Making A Difference With Difference Blending

Making A Difference With Difference Blending
You might think that the image that leads this post is missing. Or maybe it’s a mistake. I can assure you that it’s not. It’s exactly the image that should be there. In fact, it’s perfect.

Before I explain the image, I want to talk about the tools that editors have at their fingertips. A lot of the tools are all about making a difference on the screen. They may be included in the editing package or they may be plug-ins added to the software.

I use some tools not to make a difference but to make sure there isn’t a difference. There are times when shots or sequences leave my realm and are returned to me later in the project. These shots might now include effects or color work. In other cases, someone might have taken my cut and recut for another use, then sent it back to me to finish.

In any case, shots or sequences that I had no control over may be returned to me and I now need to incorporate them into my sequence. In doing so, I want to make sure that the only changes that were made outside my control were done on purpose.

One of the tools I use is found in the composite method assigned to each clip in the timeline. Depending on the application this setting might be called Composite or Blend mode. Usually, a pull-down menu appears when you click on it, displaying a dizzying array of options for this tool. Each one is a formula for overlaying the current clip on the clip below it.

I use some of these selections in order to composite graphics or footage to achieve a certain look, but that’s for another discussion. The option I use for comparison is the Difference mode. Difference mode causes the software to compare the RGB values of each pixel in the clip with the clip below it.

It’s really a simple mathematical equation. Each pixel is defined by its red, green and blue values (RGB). If the RGB values of a pixel in the top clip are R123, G210, B27 and the value of the pixels of the bottom clip are also R123, G210, B27, then when you subtract the RGB values you get 123-123=0, 210-210=0 and 27-27=0. That leaves R0, G0, B0. That represents black. That’s what the image is at the top of this post. That result, from a comparison perspective, is perfect.

So, if nothing changed between the top and bottom clip, you get black. If something did change, you’re left with a different kind of image. That image can be strikingly psychedelic or it can be more nuanced.

Making A Difference With Difference Blending
The Difference mode shows that one version of the shot has been changed. In this case, it has been repositioned horizontally compared to the original.

Using Difference blending, I can quickly compare an offline cut with a finish cut to look for flash frames where edits don’t line up or shots are wrong. Sometimes, there might be accidental scaling applied where the clip has been enlarged or reduced by 1 percent. Or, there might be a clip that has a different speed change applied to it.

Making A Difference With Difference Blending
Even a tiny scale difference of 1 percent will show up.

Quickly scrolling through a sequence that’s mostly black makes the “mistakes” pop out. A 1 percent scale difference is obvious using Difference blending but difficult to see when just looking at the scene.

Making A Difference With Difference Blending
Sometimes speed changes are altered as a project moves through different software. A quick comparison with Difference blending can help sort out the problems.

There are also times when a client might ask if a particular version of a shot was used. A client might call and ask, “Is that the version where we went back and made his eyes a little more intense?”

Making A Difference With Difference Blending
Difference blending can help confirm the use of the right version. Subtle color correction changes don’t look so subtle when compared.

If I wasn’t involved in that revision to his eyes, it might be hard for me to see the difference. Using Difference blending, I can compare the versions of the clips to see changes—they’ll be subtle but easier to see than looking at a split-screen.

Making A Difference With Difference Blending
Need help seeing where compression affects your export?

There are lots of tools available to an editor, but not all their effects show up on the screen. I rarely use Difference blending as part of a composite on a clip, but it is still an indispensable tool. With it, I can make sure I get things right when finishing.

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