Understanding Tone Mapping - Via Macphun

Here's another from my series of Educational blogs with Macphun!  Be sure to check them all out on https://aurorahdr.com/blog


What's the first image that pops into your mind when someone says HDR? For most people, they think of dark, grungy, over-saturated images with halos surrounding every hard edge in the photo. The truth is, that's not actually HDR, but in fact, a form of Tone Mapping.

Understand HDR first!

To understand Tone Mapping we need to first explain HDR. So what is HDR exactly? Well High Dynamic Range images simply mean that the photo has more dynamic range in it than any camera (currently) can capture in one single shot. To be able to create a real HDR image, you have to take 3 or more photos at different exposure values. 

 

Typically one frame will be at a proper exposure, then the rest are overexposed and underexposed at various increments to let you capture the details in the brightest highlights and the darkest shadows. Then these images are merged together to create a 16-32 bit file.

As I've explained in a few other blog posts and videos on our YouTube channel, most every camera available on the market currently has a sensor in it that can record information with a very specific dynamic range, (somewhere between 5-14 stops of light), which is FAR LESS than what the human eye can perceive. Therefore we combine these bracketed exposures together to create an HDR image.

If you try to take a high contrast shot with a single exposure you typically end up with either completely blown highlights, or all details lost in the shadows.

When you've taken a proper mix of exposures (under, balanced, and over), and merged them with an HDR software app, you're left with a relatively flat and low contrast image. This is where tone mapping comes into play.

What is tone mapping?

It's the process of converting the tonal values of an image from a high range to a lower one. For instance, an HDR file merged from multiple images with a dynamic range of 100,000:1 will be converted into an image with tonal values ranging from around 1 to 255.

Why do we want to reduce that tonal range so much? Well the reason is simple. Most standard display devices (and printers) can only reproduce a low range of dynamic values (between 100 or 200:1 or lower). The goal of tone mapping is to reproduce the appearance of images having a higher dynamic range to fit/display properly on standard display devices, thus keeping the image looking realistic.

The algorithms that tone mapping use to scale the dynamic range down attempt to preserve the appearance of the original image captured by breaking the information up into two categories: global and local.

Global operators map each pixel based on it's intensity and global image characteristics. The process ignores its spacial location or if it's in a dark or light area. Using global only tends to leave you with a flat non-contrasty image after the conversion process.

Local Operators uses the pixels location in the image when analyzing the appropriate scaling for it. This allows each pixel of a given intensity will be mapped to a different value depending on whether it's found in a dark or light area. Local tone mapping requires the system to look up surrounding values for every pixel mapped. This makes it slower (and more memory/system intensive), but leaves you with a much richer and eye pleasing image when correctly done.

Does shooting in RAW matter?

Short answer - YES! While you can still get amazing images from JPGs, tone mapping using RAW files provides much much much more information for the HDR program to work with. While it may take longer to process, you'll be left with a much more accurate image when working with RAW.

When using Aurora HDR to merge your brackets, you're given one of the most human-realistic representations of the scene possible, and when using raw files, you have a TON of information and tools within your 32-bit image to generate a finished photo with the exact look and feel that you desire!

The reality is, you don't need multiple exposures to tone map your image. You can have a single frame and create just as epic and breathtaking details, however, you will have less dynamic range to play with so remember that.

This is why Tone Mapping and HDR are not the same thing! To create an HDR image, you need at least 2 bracketed images. It boils down to availability and style. It's up to you to create an image that's either realistic, or fantasy like.