Timelapse of Tappan Zee Bridge
Neither automatic alignment with Photoshop panorama tools nor align_image_stack from Hugin worked, probably because there are too many differences between the first and the last frame. So I had to align manually using the following process:
- Create a spreadsheet with the list of image files;
- Find two easily identifiable key points, one near upper left corner, another near upper right corner;
- Open each file in Photoshop, hover mouse pointer over the left key point, record x0 and y0 coordinates (in pixels) from the info pane, do the same with the right key point, record x1 and y1;
- Using formulas in spreadsheet, create ImageMagick commands;
- Copy and paste commands from the spreadsheet in terminal.
convert orig/2013_0527_1104.tif -extent 5178x3652-241-1231 -rotate 0.368461030248915 -crop 3008x1692+398+1500 -resize 1920x1080 aligned/frame11040.png
This command does the following:
- Add border around the image so that the right key point is in the middle;
- Rotate the image so that the line between the key points is at the same angle relative to the horizon;
- Crop the image to target aspect ratio;
- Resample to video resolution.
To get rotation angle, start by calculating the angle α between the horizon and the line connecting the key points: α = 180° · atan((y1 - y0) / (x1 - x0)) / π, then subtract average α. To cancel out this rotation, rotate the image in the opposite direction (flip the sign).
To add panning, keep shifting the frame by a small amount when cropping.
To get smooth transitions, add interpolating frames obtained by blending consecutive frames. Example:
composite -blend 66x34 aligned/frame11043.png aligned/frame16170.png -alpha Set inter/frame11041.png
When all frames are ready, combine them into a video using ffmpeg:
ffmpeg -framerate 6 -i 'inter/frame%*.png' -pix_fmt yuv420p -c:v libx264 -r 6 tz_timelapse.mp4
Direct link to video file.