The Foundry's powerful tool Nuke can be used for lots of things, one of them being great upscaling of HD footage to 4K without any plugins.
This quick tutorial will show you how to upres HD footage to 4K using Nuke and the included TVIScale node.
Here are the steps:
1. Blur Blue
The first step is to add a Blur or DenoiseBlue node. Be sure to only blur the Blue (RGB, B) channel.
2. Change the ColorSpace
Add a ColorSpace node and set its' "Out" value to "YCbCr".
3. Blur the Green and Blue channels
Add another Blur node and check the channels Green (G) and Blue (B). Leave the settings at default or adjust them to your preference.
4. Change ColorSpace to Linear
Add another ColorSpace node and set the node's input to "YCbCr" and its' output to "Linear".
5. Add TVIScale node
Now add the TVIScale node to your project. No settings are needed.
6. Enter the Matrix
Now, add a Matrix node and enter the following values:
1 - 2 - 1
2 - 4 - 2
1 - 2 - 1
This ensures that the pixels are distributed evenly across the new resolution.
Add a Sharpen node and set it to whatever you feel works for your footage. This can be left at default if you just want a tiny bit of sharpening.
8. Change output format
Lastly, add a Reformat node to change the current resolution to your desired output resolution.
While it's difficult to make low-res footage to look hi-res, this upscaling method gives you the ability to both sharpen and smoothly upscale the footage to 4K. I've tried this method on loads of stuff, and it works like a charm.
Zoom in closely if you can't see the difference, and you'll start noticing that the image is as smooth as it can be in the new upscaled resolution.