How to center images in gif using convert from imagamagick? - bash

Question

My goal is to create gif image from jpg which I will rotate on angle A with delay D imagemagick. So, i've got this code:
for (( i=0; i < 360; i+=$a )) do
name=%(printf "%03d" $i)
convert $image -rotate $i "temp/$name.jpg"
done
convert -delay $d "temp/*.jpg" out.gif
The problem is that images in out.gif is not centered. How can i center them?
(image should be rotated relative to the center)

Answer 1

The problem is that images in out.gif is not centered.
It just seems that way when you look at the final gif. However, the image already rotates around its center. Example: I have the image 1.png and rotate it in steps of 15°.
convert 1.png -rotate 15 2.png
convert 1.png -rotate 30 3.png
convert 1.png -rotate 45 4.png
Now let's have a look at the images 1-4.
As we can see, the image rotates around its center, but the image dimension changes.
When we convert those four images into one gif animation, we have to set a fixed dimension for the whole animation and a position of each frame on the canvas. Without giving further information, ImageMagick uses the first frame's dimension as the dimension of the animation and places each frame at the top left. Since the first frame (rotation 0°) is the smallest, the other frames get cropped. The following illustration shows what will get cropped.
To solve the problem, we have two options:
When creating the animation, center each frame.
When rotating, crop the result such that all frames have the same size.
I would go with 2. ImageMagick has the operator -distort SRT <ANGLE> which rotates and crops in one step. Using this operator will also look smoother in animations, see here. Replace your -rotate $i with -distort SRT $i.

Answer 2

The ImageMagick "-rotate" operator will increase the output dimensions so the entire input image still fits within the output. That produces results with various dimensions for different angles of rotation. Using the plus form of "+distort SRT" will have the same effect.
And "-distort SRT" will rotate the image on center inside its input dimensions, but that will clip off the corners as they rotate outside the original frame.
There is a way to rotate an image several times, pivoting on the center point, and in a viewport that accommodates the entire image even when rotated.
By setting the input image name "$image", the number of frames "$f", and the delay per frame "$d", a command like this can do everything your "for" loop is doing while eliminating the writing and reading of temporary files. It looks more complicated than it really is...
image=input.jpg
f=20
d=50
convert -delay $d -dispose previous "$image" -background none -virtual-pixel none \
-duplicate $f +delete -set option:distort:viewport "%[fx:hypot(w,h)]x%[fx:hypot(w,h)]" \
-distort SRT "%[fx:w/2],%[fx:h/2] 1 %[fx:360/n*t] %[fx:hypot(w,h)/2],%[fx:hypot(w,h)/2]" \
out.gif
That command will read the input "$image", duplicate it to create a total of "$f" frames, set the delay to "$d", rotate each image "360/$f" degrees more than the previous, and output the final result as an animated GIF.