ffmpeg - Apply padding to input images while combining multiple images and audio track

Question

I am using below command to create a slideshow using ffmpeg:

ffmpeg -r 1 -i IMG_%04d.jpeg -i audio.mp3 -c:v libx264 -c:a aac -pix_fmt yuv420p -r 24 -y output.mp4 -vf pad=ceil(iw/2)*2:ceil(ih/2)*2

I had to add pad=ceil(iw/2)*2:ceil(ih/2)*2 to make width or height even number because if one of the input image width/height is odd, ffmpeg throws error. Now my problem is; input images can be of different resolution and aspect ratio. I want to add padding (left/right or top/bottom) to all images to make them of equal size before they are converted into video frames. How can I achieve this in efficient way?

Edit: It would be nice if I can specify output resolution and all images are either scaled down or up keeping their aspect ratio intact according to that resolution.

Answer 1

If your output resolution is AxB, then your filters would be

-vf "scale=A:B:force_original_aspect_ratio=decrease,pad=A:B:-1:-1"

Answer 2

I have got it working through this way:

ffmpeg -r 1 -i image%4d.png -i audio.mp3 -filter_complex scale=w=%d:h=%d:force_original_aspect_ratio=1,pad=%d:%d:(ow-iw)/2:(oh-ih)/2 -c:v libx264 -c:a aac -pix_fmt yuv420p -r 1 -y output.mp4

This way aspect ratio of input images remain intact and also it solves odd image width/height issue. I hope this answer would help other users facing same issue.