Filmic Tone Mapping Questions

Frank Sullivan · 2014-01-09T00:35:39

Greetings, all. I am currently trying to implement Hable's filmic tone mapping as described in this blog post. There are a lot of parameters to tweak, though, and I'm having trouble getting it right. But first, would anyone mind taking a look at my algorithm and seeing if I'm basically doing things right? I'm using OpenSceneGraph for this project, but I'll describe everything in terms of the underlying OpenGL. Right now, my scene is just a skybox using SilverLining. According to the SilverLining docs, everything is rendered in units of kilocandelas per square meter. Here is what it looks like with SilverLining's built-in tone mapping: Which, in my opinion, looks very nice. Sundog's tone mapping operator looks very complicated, though, and difficult to reason about. It looks very different than the tone mapping operators that I have seen used here and elsewhere on game development blogs. But...luminance is luminance, right? I feel like I should be able to get good results with, say, Hable's filmic tone mapping operator. So here is the algorithm I'm using. 1. Render everything to a floating point texture. I've created an FBO to render everything to. It has a 16-bit floating point color attachment (GL_RGBA16F) and and depth stencil attachment (GL_DEPTH24_STENCIL8). I am rendering the scene into this FBO. The dimensions of each are 1920x1080. 2. Calculate the average luminance of the scene. For this, I'm binding the aforementioned floating point texture, and rendering it into another floating point texture. This one is 512x512. However, I am not storing the color values in this texture, but rather the log of the luminance. Here is my fragment shader code: uniform sampler2D baseTexture; varying vec2 texcoord; void main() { vec4 color = textureLod(baseTexture, texcoord, 0); float L = 0.2126 * color.r + 0.7152 * color.g + 0.0722 * color.b; float r = log(L + 0.001); gl_FragColor = vec4(r,r,r,1); } I think call glGenerateMipmap to handle the averaging. Incidentally, I don't see how this algorithm matches what is called for in equation (1) of Reinhard's paper. It seems to be more similar to this (I'm including the exponentiation, which takes place in the next step): Although I admittedly have not done the algebra yet, this does not seem to be the same as what Reinhard suggested, although it does seem to be a more proper geometric mean than what Reinhard suggested. 3. Exposure Control and Tone Mapping I'm including these as one step because I'm doing them both in the same shader. For exposure control, I'm using equation (2) from Reinhard's paper. In order to get the average luminance, I sample the texture from step 2. In doing so, I sample the second-to-last mipmap level (the 2x2 level). This is to give the tone mapping some semblance of locality, although I suppose I should probably worry about such flourishes when I have the algorithm working properly. To do the tone mapping step, I am using Hable's filmic curve. I pulled the meaning of the constants from his presentation. Here is the fragment shader code: uniform sampler2D baseTexture; uniform sampler2D avgLumTexture; varying vec2 texcoord; uniform float A; uniform float B; uniform float C; uniform float D; uniform float E; uniform float F; uniform float W; uniform float key; vec3 Uncharted2Tonemap(vec3 x) { return ((x*(A*x+C*B)+D*E)/(x*(A*x+B)+D*F))-E/F; } void main() { vec3 texColor = texture2D(baseTexture, texcoord).rgb; float avgL = exp(textureLod(avgLumTexture, texcoord, 8).r); float L = 0.2126 * texColor.r + 0.7152 * texColor.g + 0.0722 * texColor.b; float exposure = key * L / avgL; texColor = texColor*exposure; vec3 curr = Uncharted2Tonemap(texColor.rgb*2.0); vec3 whiteScale = 1.0f/Uncharted2Tonemap(W); vec3 color = curr*whiteScale; vec3 retColor = pow(color, 0.45); gl_FragColor = vec4(retColor, 1.0); } A quick note about gamma correction: according to the SilverLining documentation, disabling their tone mapping (which is what allows me to use my own) also disables gamma correction. So, all of the values I get from them should be linear. Here are the results, using the values for A,B,C,D,E,F,W that are hard-coded in the code sample on Hable's blog (which I did not expect to work for my own scene, but I use them as a starting point: In the upper right corner, you can see my average luminance texture. It's using the 2x2 mipmap level, as you can see. When displaying that little debug square, I am exponentiating my sample so that it's back in linear space. I am also multiplying by 0.1 because most of the luminance values in this scene are between 0 and 10 and so it gives me a good idea of what I should be seeing. In the bottom left corner, you can see the values I'm using for Hable's coefficients, as well as a graph of the curve. I should probably put axes and guidelines on that graph so I get more information than just the shape, but this is what I have for now. In any case, I can edit these values on the fly, and sort of play with things until they look right. They "key" value (variable a in Reinhard's equation) can also be modified on the fly. My problem is, those cloud bottoms are extremely dark (about 0.4 kilocandelas per square meter, according to gDEBugger) while the cloud tops are extremely bright (about 5.5 kilocandelas per square meter). Any attempt I make to lighten those blacks seems to wash everything out and desaturate the whole scene. If I then modify another parameter (say, the white point) to make things less washed-out, it invariably has the effect of darkening those cloud bottoms again. So I guess I have a few questions: a) Do I appear to be doing this all correctly? I guess there is no sense in sitting here, tweaking the numbers, if I don't have the basics right. b) Does anyone know of any parameters I can use that should be reasonable for outdoor scenes, especially for those using SilverLining as a skybox? c) If not c, does anyone have a better intuition for how these parameters work so that I can find the right combination? Will I need an entirely different combination for nighttime scenes?

Graphics and GPU Programming Programming OpenGL

Started by CDProp January 07, 2014 01:42 AM

9 comments, last by Hodgman 10 years, 3 months ago

Hodgman

52,717

January 09, 2014 12:35 AM

Indeed the graph has been very helpful. I have one (see my second screenshot in the OP), but it uses a linear scale. Do you mean that I should graph it with a log2 scale on the y-axis? Won't that look weird for values between 0-1?

The y-axis is from 0-1 and the x-axis is from 0-maxLuminance. I think MJP is suggesting to change the x-axis to be from 0-log2(maxLuminance).

e.g. here's my tone-mapper with luminance on x and final pixel value on y:

(left is 0-1, middle is 0-10, right is 0-1024 input range)

Note how I need to zoom into the graph in order to see the detail at the bottom-end, and zoom out to see the detail at the top-end.

And here's the same function graphed with the x-axis changed to be logarithmic (still 0-1024 input range)

As you can see, the small bottom-end toe of the graph is now easily seen, along with the linear section and the top-end shoulder. It gives a better view on how the tone-mapper treats all inputs.

I don't know that I want the exposure to change that much just because a dark (or light) object moved into view. Is there a standard way of limiting the effect of the exposure control, or should I just get creative and fiddle with the formula a bit?

I'm not sure what the standard solution is to prevent your camera from having infinite sensitivity (a real camera has limits on exposure settings, or at least side-effects -- e.g. long-exposure photos introduce motion blur, or high-ISO settings increase image noise, or wide-apertures reduce the amount of depth that's in focus, etc). My solution was to give artists control over a minimum and maximum average luminosity value, which were used to clamp the actual average lum.

e.g. if the artists set the minimum-avg-lum to 0.5, but the actual measured avg-lum is 0.001, then the tone-mapper uses 0.5 as the avg-lum value.

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Filmic Tone Mapping Questions

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