4.2. Tone Mapping Practice

There are lots of different possibilities, with a wide scope of results and more powerful options than ever before in digital imaging. And to quote Uncle Ben, "Remember, with great power comes great responsibility".

True tone mapping: True tone mapping is an invisible art, very much akin to the art of visual effects. The goal is to have the final image appear as natural as possible, natural being defined here in the sense of appearing as if the image had never been touched. It's not about artistic expression. It literally is about creating an image that looks like it was shot with an ordinary camera but incorporates more dynamic range than a camera could actually handle. Here HDRI and tone mapping are just tools used to get around hardware limitations. The advantage can be highlight recovery, noiseless shadows, or mastering difficult lighting situations. Processing artifacts that draw the attention away from the main subject is avoided at all costs. In tone mapping, the most common artifacts are halos, excessive coloration, and when all the tonal range is spent on details but none on the global contrast.

Excessive digital processing is nothing new. Since the invention of the Saturation slider, it's been very easy to overdo an image. With an HDR image as the starting point, there is just more information to pull from, and especially with local tone mappers there is a huge temptation to go too far. Global tone mappers are generally safe to use for this because they are pretty immune to artifacts. Yet there is more to gain from local TMOs. It is a very thin line between too little and too much, and it's certainly a matter of personal opinion.

True tone mapping is probably the hardest and yet least-appreciated application. Drawing the parallels to visual effects again, it's the invisible effects that require most work and carefulness. Everybody can see the special effects in a space battle. It just has to look cool. But let me tell you from experience that it is much harder to digitally replace a live-action airplane with a CG model, just because the story calls for a 747 instead of an Airbus. It takes immense effort, and you have been successful only when nobody sees that you did anything at all. That's why this kind of tone mapping is not in the public spotlight. It has less of a wow effect. Someone barely ever takes notice. Only when you see a before and after can you appreciate the achievement.

Figure 4-31. True tone mapping: The invisible art to create a natural looking image.

New impressionism: If you don't care about the natural look but rather want to create a piece of art, then you will just love how local TMOs allow you to squeeze every little detail out of an image. Once you completely twist the ratio of detail contrast and overall scene contrast, the image turns into something painterly surreal. We are not used to seeing that in a photograph, but painterly is the key word here. Painters cheat the scene contrast all the time; it's part of their artistic arsenal. With advanced local tone-mapping operators, this is finally part of a photographer's toolset as well.

We've discussed before how traditional photography and the sensation of human vision are two different sides of a coin. And when an artist creates an image that matches his personal impression of a scene, there is no point in arguing that it doesn't look real. If you're looking at a Vincent van Gogh painting and saying, "This is crap—sunflowers don't look like that", then you're missing the point of what art is all about. And people would look at you funny. At least now they would; back in the days when painting started to convey these new forms of artistic expression, there was a lot of resistance from the traditionalist front, calling it a trend that would fade soon. History repeats itself, but this time it is photography that is evolving away from purely realistic imagery, towards something that is more of an artist's personal impression of a scene.

Figure 4-32. Impressionist tone mapping: An artistic interpretation of a scene.

Local tone mappers are specifically designed to emulate aspects of human vision. It's worth exploring their possibilities and taking photography to the next level of artistic expression. So there you go: start playing, express emotions, find new styles! Just do me one favor: Don't call it HDR anymore. New styles deserve a new name. Call it impressionist photography, or hyper-realism, or anything original. HDRI is a tool, not a particular style.

Texture extraction: When people think about using HDRI for computer graphics, they often overlook the applications of tone mapping for preparing reference images. The importance of good reference material can not be overstated. Good reference can only be beaten by better reference. Ideally, it should be free from direct illumination—nothing but texture, detail, and shape.

It's not always that easy to shoot. What do you do on a car commercial spot in the middle of the desert? The car doesn't actually run; this is going to be your visual effect. It's noon and there is no shade anywhere. There is a group of people called crew, but they don't listen to you, and they certainly won't push that car around just so it's lit nicely for you. Instead, they give you precisely 15 minutes to do your thing and then they leave for lunch break. What do you do? Well, you take what you can get and shoot brackets like a maniac. Back in the office you have all the time in the world to merge, stitch, and tonemap.

Figure 4-33. Texture extraction: Eliminating light and shadow influence via tone mapping.

For example you could stitch a 7,000-pixelwide panoramic strip out of three views, with nine exposures each. Like this one.

The point of tone mapping in this context is to take the lighting out. Once the image is texture-mapped on a 3D object, you will be applying your own light anyway. Shading, shadows, glare, reflections—all these things do not belong in a texture map. So, tonemap like the devil, and use local operators to suck all texture details out.

See you later: At this point I would like to clear the stage and leave you in the capable hands of two professional photographers. From a photographer's standpoint, tone mapping is the creative heart of high dynamic range image processing. You can create or emphasize the message of your images by opting for contrast augmentation, for creating images that look realer than real, or maybe heading for an impressive painterly look. The image data, the "meat", is there—inside your prepared HDR files—sitting and waiting for you to let them shine through.

To give you a taste of what is possible, and how to do it, on the following pages you'll find some inspiring and challenging HDR/tonemapping projects done by Uwe Steinmüller and Dieter Bethke. Let's find out how HDRI and tone mapping have changed their creative flow and how all these new opportunities are applied to fine art photography and commercial photography.

Ladies and gentlemen, please welcome Uwe Steinmüller and Dieter Bethke!