IOI released an FAQ detailing some of the technical changes. They also commented on the lack of PC version.
https://www.ioi.dk/hitman-hd-faq/
They also shared some screenshots at the link.
https://www.ioi.dk/hitman-hd-faq/
Controls: What has changed?
- To ensure a more fluid experience, we have updated the controls to fit current gen consoles with smoother analog stick handling and better response.
- The button mapping has been updated to match current gen controllers and maintain peak performance.
What has been done to enhance the tech specs?
4K visuals at 60 frames per second:
Increased texture resolution:
- Depending on your console and on your TV different approaches are taken. If you have a 4K TV and a XBOX ONE X or PS4 PRO we upscale the picture to 4K and then draw a crisp 4K GUI on top.
- If you have a 1080p TV we will render in higher than 1080p resolution internally and then downscale to 1080p to increase quality and reduce jaggies. On XBOX ONE though we're rendering in 1080p/60 FPS.
Upgraded texture formats:
- Textures are usually authored in higher resolutions and then downscaled to fit in memory.
- With more memory available on the newer consoles this downscale is not necessary anymore and the textures can be enjoyed in their authored resolutions.
Super-sampling & upscale support settings:
- The new consoles have support for newer and improved texture formats.
- We use these where applicable for better-looking textures with fewer artifacts.
Heightened shadow map and mirror resolutions:
- Xbox One 1920 x 1080
- PS4 Base 2304 x 1296 (44% more than 1080p)
- PS4 Pro 2560 x 1440 (+177%)
- Xbox One X 3200 x 1800 (+277%)
Improved lighting:
- We have increased the resolution of shadow maps to get better and crisper looking shadows.
- Likewise, for mirrors – reflections are sharper than ever.
- Lighting calculations have been re-engineered. Back when the original launched it was a popular optimization technique to bake some lighting calculations into textures to save work on the GPU.
- These days it is more effective to just do the calculations and it also avoids the banding artifacts caused by using a texture. Together with higher resolution and higher precision internal render buffers it gives you a better quality image – especially noticeable with specular highlights which are much smoother now.
Contracts Mode: Where is it? Why is it gone?
Ever since the change to remove contracts mode from Absolution was forced upon us due to GDPR rules and the fact that we do not have ownership of the Absolution Contracts Mode servers, we've been looking into ways to restore it. We will continue to investigate other possibilities, but we can't promise anything specific at this point in time.
Why no PC version?
The key focus with this collection was to bring these classic games into the modern age to be playable on current gen consoles (PS4 / PS4 Pro and Xbox One / Xbox One X).
Since PCs have been able to keep up with modern technical demands, we wanted to give our console players the opportunity to enjoy these games in high definition quality.
They also shared some screenshots at the link.