Hi,
I'm gathering all the information I can get my hands on in the prospect of implementing a simple spectral ray tracer. Currently I'm stuck on HDR - I understand the concept but I have some trouble grasping the theory. I am working on Reinhard's operator at the moment.

As I understand, HDR images cannot be displayed on LDR monitors because the luminance range in the image exceeds what the monitor can display - so the luminance has to be scaled somehow. This is done by considering the per-pixel luminance in the image and processing them to compress the HDR image's luminance range in a way that's perceptually acceptable.

I have done some tests with this algorithm, which would seem to be the real deal as it produces fairly nice tone-mapped images but some things are still unclear:

1. calculate the maximum luminance in the image, using the standard CIE luminance curve [eqn]L = 0.2126R + 0.7152G + 0.0722B[/eqn]
2. for each pixel, scale its luminance [eqn]L[/eqn] as:

[eqn]\displaystyle L_{\mathrm{scaled}} = L \left [ \frac{1 + \frac{L}{L^2_{\mathrm{max}}} }{1 + L} \right ][/eqn]
3. thus scale the pixel's RGB components by multiplying them with the luminance coefficient: [eqn]\displaystyle k_L = \frac{1 + \frac{L}{L^2_{\mathrm{max}}} }{1 + L}[/eqn]
[/quote]

First, should the maximum luminance in the image be used for [eqn]L_\mathrm{max}[/eqn]? In this reference they call it [eqn]L_\mathrm{white}[/eqn] and state it should be the luminance that will be mapped to a LDR luminance of 1 (i.e. maximum). This suggests that it could be anything, depending on the exposure we want to obtain - should the maximum or the average be used? Or perhaps the median? Maximum seems to produce the nicest results on most of the HDR photos I tested but is sometimes too dark. Average is often far too bright, but is sometimes spot on. What gives?

Thanks!

In Reinhard's original paper he referred to it as Lwhite, because its "the smallest luminance that will be mapped to pure white". He came up with that formulation so that you could allow the image to "burn out", such that a certain subset of high values mapped >= 1.0. Using your maximum luminance means that absolutely everything will end up in the [0, 1], but that's already what you get (1 /(1 + x)) which is his original formula. Ultimately some amount of burn out is usually desirable, so that you don't compress your mids and lows too much.

In Reinhard's original paper he referred to it as Lwhite, because its "the smallest luminance that will be mapped to pure white". He came up with that formulation so that you could allow the image to "burn out", such that a certain subset of high values mapped >= 1.0. Using your maximum luminance means that absolutely everything will end up in the [0, 1], but that's already what you get (1 /(1 + x)) which is his original formula. Ultimately some amount of burn out is usually desirable, so that you don't compress your mids and lows too much.

Oh, you are correct, I had gamma correction in one test and not the other so they came out different - maximum is the same as the simple formula, indeed.

So I take it I need to take the maximum luminance and multiply it by some "burn out" coefficient (between 0 and 1), and then use that as Lwhite? Is there some way to estimate a good coefficient or is it really trial and error? I understand Reinhard is a global operator and as such won't always look nice on every image (an adaptive algorithm would be a more sophisticated solution).

Ah, I missed the "key" part in the paper - I need to use the summation formula to estimate the total luminance in the image on a logarithmic scale and work with that, if I understand correctly.

EDIT: ok, I got it working - Reinhard works superbly when the luminance range isn't excessive, after which it starts to break down (everything becomes dark) - I think a local operator is required for those pathological cases, as indicated in the paper.

When I last worked with Reinhard we just let the artists pick the white point, which they chose according to taste. The only attempt we ever made at automatically picking or guiding a value was to determine the maximum luminance, and that doesn't work out well. We've been using a filmic curve for a long time now at work, and that's been much easier for the artists to work with. We just settled on a curve they were happy with, and now they just tweak exposure settings rather than the curve itself. I'm not sure if your idea with the burn out coefficient would work well in practice since I haven't tried it, but it sounds like it could work.

