Photorealism can be defined as the ability to create images or models that are so real that they could easily be mistaken for photographs. Whether in video games, cinema or 3D, the boundaries between digital art and reality are constantly being pushed back. But in practical terms, what makes an image photorealistic? And how do you achieve this level of detail and precision? That's what we're going to find out in this article.

Understanding photorealism

Photorealism is more than just a realistic rendering, and achieving it is not as easy as you might think.
In fact, to achieve perfect photorealism, a large number of steps and simulations are required. These include simulations of light, textures, shadows, transparency, translucency, diffusion and many others. These sometimes complex simulations enable the subtleties of the real world to be faithfully recreated. All these steps make photorealism difficult to achieve, but not impossible!

As you can see, photorealism is all about the details. The concept first emerged in the 60s and 70s as an artistic movement, particularly among painters, and then rapidly took hold in digital production, becoming a benchmark for 3D image creators.

How to achieve photorealism: the ingredients of a photorealistic image

To achieve the Holy Grail of 3D, i.e. photorealism, you need to master a number of essential elements:

1. Light and shadow, the basics

It all starts with good light management. To do this, you need to observe and ask yourself:

• What is my light source? Is it primary (e.g. a lamp), secondary (e.g. light reflected from a spotlight), extensive (e.g. sky light), local (e.g. a point of light), etc.?
• How do the shadows behave? Are they sharp, blurred, contrasting, light...?
• How does this light behave on the materials? Does it bounce? Does it go through?

As you can see, calculating the simulation of light is fundamental to obtaining a photorealistic result. Today, the most efficient way of calculating light is RayTracing. It simulates the natural behaviour of light in an ultra-realistic 3D environment on materials.

Still based on ray-tracing, we use Global Illumination (GI) techniques to simulate the way light bounces off surfaces to produce more natural lighting. By asking the right questions and using the right techniques, we can achieve a more realistic and natural result on objects, in addition to working with materials.

And let's talk about materials. Yes, light lets us see them, but it's the materials themselves that change and adapt the way light interacts with objects!

2. Surface appearance (materials, texture, PBR, SSS)

First of all, we need to define what a material is in 3D. A material represents the physical nature of an object or element. It can be wood, skin, forest floor, fabric, etc., any existing element that can be reproduced. But for a material to come fully to life, it needs one or more textures. Textures play a key role in bringing an image to life by 'dressing up' the material, defining its colour, shine, roughness, relief, and so on. One of the important parameters of a texture is its resolution, i.e. the higher the resolution, the more detail and subtlety you get, even in close-up. You can also use techniques such as normal maps or displacement maps, which add realistic irregularities: the grains of aged wood, the micro-cracks in a concrete wall, etc. without weighing down the scene.

This skilful blend of textures creating materials gives a 3D object character and credibility. But that's not all.

Reproducing a material, as well as making it 'pretty', also means understanding how it reacts to light. A material can sometimes react in complex ways to light, resulting in its transparency, or even its internal scattering properties, such as subsurface scattering for human skin, for example.

Today, the PBR (Physically Based Rendering) standard guarantees that a material has the basic information it needs to look realistic. These parameters/aspects are taken into account:

• Broadcasting (colour),
• Roughness,
• Emissivity (the ability of a material to emit its own light),
• Metalness
• Relief,
• And any other aspect deemed useful for the materials in question.

3 . Les technologies/techniques pour atteindre le photoréalisme

As we've seen, to achieve photorealism, you need to take into account multiple physical effects. However, to achieve this, your rendering engine needs to be able to take them into account. The rendering engine is the algorithm that will synthesise the image by simulating the rays of light. The more the rendering engine is able to simulate physical effects, the closer the rendering will be to photorealism. Here is a list of effects that a renderer should take into account:

• Shading (lit or unlit object)
• Drop shadow (point or extended source)
• Réflexion (Ray tracing)
• Réfraction (+ version glossiness)
• Relief (bump map ou normal map)
• SSS (Sub Surface Scattering)
• Metalness (métallisé ou non)
• GI: Bouncing light off surfaces
• Caustics (convergence or divergence of light rays)
• Motion Blur
• Profondeur de champ (flou d’optique)
• Volumetric: smoke, fire, volumes...
• Etc.

4. Careful composition and final touches

The layout of objects, perspective, choice of angles, etc., all contribute to the immersion and credibility of a scene. Just as in film or photography, good framing can transform a simple scene into a unique and impactful image.

And even the best renderings benefit from going through a post-production phase! This phase enables :

• Colour adjustment,
• A subtle addition,
• Lens effects
• Slight chromatic aberrations.
• Etc.

All these little details simulate the natural effects of a real camera, making the images much more believable to viewers.

Beyond the technical retouching, what really makes an image real are the little defects added to it.

Now you've achieved your goal of having a perfect, correctly simulated effect, and yet... It doesn't look believable! The fault lies in the lack of imperfections, let's see that straight away.

5. The perfection of imperfections

A rendering that is too perfect can sometimes seem artificial, because our world is full of details, little flaws and imperfections. Quite the opposite of a virtual world, which is mathematical, strict and orderly. To improve the illusion of reality, it is crucial to incorporate imperfections, and here is a non-exhaustive list:

• Micro-scratches and wear on surfaces.
• Slight blurring or chromatic aberration in the final image.
• Subtle misalignment in the objects to break the perfect symmetry.
• Image grain to simulate real camera sensors.

As you can see, it's these little irregularities that make an image more credible by imitating the imperfections of the real world. Adding all these details may sometimes seem like a constraint, but it's a necessary one if we are to achieve the ultimate goal: photorealism!

Photorealism is a skilful blend of advanced artistry and technical rigour. It's a constant quest to capture the essence of reality and transcribe it with pixels. It takes time, expertise and the right tools, but you can be sure that the result is always worth the effort.

So, are you ready to blur the boundaries between reality and the virtual world?