In photography, HDR can be obtained using a wide range of techniques. A composite image is typically created by fusing many exposures to highlight contrast and minor details. Postproduction can use the tone mapping technique, also referred to as local tone mapping. When working with applications demanding high dynamic range photographs, the idea of tone mapping is essential. Tone mapping can be completed with a single exposure, negating the requirement for multiple exposures.
What is Tone Mapping?
Pixels, the building blocks of an image, come in a variety of colors, including black and white. Tone mapping is a technique for digitally modifying the tonal values of individual pixels that involves converting digital signals to the proper light levels depending on the HDR meta-data.
The dynamic range of real-world scenes is significantly higher than what a camera can capture. We now have sensors and ISPs that enable us to capture photos with a considerably higher dynamic range due to technological advances. However, the dynamic range of display devices is limited.
Although the fundamentals of this technique are relatively complex, knowledge of high and low-dynamic range images can make things simpler. A high dynamic range image, or HDR, has a variety of tones, or levels of lighting and darkness. This implies that there is a significant difference between the lightest and darkest tones of color in an HDR photograph. To display HDR photographs on low dynamic range displays, the captured high dynamic range should be scaled down to the display device’s available range. This technique is referred to as “tone mapping.”
Tonal Values: an Insight
Tone mapping is the process of lowering an image’s tonal values to make them appropriate for viewing on a digital screen as discussed above. But this instance explains the tonal values, such as a typical LCD monitor has a 1000:1 contrast ratio. An HDR image’s contrast ratio is about 250000:1. We lose contrast and details when we display the HDR image on the screen. Tone mapping is required to preserve contrast and details in images.
To put it another way, this technique is modifying the tonal values of a high dynamic range image to make it viewable on digital displays. It reduces the dynamic range while preserving the original image’s appearance. Thus, tone-mapping is used on HDR photographs to bring out all of their nuances, give them a lively twist, and give them a genuine appearance.
This technique which enables you to modify the luminance and color of your image, is a crucial stage in the HDR process to get the finest presentation possible.
Local vs Global Operators
There are two types of tone-mapping algorithms:
Local Operator
Local tone mapping is spatially variable based on the pixel’s surroundings. The local operators localize each pixel in the image. Then, pixels are handled by their spatial properties, and significantly more detail is extracted from them as a result.
In other words, the location of each pixel in the image defines the luminance adaptation for that pixel. Although processing time may be longer, the final image contains more details and is easier for the human eye to read.
Global Operator
The tone mapping function, in contrast, is applied to every pixel in the global operator. Independent of their localization, global operators map each pixel according to their global properties. With this technique, a single matching luminance is computed for the entire image. As a result, the location of a pixel in a bright or dark area of the image is not taken into consideration. As a result, the processing is completed more quickly, but the resulting image has less information.
When working with a camera that supports HDR, we have to consider the onboard Image Signal Processor’s (ISP) capability to perform a tone-mapping process. On the other hand, the same would need to be included in the application running on the host.
Wrapping Up
As you can see, tone mapping is just a component of HDR processing. Both single photos and HDR images produced from numerous merged images can be tone-mapped. Keep in mind that if you shoot in RAW and combine at least two images with varied exposures, your final image will have considerably more detail.
Anyway, this technique is beneficial, but if it’s used excessively or aggressively, it can muddy the picture and make it look bad. You should also be aware that tone mapping does not perform color correction. As a result, color grading a photograph is a whole distinct process for editors and retouchers. This is crucial because tone mapping can change the way that colors appear. Tone mapping cannot provide the accuracy needed to get the right colors. Still confused? We are here to help you with camera technologies and components.
Understanding the technologies of a camera and its components can help you find the most suitable camera for your application. If you’re ready to talk about your camera needs, Vadzo’s team will be happy to assist you. Vadzo has the pleasure of working with Image Signal Processors from businesses such as On Semiconductors, Sony, Omnivision, and others.
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