Thursday, September 25, 2014

Restoring Old Movies, Part 2

Here's Part 1,

Definition: Dynamic range is the difference between the whitest whites in a picture and the darkest blacks. In some old videos, like the one I showed from The Adventures of Richard the Lion Heart last time, everything has mushed towards a middle gray, muddling contrasts. Contrasts are what give you discernible features in an image. Without sufficient contrast, everything blurs together into a blob.

Last time, I played with modifying brightness and contrast. I've since found out those are tools for illiterate savages. Brightness takes your pixels and makes them lighter or darker. That may sound fine, but it has a tendency to saturate your extremes. Saturated colors show no features at all since they're, well, saturated and can't get any whiter or darker. Here's an annotated color analysis from one still of that scene where I blundered about with brightness and contrast.

A graph of light intensity on the left and the still image on the right. I left this large, so clicking on it will make the situation more obvious.
Increasing the brightness did indeed give me more dynamic range in the image, but it also smashed the lady's dress right up against the stops. You can see the folds of her gown in the original image, but not in my "improved" one. Dittos for King Richard's hair and clothes on the far right, although they fare a bit better.

You want to increase the dynamic range of the scene so you can get every last drop of contrast out of it and clear things up, but there are better ways of doing it than boosting or quenching every pixel identically. The link above takes you to a tutorial that shows you how to do it with more finesse using Adobe Premiere. Adobe Speedgrade has even better tools than those shown in that tutorial.

Instead of crudely changing every pixel, you can modify them based on where they are in intensity. That is, the bright ones can be modified differently than the midrange ones and differently from the dark ones. Nothing needs to be saturated unless you want it to be, I'm still playing with it and hope to show off some cleaned up video from this scene in the near future. So far, I really like what I'm getting out of it.


tim eisele said...

Now that you mention it, I should probably look into getting a more recent version of ImageJ. I'm still using the "illiterate savage" tools probably more often than I should, and they've probably gone beyond the simple linear contrast/brightness controls since I last updated.

The Fiji package looks promising.

lee said...

Is this like adjusting the curve in Photoshop?

K T Cat said...

Lee, bingo! I think we're talking about the same thing - a curve that maps input values into output values, right?

lee said...

Right. You can do a much better job of adjusting congrats on Photoshop by adjusting the curves than by sliding the dohickey over on "contrast." My fast and easy audience is to add a slight s to the curve. For things o want to spend more time on (and warrant having more time spent on them) I'll tweak the curve awee bit in a variety of places. "Control + Z" is your friend.

lee said...

Contrast not congrats!