Camera white balance

Courtesy Mihai Iliuta

The term white balance in photography refers to removing any color casts from an image which may be due to the spectrum of light that was illuminating the scene. You adjust the white balance so that white objects in the image appear white and thus all other colors in the image also display correctly. Maxwell Render has a white balance tool in its camera parameters since V3.1.

Why the need for white balance in film/digital cameras

To understand why a white balance tool is needed in the real world, we must first look at how different light sources emit light. For example in a light bulb, a metal filament heats up so much that it starts to emit light. The temperature of this filament, is said to also be the "temperature", or spectrum, of the light that's radiated from it, noted in Kelvins, or "K". The lower the temperature (for ex. a "3200K" light source), the more red/yellow the light will be, and the hotter it is (for ex. a 6000K light source), the bluer the light will be. Other light sources, such as fluorescents, emit colors not only in the yellow to blue range, but also in the green to magenta range. The human eye can adapt very well to these different colors because our brains interpret it as "white" light. However for a film or digital sensor it poses a problem because it can't do this adaptation by itself. This is where a white balance tool comes in handy, which tells the camera - even though the light you are receiving is in the "yellowish" spectrum, transform your color response to it, so that it registers as white.

Usefulness of a white balance tool in Maxwells virtual camera

In the real world, you can't change the temperature of a light source freely, thus you must be careful to properly adjust your cameras white balance to avoid any strange color casts, or white balance your photos in post, using the RAW file of your camera. In Maxwell however, you are free to specify any temperature (color) for your emitters, including the Sun in the Physical Sky. The Maxwell camera is set by default to a white balance of 6500K, which is considered a natural daylight white balance, so if you wish to have all your emitters emit perfectly white light (as far as Maxwells camera is concerned), simply set them all to 6500K and you will have a perfectly white balanced render. So how can a white balance tool still be useful when rendering? Here are a few reasons:

• Purely artistic reason: You may not always wish to have a perfectly white balanced render as it may look too clinical and boring. A slightly warmer tone for example may be more appealing in some cases. In other cases maybe a strong blue cast is what you want.

• In situations where you have mixed lighting temperatures (Physical Sky (around 6500K) mixed with interior incandescent lighting (around 3000K)), it's nice to be able to adjust the white balance depending on if you wish the incandescent lights to appear colder, or the Physical Sky light to appear warmer etc.

• Have a more accurate "starting point" when compositing a rendered object with a photo. It may be the case that the photo was taken in an interior with low temperature lighting (for ex. 2400K) and then the white balance adjusted to make it look less yellow, but still maintain a distinct warm look. By starting in Maxwell with emitters set to 2400K, and then white balancing the render, you will have a more accurate color response and also know to set the white balance to the same value as for the photo, thus making the composite easier and more realistic.

In all of the above examples it is important to note that white balancing directly in Maxwell (during or after the render is done) is more beneficial than doing it in post. This is simply because Maxwell internally works with a very large color spectrum, much larger than even AdobeRGB or Prophoto color spaces and all this color information is stored in the MXI format. Thus the white balance can work more accurately as it has more color info at hand. This is similar to white balancing a RAW file from your digital camera, which also contains more color information than even the Prophoto color space. If you however first export your render, saving it even with a wide color space such as AdobeRGB, and then do the white balancing in in image editing application, the result will not be as accurate - some colors will still look strange (depending on what the initial temperature of your emitters was set too). When dealing with white balance, it is the color space that is important, and not necessarily the bit depth. A tiff file of your render for example, saved in the sRGB color space, even if saved as 16bits, will have discarded a lot of color information.

Parameters

These can be found under the "Tonemapping" section of the Camera panel in Maxwell Render. Please note that they do not appear in the camera panel in the Maxwell Studio interface.

White point

Lets you adjust the white point of the render, meaning the yellow to blue range (lower K values make the render bluer or "cooler", higher K values make it more yellow or "warmer").

Tint

Controls the green to magenta range. Emitters in the real world not only emit bluer or more yellowish light, but can also emit a more greenish light, especially fluorescents. For this reason a complete white balancing tool also needs to control the green to magenta color cast in a photo. Negative values add a green tint, positive values add a magenta tint.

From left to right: Default white balance (6500K), colder render by setting the white point to 3000K, warmer render by setting the white point to 10 000K. Click to enlarge.