Imagemagick - how to get largest layer and compress? - imagemagick

Question

Please help. Is there an easy way to take the largest layer of a tiff and zip compress it back as a single layer tiff again with imagemagick or similar?

Answer 1

Just a slightly easier version of Fred's answer. You can generate a list of the area (in pixels) of each layer in a TIF followed by the layer/scene number like this:
magick identify -format "%[fx:w*h] %s\n" image.tif
Sample Output
240000 0
560000 1
200000 2
So, if we do that again, sort it reverse numerically and take the second field of the first result, we will get the number of the layer with the largest area:
layer=$(magick identify -format "%[fx:w*h] %s\n" image.tif | sort -rn | awk 'NR==1{print $2}')
So, the complete solution would look like:
#!/bin/bash
# Get layer number of layer with largest area
layer=$(magick identify -format "%[fx:w*h] %s\n" image.tif | sort -rn | awk 'NR==1{print $2}')
# Extract that layer and recompress as single layer
magick image.tif[$layer] -compress lzw result.tif
If you are using ImageMagick v6 or older:
magick identify ... becomes identify ...
magick image.tif ... becomes convert image.tif ...

Answer 2

In concept, using ImageMagick this can be done in a single command. Here's an example...
magick input.tif -background none -virtual-pixel none ^
( -clone 0--1 +repage -layers merge ) ^
-distort affine "0,0 0,%[fx:s.w==u[-1].w&&s.h==u[-1].h?0:h]" ^
-delete -1 -layers merge output.tif
That starts by reading in the original TIF and setting the background and virtual-pixel settings to "none".
Then inside the parentheses it clones all the layers of the TIF, repages them, and merges them into a single image with the dimensions of the largest layer. That will become a gauge to measure with.
Next it uses "-distort affine" to slide each image out of the viewport and leave it transparent unless the image matches the width and height of that gauge. So after that distort, the largest image will remain unchanged, and all the others will be transparent.
Finish by deleting that gauge image and merging the rest. All the layers are transparent except the largest one, so merging them leaves just that visible one as a single layer.
The command is in Windows syntax using IM7. If you're using ImageMagick v6, use "convert" instead of "magick". To make it work in *nix, change the continued line carets "^" to backslashes "\" and escape the parentheses with backslashes "\(...\)". There may be other issues I've overlooked.
Obviously if there are two or more layers matching the largest dimensions, the output result will only be the first one from the original TIF.
Edited to add: This method will only work if both the greatest width and greatest height are on the same image.

Answer 3

How do you define largest? Width, Height, File size? If the largest dimension from width and height is used, then in Unix, you can do the following on a 3 layer tif file. Get the max dimension of each layer. Then find which layer is the largest. Then use just that layer when reading and writing the file.
Arr=(`identify -format "%[fx:max(w,h)]\n" img.tif`)
echo "${Arr[*]}"
500 1024 770
num=${#Arr[*]}
dim=0
for ((i=0; i<num; i++)); do
if [ ${Arr[$i]} > $dim ]; then
dim=${Arr[$i]}
index=$i
fi
done
echo "$index"
2
convert img.tif[$index] -compress zip newimg.tif
identify newimg.tif
newimg.tif[2] TIFF 770x768 770x768+0+0 8-bit sRGB 3662B 0.000u 0:00.000
I cannot think of any direct and simple method to find the largest layer and extract it in the same command line.