The motivation for HDR displays is pretty straightforward. The classic HDR problem is that the world has more range than our monitors can display. So if we can finally buy displays that have that much range, then that display “solves” the problem. And thus, an image on a TV set would look like how our eye would see it if we were actually standing there. Right? Unfortunately, that’s not necessarily true because of the FOV change when you look at a TV versus the real world.
First, here is a homework assignment for you. You don’t even need a camera. Next time it’s about an hour before sunset, look at the horizon, like this:
But this scene isn’t actually what our eye sees. The sun is so bright that our eye does some magic local adaptation, and what my eye sees looks more like this. Please excuse the terrible Photoshop skillz…I did this on my laptop with a trackpad.
The sun is so bright that the trees in front of the sun look nearly black. But the trees that are on the left look normal. The really bright areas “contaminate” the nearby areas which makes them too bright.
There is a biological reason for this effect. Wikipedia as always has a lot of information on the Human Eye. The eye has a static contrast ratio of only 100:1. So if you focus intently on one point and don’t let your eye move, you can only handle a 100:1 ratio, or 6.5 stops. However the eye is constantly moving and it changes the exposure both chemically and by adjusting the iris.
We all know that our iris changes size when we go from a bright area to a dark area, but in reality it is also changing as we look at a static scene. But then our brain essentially stitches together a panorama of images with different exposures. There are two important takeaways here.
- The human eye has a maximum dynamic range of up to 20 stops (1,000,000:1 contrast ratio) for objects that are far apart in the FOV.
- The human eye ohas a dynamic range of only 6.5 stops (100:1 contrast ratio) for objects that are close in the FOV.
Now let’s get back to why HDR Displays are not the end-all-be-all solution. According to the THX HDTV Setup Guidelines you want the TV screen to have less than a 40 degree field of view. Keep in mind, that’s far larger than what your average consumer has. I don’t know many people sitting 6-9 feet away from a 60 inch TV.
There are various estimates for the FOV of human vision but a reasonable, conservative estimate is 120 degrees horizontally. So if you are sitting really close to a HUGE television, you have compressed your FOV by 3x. The really bright sun is now going to “contaminate” 3x as much of the image. In our previous example, the distance between the left and right edges of the screen is too small. The dynamic range of our eye will be closer to 6.5 stops than 20 stops. It would look something like this:
When I actually looked at this scene the trees on the left were perfectly visible, but in this hypothetical case they would be nearly black like the trees just by the sun. Of course, very few people have THX-sized screens. I think of the typical setup as a 42-inch screen 12 feet away. Assuming a 16:9 aspect ratio, that TV would be 36.6 inches wide, and take up a FOV of about 14 degrees as opposed to our 40 degree FOV THX television. In that case, our “contamination radius” would be 8.6 times larger than the original scene.
So here’s how I look at it.
- HDR Displays are awesome. I’ve actually seen a few HDR displays, and they are really cool. Getting more range from TVs is a very, very good thing.
- That being said, perfect HDR displays are not magic. If you have scenes with high contrast ratios, you will still need to massage your data for it to look good on the smaller format. In many ways, it’s similar to what the 3D guys have to go through. We can perfectly simulate an image from two eyes 3 inches apart with a beam splitter. But when you are showing a 6 foot man fighting a 30 foot monster on a 42 inch screen 12 feet away those guys will look like miniatures. While HDR displays are awesome, we’ll still have to massage our data to look right.
- Our eyes don’t have much contrast when things are really close together. Monitors have gotten so bright that I usually have to turn down the contrast when I buy a new monitor. Otherwise I get a headache. It used to be ideal to have black text on white background. But with how bright most monitors are, that’s why more websites now go with off-white text on a dark grey background or vice-versa.
- It’s going to be a long time before we have a complete pipeline for HDR content. You have to change cameras, post production workflow, cable head-ends, satellites in the sky, set top boxes, and televisions. It’s non-trivial to get all of those people to upgrade the software/hardware for HDR programming.