Overlay & GreyScale Worked - I just don't Understand Why

[lv2race]

I've been converting my VHS tapes to digital and have been using Avisynth to do minor color corrections. For the most part it works out OK using ColorYUV(cont_u, cont_v) but sometimes the colors don't look as good as I would like.

Last night I was reading some threads on Overlay, and decided to give it a try. I've never used it before, more or less understand the concept, but not the application. I copied a script from one of Jagabo's posts, edited the settings, and low and behold - like a miracle, everything looked better. I just don't understand why, and really what I'm doing.

Original video:


[Attachment 68054 - Click to enlarge]

Script using Overlay and Greyscale:

org=last
ColorYUV(gamma_y=0,off_u=-35, off_v=20, cont_U=51, cont_v=51)
StackHorizontal(Subtitle(last, "without overlay"), Overlay(org,last,0,0,GreyScale(org).ColorYUV(cont_ y=100, cont_U=0, cont_v=0)).Subtitle("with overlay"))

Resulting video:


[Attachment 68055 - Click to enlarge]

You can ignore the image on the left, that's what it would look like without Overlay, but I was surprised with the image with Overlay that it looked good to me.

Can someone explain to me what's happening? The help file on GreyScale is pretty sparse.

Thanks

[_Al_]

This is mask:

GreyScale(org).ColorYUV(cont_y=100, cont_U=0, cont_v=0)

This is color change for original

ColorYUV(gamma_y=0,off_u=-35, off_v=20, cont_U=51, cont_v=51)

Mask is black and white video, no colors, then further whites are more blown (cont_y=100). The way mask works in Overlay, if pixel is more white then you end up with more pasted pixel (from last) instead of original (from org).

So effectively, highlights from your original video are replaced more than darker areas.
If you'd have a videoeditor and doing color corrections, you could tell to change colors in dark areas, midtones or highlights only. In Avisynth you can use Overlay to do that by defining a mask.

[lv2race]

Thank you for the explanation!