Understanding Tone Mapping in 3D Rendering and Games

Tone mapping (also known as view transform in Blender) is a technique used to convert high dynamic range (HDR) images or renders into a format that can be displayed on standard dynamic range (SDR) displays. This process is essential for making the final image look visually pleasing on devices with limited brightness and contrast capabilities, such as computer monitors or televisions.

If you’re new to this, you may ask, “Why do I need tonemapping? I’m not using HDR.” The reality is that most of the renders you make in your 3D software (such as Blender) are HDR because they contain pixels that are brighter than the maximum brightness displayable on your monitor. Let’s take a look at this example:

The above render doesn’t have tonemapping; RGB values higher than 1 (the maximum) are clipped to 1. As you can see in the area around the bright sun, the clipping doesn’t look very good. Tonemapping takes care of this by compressing the dynamic range.

If you’re wondering what the scene would look like if you simply lower the exposure until there is no clipping, it’d have to be extremely dark because of how bright the sun is:

Clipping also is the cause of what’s known as the 6-color problem. With hard RGB clipping, saturated bright colors approach six colors the more they clip:

Red
Yellow
Green
Cyan
Blue
Magenta

In my opinion, an essential quality of good tonemappers is their ability to avoid the 6-color problem.

There are many tonemapping algorithms out there. Here are 3 tonemappers compared to standard RGB clipping:

Each of these 3 tonemappers must trade off between saturation, perceived hue accuracy, and perceived brightness, and they do so in different ways. Why? Because there comes a point where a color (such as a saturated red) cannot be brightened any more without clipping. In order to make the color appear brighter, math must be done on the RGB values. One visually pleasing way to do it is to gradually desaturate the color to white, for white is the brightest displayable color.

Which tonemapper does the best job of “rolling off the highlights” as we call it? First of all, it’s definitely not Filmic; Filmic suffers from the 6-color problem. It’s one of the reasons why Blender 4.0 made AgX its factory default view transform. AgX doesn’t have the 6-color problem, and it is very smooth, but it lacks in saturation. Khronos PBR Neutral is another tonemapper, added in Blender 4.2, that allows for high saturation at the cost of some unpleasant artifacts. Third-party tonemappers (e.g. AgMax, which is our tonemapper offering at CG meerkat) exist too. The rest of this article compares 2 built-in (AgX and Khronos PBR Neutal) and 1 third-party tonemapping solution in Blender (AgMax).

Qualities of Good Tonemappers

1. Avoidance of the Six-Color Problem

Being able to compress the dynamic range of bright colors without the 6-color problem is the most important quality of making a visually pleasing tonemapper.

2. Maintaining Relative Brightness

A tonemapper should maintain relative brightness. What does this mean? If color A in an image is brighter than color B before tonemapping is applied, color A should still be brighter than color B after tonemapping. This makes sense intuitively, but some tonemappers fail at it.

In the above comparison, the tonemapper on the left makes the blue of the water around the reflected trees appear darker than the trees. Looks strange and unrealistic. In the original scene, the blues around the reflections of the trees in the water are lighter than the greens of the trees’ reflections. The tonemapper on the right maintains relative brightness.

3. Accommodating for Quirks in Human Visual Perception

This point is up for debate, but I think tonemappers ought to compensate for quirks in the way humans perceive color. Quirks like what? The Abney Effect.

The Abney effect, also known as the Abney effect or Abney's effect, refers to a phenomenon observed in color vision where the perceived color of a light can shift when the intensity of the light changes, particularly when the light is mixed with different levels of white or gray.

When a pure color light is mixed with white light, the resulting color can change in hue depending on the intensity of the added white. For example, mixing white into a blue color makes it appear more violet or purple. Even though the RGB hue is the same, the perceived hue is not pure blue.

Because tonemappers roll off bright colors towards white, an Abney Effect that doesn’t exist in the colors of 3D scenes is produced through the tonemapping process. Visually pleasing tonemappers compensate for the Abney Effect by rotating hue the more a color gets whitened by tonemapping.

Let’s look at a few examples.

AgX, Blender 4.0+’s default tonemapper, almost compensates for the Abney Effect. To my eyes, a little bit more hue rotation is needed for the blue. Khronos PBR Neutral, a tonemapper included in Blender 4.2+, does not compensate for the Abney Effect whatsoever, so the bright blue appears purpleish.

4. No Hue Shifts Unless Compensating for Perception

While a tonemapper needs to perform hue rotation to compensate for quirks in humans’ vision, it should not do hue rotation where such compensation is not needed (such as on a dark color). Unnecessary hue rotation is a side effect of less-than-ideal tonemapping algorithms. AgX suffers from this:

A BlenderArtists user reported that AgX changes the hue of a color that is not bright enough to need to be even touched by tonemapping.

5. Vibrance

Visually pleasing tonemappers must desaturate colors to avoid the 6-color problem and to smooth out the rolloff curve (see #6). However, it’s important for colors to not be desaturated so much that the tonemapping takes away from the inherent vibrance of the 3D scene.

AgX strongly desaturates. Khronos PBR Neutral is optimized for maximum saturation, but it fails the smoothness test (see #6).

6. Smoothness

Tonemappers shouldn’t have abrupt changes in the ways they deal with compressing dynamic range. See below a shortcoming of Khronos PBR Neutral:

Hands down, without a doubt, AgMax is the best all-around tonemapper for Blender today. On top of all the things it excels at, its parameters are adjustable.

Get AgMax

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