[Warner306]

@madjock,

I missed your bold reply. I don't think anyone would argue that a true 4K master is superior to a 1080p upscale. But that doesn't mean that upscaling doesn't have significant value. And good, sharp upscaling can can sometimes be superior if the 4K source is soft and lacking detail. Not all 4K content is razor sharp.

HDR -> SDR. The tone mapping curve reduces the luminance (Y) of all pixels to fit the value set in target peak nits. The default curve compresses everything into the available luminance of the display. You will obviously lose some highlight detail by doing this, so there is an option at the bottom to sharpen these pixels to make them stand out a little more at a lower luminance.

The gamut mapping algorithm corrects any RGB pixels that don't fit into the gamut after tone mapping. By focusing on luminance reduction and then creating colors, you will end up with some pixels that are out of gamut because each RGB color contains different amounts of white, so they won't scale linearly with reductions in white (luminance). They need to be corrected (estimated) into a value that can be shown by the display's available colors.

The way the pixels that are too bright & too saturated are corrected depends on a balancing act between luminance and saturation. This only applies to any of the scientific tone mapping algorithms. The dumb method simply clips the offensive pixels to fit into the gamut. You can't have a perfect balance of hue, saturation and luminance, so you have to adjust each in certain amounts to make the pixel fit. This method of estimating the pixel color is called hue preserving tone mapping, as the goal is to preserve the hue while manipulating saturation and luminance to find the best balance between the two. Some people don't like the hue preservation method and instead prefer dumb tone mapping. It is all subjective as there is no way to perfectly recreate the original highlight color. It won't scale linearly with luminance. Trying to preserve the hue when tone mapping RGB pixels is recommended by Dolby and other white papers that have been written on tone mapping techniques. Hence, madshi's desire to use this method.

The option to measure each frame's peak luminance should eventually lead to the creation of dynamic tone mapping like Dolby Vision or HDR10+, where the brightness changes based on the max luminance of the scene rather than use a global value like the current HDR10. I don't think this is working, yet, but should be a significant advantage when it is available.

Someone can correct me if any of that is not 100% accurate.

Edit: To add to that, the quick math showing how far off colors can be would be as follows:

- If colors are calculated as floating point values where 0 = black and 1.0 = white, then the PQ gamma states 1.0 = 10,000 nits.

- A 2,000 nit master then has an approximate (likely not exact) maximum value of 0.2 (0.2, 0.2, 0.2), which is pure white.

- If the target is 100 nits, then the scaling factor is 10,000/100 = 100 times.

- If you multiply 0.2 x 100 = 20. This is 20 times the maximum displayable value of 1.0, so this pixel will have to be tone mapped.

- If the max value is twenty times larger than the target gamut (0.0 to 1.0), then you can imagine how many values have to be reduced to fit a 100 nit gamut.

So tone mapped images will never look 100% identical to the 4K master because so many values have to be changed to fit into the gamut.

Well, I hope that math is right. The 10,000/100 might not be correct.