! -append -resize 16x512! newgrad.png


    enter image description here

    Note that proper Imagemagick syntax reads the input first.

    ADDITION:

    Given your new information in your comment, try this:

    convert grad.png -crop 16x16 -resize 16x8 -gravity northwest -background none -extent 16x16 -append newgrad.png


    enter image description here

    Change the background color as desired and the gravity setting as desired for positioning.