What is 3D rendering?
Quick answer: 3D rendering is the calculation that turns a three dimensional scene into a flat image. A computer is given geometry, materials, lights and a camera, and it traces how light would travel through that scene to work out the colour of every pixel. The output is a single image, or a sequence of images that becomes an animation. In architecture, it is how a building that does not exist yet gets photographed.
The idea in one paragraph
Imagine building a physical model of a room, placing a lamp in it, and taking a photograph. 3D rendering is the same thing done in arithmetic. The model is geometry stored as numbers. The lamp is a light with a measured intensity. The camera is a position, a direction and a lens. The renderer works out what that camera would see, and writes the answer out as an image.
Everything else is detail. But the detail is where the money and the time go.
The four things a render needs
Geometry
The shapes. Surfaces described as triangles or as mathematical curves, positioned in space at real dimensions. In architecture the geometry comes from the drawings: walls, slabs, openings, stairs, railings, site. Geometry is where accuracy lives. If the geometry is wrong, no amount of lighting saves the image.
Materials
What each surface is made of, described in terms of how it behaves toward light. A material says how much light bounces, in what direction, how rough the surface is, whether light passes through it, and whether it bends on the way. Concrete scatters light in every direction. Polished stone scatters it mostly one way. Glass lets most of it through and bends it.
A common mistake is to think of a material as a colour. Colour is the least important part. Roughness and reflectivity are what make a surface believable.
Lights
Where the light comes from and how much there is. A sun, a sky, interior fixtures, a glowing screen. In architectural work the sun is placed by real date, time and compass orientation, because the shadow it casts is a factual claim about the building.
A camera
Position, direction, lens, and how long the shutter is open. These are the same decisions a photographer makes, and they do the same work. A wide lens at a low position makes a building look monumental. A normal lens at eye height makes it look like a building.
How the calculation actually works
The dominant method today is to follow light backwards. For each pixel, the renderer fires a ray out from the camera into the scene and finds what it hits. Then it asks a question that sounds simple and is not: how much light is arriving at this point, and from where?
To answer that, it fires more rays from that point: toward the lights, and out in other directions to find light that has bounced off something else. Each of those rays may hit a surface and spawn more rays. This is why a white room is harder to render than a dark one. White surfaces keep passing light along.
Bounced light is the whole trick
Light that has bounced is called indirect or global illumination, and it is the single biggest reason modern renders look real and older ones did not. In a real room, most of the light reaching the ceiling never came from the lamp directly. It came off the floor. Simulating that is expensive and it is what makes the difference between an image that reads as a photograph and one that reads as a computer drawing.
Sampling and noise
The renderer cannot trace every possible light path, so it takes samples: a few hundred or a few thousand random paths per pixel, then averages them. Too few samples and the average is rough, which shows up as grain, called noise. More samples means a cleaner image and more time. Roughly, halving the noise costs four times the computation.
This is the fundamental trade of the whole field. Quality, time and cost are one dial, not three.
Why renders take so long
- Resolution. Every pixel is an independent calculation. A print size image has several times the pixels of a screen size one.
- Glass and metal. Reflective and transparent surfaces multiply the number of rays, because each hit spawns more.
- Interior scenes. Light entering through a window and bouncing around a room is many bounces. Exteriors lit by an open sky are much cheaper.
- Soft shadows and depth of field. Both are produced by averaging many slightly different samples, so both cost samples.
- Animation. A ten second clip at thirty frames per second is three hundred images. This is why animation pricing is a different conversation. See 3D animation.
Modern denoisers, which clean up a noisy image after the fact, have cut render times dramatically. They have not changed the underlying trade.
Real time versus offline rendering
There are two worlds, and confusing them causes most of the misunderstandings about what a render should cost.
Offline rendering takes minutes to hours per image and prioritises accuracy. This is what produces a still for a brochure or a sales centre wall. Light is simulated properly and the result holds up at print size.
Real time rendering produces thirty to a hundred and twenty images per second, because a person is moving the camera. It approximates aggressively. This is what powers interactive walkthroughs, configurators and virtual reality, and it is why those experiences look slightly softer than a still. See 3D walkthrough and 360 virtual tours.
Both are 3D rendering. They buy different things. A client who has seen a real time demo and expects a brochure still at the same price is comparing two different products.
Rendering is not modelling
People use the two words interchangeably and it causes real confusion in quotes.
Modelling is building the geometry: a person constructing the building in 3D from your drawings. It is skilled human time, and on an architectural project it is usually the largest part of the cost.
Rendering is the computer calculating the image from that model. It is machine time, and it is comparatively cheap.
So when someone says a render takes four hours, they probably mean the calculation. The project still took a week, because someone had to build and light the scene first. This is also why extra views of the same building cost far less than the first view: the modelling is already paid for. See 3D rendering cost.
The vocabulary, decoded
Quotes and conversations about rendering are full of terms that sound like jargon and are actually simple. These are the ones that come up most.
Ray tracing and path tracing
Ray tracing means following light rays to work out what a pixel sees. Path tracing is the modern version that follows whole chains of bounces, which is what produces realistic indirect light. Almost every architectural still you have seen in the last decade is path traced.
