Gaining shadow detail with Contrast Mask.

[racer-x]

A very good trick to improve shadow detail in an image without blowing out highlights is to use a Contrast Mask. You can do this in any image editor that supports layers and blending modes. Paint.NET and Gimp are good free ones that can easily do this. It's basically a 4-step process. It also works in video if your video editor supports multiple tracks and blending modes. A good free one is Aviutl.

1) Duplicate layer with image or video.
2) De-saturate (turn black and white) overlaying layer.
3) Invert colors on overlaying layer and switch blending mode to "Overlay".
4) Apply a Gaussian blur to the overlaying layer to bring back sharpness.

I made the before/after image in Paint.net and video in Aviutl: