Flamingo nXt

easy rendering for Rhino in Windows

What is HDRI?

This page may help explain HDRI (High Dynamic Range Imaging) and how HDRI is used in Flamingo nXt. EN  DE  ES  FR  IT TW

A short history of color and computing:

We are all familiar with digital photographs, renderings and other images we use every day. These files are saved in the JPEG, BMP, TGA and PNG formats. Classified as bitmap formats, these images contain 24-bit per pixel (24bpp), by storing the color of 1 pixel by mixing 3 colors channels Red, Green, Blue. Each channel can contain a value between 0 - 255. This means bitmaps can support 16,777,216 distinct colors by mixing the 3 colors together. It is no coincidence that this is also the way most computer monitors also mix color using red, green and blue. While 16 million colors may seem like a lot of colors, it is a very small number when compared to the actual colors we can perceive. Bitmaps can be considered a low dynamic rangeformat.

We perceive light in our environments not only through color, but strength of light and wavelength. All devices we use to view images (printers, monitors, photographs) can only display a very small portion of the light and color we can perceive. So with that said, all printers, monitors and photographs are also considered to be low dynamic range.

In addition, many older rendering technologies use this same RGB color spacefor all calculations. They too were considered low dynamic range, clamping all values within the 16 million colors of the RGB space.

What is special about HDRI?

HDRI is a much higher dynamic range of color and values than traditional bitmap formats. Instead of encoding colors like a computer monitor, using 24-bits of color for each pixel, HDRI is modeled after trichromatic base of the human eye and store actual luminance values in each pixel. So not only is color contained, but the strength and brightness of the light at that point in the map. The range of color and light that can be contained in this format is much greater than the RGB scale of traditional computer graphics. nXt uses HDRI values throughout the rendering process, putting nXt into the classification of HDRR (High Dynamic Range Rendering)

In HDRI, each channel holds more accurate values. For instance, if you look at a 60 watt light bulb, it will be white. But if you look at the sun it is also white, but 10,000 times brighter. If you look at a star in the night sky, it is also white, but may be 1,000 time dimmer than the light bulb. In RGB, white is white, in HDRI, light has a much greater range and can capture the actual amount of light coming from each of these sources.

How does NXT use HDRIs?

One common way nXt uses HDRI is for lighting environments. If we use HDRI technology for a complete environment, HDRIs can be used to capture the amount and direction of light in a scene. HDRIs commonly formatted as lighting environments are called lightprobes and are saved normally in the .HDR format or the .EXR format. nXt can wrap these probes around and environment and sample the scene for the lighting values. Used this way, HDRI probes will supply the lighting to a scene.

 

You can see that not only does the light from the HDRI render, but also in the reflections of the objects within the scene use the lightprobe.

HDRI environments come in two aspect ratios, the Equarectangular and the Spherical projection. Equarectangular HDRI environments are a 1:2 aspect ratio:

Spherical HDRI Environments are square in aspect ratio.

Both will wrap completely around the scene. Both layouts have the same data.

 

There are many sources for HDRI environments. You can select environments that best fit your renderings. Go to the HDRI Environments page to see more.

How to select an HDRI?

In Flamingo nXt, HDRI environments are selected by clicking on the HDRI in the Sky Tab:

 

There are a number of features with an HDRI to consider. How will it light the scene? How will the colors reflect in the objects? Here are three different HDRI environments and how they affect the scene.

 

 

 

An HDRI environment that is dark on one side and bright on another (as in this environment taken along the edge of a pond) will cast darker shadows and depending on where in the environment may have both dark and light reflections.

This HDRI with its relatively clear sky will cast very soft shadows. The reflection will be quite blue based on the reflections of the blue sky.

 

This interior brings softer lighting with neutral grey reflections.

Selecting the best HDRI for your renderings will become quite important. A collection of 3 or 4 favorties is typically what develops over time.

Tone Mapping

If you have not figured it out yet, there is a problem that HDRIs present. All HDRIs are High Dynamic range, and all display methods we use to view the HDRIs are low Dynamic range. There is no way to view HDRI information directly. Monitors do not have the range, printers do not have anough colors. So, in order to view HDRIs on any device, there needs to be some type of conversion from HDR to low dynamic range. This conversion is done through an algorythm called a Tone Operator. Naturally if we're moving from a high dynamic range to lower, there will be some loss of information or compression of information.

A tone operator does not change the actual amount of light in a scene, but will change the way that scene is exposed or compressed to get onto the screen. For example take this HDRI:

 

By simply changing the tone operator inputs, these images are possible:

 

 

In nXt you can control how the Tone Operator compresses an HDRI image to get it on the screen, or to save to a stanadard bitmap format. The controls for this are contained in the Adjust image Panel in the Render window. You can control the Brightness, Burn and Saturation of the final image. For more information on Tone Operator, go to the Adjust image article ...

Understanding how to use the Tone Mapping controls can make a big difference in the quality of you renderings. The images below are lit exactly the same, only the exposure of the scene has changed.

 

Additional notes:

nXt assumes that HDRIs contain radiance values expressed in units of watts. All of the HDRIs available via our website are built this way unless otherwise noted. However, this is often not the case with HDRIs from other sources. The units in many HDRIs are not calibrated to real-world units of any sort. The intensity of these HDRIs may need to be adjusted in order to achieve proper illumination levels.

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