Samples and passes
A sample is one traced light path. A pass is one complete sweep of samples across the image. More passes means a cleaner picture. When a studio says a frame took six hours, they mean it kept adding passes for six hours.
Denoising
A cleanup step that looks at a grainy render and predicts what the clean version would be. Good denoisers have cut render times by large factors. Pushed too hard they smear fine detail, which is why some renders look slightly plastic.
HDRI
An image of a real sky, captured across a very wide brightness range, used to light the scene. It is the fastest way to get believable exterior light, because the light is real light that was measured somewhere.
Resolution and DPI
Resolution is the pixel count of the image. DPI only matters when printing, and it is what decides whether a given pixel count is enough for a given print size. If your render is going on a sales centre wall, say so before it is rendered, not after.
Render farm
A group of machines splitting the work. Essential for animation, where hundreds of frames can be calculated in parallel, and rarely worth the setup for a single still.
Where 3D rendering is used
- Architecture and real estate. Selling, permitting and approving buildings that do not exist yet. See 3D rendering services.
- Interiors. Testing finishes, furniture and lighting before anything is ordered. See interior rendering services.
- Products. Catalogue and advertising imagery made from the engineering model rather than a photo shoot.
- Film and television. Effects and fully animated features.
- Games and virtual reality. Real time rendering, every frame, forever.
What to judge in a finished render
If you are paying for renders and want to know whether you got good ones, ignore how pretty the image is and check these:
- Do the shadows agree with each other? One sun means one direction. Shadows pointing different ways mean the scene was faked.
- Do reflections show the real surroundings? Glass should reflect the actual building across the street, not a generic sky.
- Is there contact shadow? Objects that meet a floor darken slightly where they touch. Missing contact shadow is why things look pasted on.
- Does the light fall off? A room lit evenly from wall to wall has no real light source in it.
- Do the proportions match the drawings? Measure a door. It should be a door.
Those five checks catch most of what separates a render you can present from one you cannot. For the wider discipline, see architectural visualization.
Common questions
What is 3D rendering in simple terms?
It is the calculation that turns a 3D scene into a flat image. The computer is given geometry, materials, lights and a camera, and it works out what that camera would see.
What is the difference between 3D modelling and 3D rendering?
Modelling is a person building the geometry. Rendering is the computer calculating the image from it. On architectural projects the modelling is usually the expensive part, which is why extra views of the same building cost much less than the first.
Why does 3D rendering take so long?
The renderer traces thousands of light paths per pixel and averages them. Higher resolution, reflective surfaces, interior scenes, soft shadows and depth of field all multiply the number of paths needed.
What is global illumination?
It is light that has bounced off one surface before reaching another. Simulating it is what makes a render read as a photograph instead of a computer drawing, and it is the main reason renders are computationally expensive.
What is render noise?
Grain caused by averaging too few light path samples per pixel. More samples means less noise and more time. Roughly, halving the noise costs four times the computation.
What is the difference between real time and offline rendering?
Offline rendering takes minutes to hours per image and prioritises accuracy, which is what a brochure still needs. Real time rendering produces dozens of images per second by approximating, which is what interactive walkthroughs and virtual reality need.
How can I tell if a render is good?
Check whether all shadows agree on one sun direction, whether reflections show the real surroundings, whether objects have contact shadow where they meet the floor, whether light falls off across the room, and whether proportions match the drawings.
Is 3D rendering only used for buildings?
No. The same calculation is used for product imagery, film effects, animated features, games and virtual reality. Architecture is one application of a general technique.
Frequently asked questions
What is 3D rendering in simple terms?
It is the calculation that turns a 3D scene into a flat image. The computer is given geometry, materials, lights and a camera, and it works out what that camera would see.
What is the difference between 3D modelling and 3D rendering?
Modelling is a person building the geometry. Rendering is the computer calculating the image from it. On architectural projects the modelling is usually the expensive part, which is why extra views of the same building cost much less than the first.
Why does 3D rendering take so long?
The renderer traces thousands of light paths per pixel and averages them. Higher resolution, reflective surfaces, interior scenes, soft shadows and depth of field all multiply the number of paths needed.
What is global illumination?
It is light that has bounced off one surface before reaching another. Simulating it is what makes a render read as a photograph instead of a computer drawing, and it is the main reason renders are computationally expensive.
What is render noise?
Grain caused by averaging too few light path samples per pixel. More samples means less noise and more time. Roughly, halving the noise costs four times the computation.
What is the difference between real time and offline rendering?
Offline rendering takes minutes to hours per image and prioritises accuracy, which is what a brochure still needs. Real time rendering produces dozens of images per second by approximating, which is what interactive walkthroughs and virtual reality need.
How can I tell if a render is good?
Check whether all shadows agree on one sun direction, whether reflections show the real surroundings, whether objects have contact shadow where they meet the floor, whether light falls off across the room, and whether proportions match the drawings.
Is 3D rendering only used for buildings?
No. The same calculation is used for product imagery, film effects, animated features, games and virtual reality. Architecture is one application of a general technique.