The bit about the "key value" and total luminance in the paper deals with exposure, or "calibration" as it sometimes referred to in literature. The goal of this part is to come up with a linear scale value for your pixel intensity so that the entire is "shifted" so that the relevant parts of the image hit the part of your tone-mapping curve that produce results with decent contrast. It can be considered independent of the tone mapping curve, and I like to think of it that way. The approach suggested by the Reinhard paper (which is commonly used in games) is to compute the geometric mean of luminance for the entire screen, which you can compute by computing log(luminance) for every pixel, computing the average, then calculating exp(luminance). The key value is a scalar that works in conjunction with this, and controls how brightly or darkly the algorithm will expose your scene. When I explain it to the artists I usually tell them that it's sort of a "mood" slider, where darker values correspond to darker, "moodier" scenes while higher key values correspond to brighter, "happier" scenes. If you're familiar with photography then you can think of it as lower values mapping well to low-key lighting and higher values mapping to high-key lighting.

We ended up with something similar to what MJP's describing, except that we were new to all this so experimented with a lot of different formulas, and might be doing things that aren't quite right
We let the artists choose between reinhard and filmic curves (they choose filmic for almost all scenes, and used a curve-editor to tweak it slightly from our initial curve values), and then let them specify the "white" saturation point and a middle-grey "key" value (pretty sure this bit is a bit fudged in our code). The "white" value is used in the tone-mapper, but they "key" value is used beforehand for automatic exposure adjustment. The geometric mean luminance and the key are used to adjust each RGB value before tone-mapping.

I plotted it and discovered that it had a s-curve as well, so I modified it a bit and got pretty much the same results as the Uncharted 2 Filmic Tone Mapping operator. I even think that my modified Reinhard operator is actually pretty good, since it's fast, filmic and is easily configurable.

Have you some comparision shots (color gradient in linear,reinhard,uncharted and modified reinhard) ? This would be quite interesting.

Why is the Reinhard Tone Mapping operator considered "non-filmic" anyway?

I don't know if this is right, but I use "filmic" to refer to "the John Hable curve".
If someone said "a filmic curve", I would think of an S-curve, but if someone said "the filmic curve", I would assume they were quoting Hable (though he called his curve "Uncharted2Tonemap", not "the filmic curve", I've still seen enough people call refer to his "Filmic Tonemapping Operators" blog post calling it the filmic curve. Also note that Reinhard is listed in that blog post).
When I was explaining hable and reinhard to our artists, I described the former as Uncharted's curve to make it look like a film, and the latter as a curve from an academic trying to make digital HDR photos look like traditional photographs.

p.s. I just had a muck around with your modified Reinhard in a graphing calculator - it looks pretty useful. I might have to try it out on some test scenes tomorrow

Why is the Reinhard Tone Mapping operator considered "non-filmic" anyway? I plotted it and discovered that it had a s-curve as well, so I modified it a bit and got pretty much the same results as the Uncharted 2 Filmic Tone Mapping operator. I even think that my modified Reinhard operator is actually pretty good, since it's fast, filmic and is easily configurable.

To me "filmic" implies that your curve is aimed at emulating the actual response of film, which was definitely not Reinhard's goal. In fact in his paper he specifically mentions avoiding a filmic s-curve in favor of a curve that only compressed higher luminances. So although the curve is still "s-shaped" when plotted on a log graph the "toe" portion is visibly less severe compared to a an actual film response curve, or even the curve that Hable originally suggested that was based on a particular Kodak film stock. The curve is also steeper in film curves, while shoulder rolls off more quickly. This is all done to boost perceived contrast, in order to combat Stevens Effect (perceived contrast is lessened at lower luminance) and Hunts Effects (perceived "colorfulness" is lessened at lower luminance). For some reference, here's a screenshot showing a Reinhard curve, and another screenshot showing a typical filmic curve. By changing Reinhard's curve to have similar properties you're really fundamentally changing it from Reinhard's original intentions, so I don't think it would be right to call it "Reinhard" at all.