Within Vray there are various processes and tools that we use to generate what we would consider to be a realistic rendered image. Options such as tone mapping, light intensity and camera exposure are fully configurable and can be moulded into what a 3D visualiser would name as their workflow. There are no right or wrong processes as they all lead to the same goal, to produce a high quality render.
If you choose not to use a linear workflow, it may not be physically correct in terms of light intensity but it might look correct and is therefore acceptable. If you are using a linear workflow, then it is not just the lighting that needs to be accurate, you must also apply the same method to your materials. If you set up everything as linear and render using correct camera exposure but you don’t know how to create a glossy plastic, you are not physically correct. You are also faced with blown out lights which can become a pain to manage.
Blown out lights are caused by the camera settings. As an example, at night look at your ceiling lamp, it doesn’t look blown out but if you then take a photo of it, it is completely blown out. This is because what a camera sees is not the same as what the human eye can see. The dynamic range of a camera is many times less than that of the human eye, algorithms have been developed so that shadows and highlights have a much greater dynamic range but it is still nowhere near the same as the human eye.
Vray renders through a camera and that is why in a linear workflow you will see blown out lights. You can control the exposure via the camera settings much like a photographer would in the real world. Professional photographers are accustomed to using all kinds of tricks to fix over exposed areas in their photos. If you are using a linear workflow for 3D rendering, you must apply the same rule. This is why linear workflow has become such a complicated and over discussed topic. It is difficult to manage unless you are 100% committed to being physically correct and understand the techniques that can be used to eliminate over exposed areas.
There are other colour mapping types available such as exponential that help eliminate blown out areas, but usually bring in a new factors that need to be controlled such as washed out renders. See below the various types of colour mapping available.
Linear multiply – This mode will simply multiply the final image colours based on their brightness are. Colour components that are too bright (above 1.0 or 255) will be clipped. This can result in burnt out spots near bright light sources.
Exponential – This mode will saturate the colours based on their brightness. This can be useful to prevent burn-outs in very bright areas (for example around light sources etc.). This mode will not clip bright colours, but will instead saturate them.
HSV exponential – This mode is very similar to the Exponential mode, but it will preserve the colour hue and saturation, instead of washing out the colour towards white.
Intensity exponential – This mode is similar to the Exponential one, but it will preserve the ratio of the RGB colour components and will only affect the intensity of the colours.
Gamma correction – This mode applies a gamma curve to the colours. In this case, the Dark multiplier is a general multiplier for the colours before they are gamma-corrected. The Bright multiplier is the inverse of the gamma value (i.e. for gamma 2.2, the Bright multiplier must be 0.4545).
Intensity gamma – This mode applies a gamma curve to the intensity of the colours, instead of each channel (r/g/b) independently.
Reinhard – This mode is a blend between exponential-style colour mapping and linear mapping. If the Burn value is 1.0, the result is linear colour mapping and if the Burn value is 0.0, the result is exponential-style mapping.
The two methods of colour mapping
- The human eye can adjust to any given light situation. To simulate this, you would then adjust the lighting, exposure and materials so that it is pleasing to the eye. The result would be exponential.
- A camera would focus on a subject and depending on the light intensity in relation to the light source, then there will be blown out / over exposed areas. You would adjust the camera exposure to compensate whilst keeping everything linear.
Reinhard tends to be the favourite for interior visualisers. If the burn value is set to 0.5 this will be half away between linear and exponential. Resulting in a less blown out image that has a nice level of contrast.
Only you as a 3D visualiser can decide which method to use. But you must stick with it and make it work to your advantage. You will eventually find your own recipe for success by either combing the two methods or bending the rules.