Compositing: Understanding Perspective

If you have taken a second to scroll down you have I'm sure noticed that what comes next is in fact...Boring. However, without understanding the theory behind how perspective works, you will never be able to composite CG elements with a photo or live action in a believable way. Bare with me, I promise that this is easy to understand and once you get it, its like riding a bike and you don't ever have to think about it anymore.

I went to school for architecture and illustration. I was taught several different ways to draw a perspective by hand that was accurate. However, these methods were a horrible pain in the ass and required that you have elevation drawings and floorplans. One of my teachers (who used to be a concept artist for the film industry way back in the days of Metropolis) sat down with me and showed me a simple trick to understanding and quickly drawing accurate perspectives without floorplans or anything else but an idea of how it was kinda supposed to look.

Here is my best recollection of his teachings.

1. Start with the horizon, cause without it you can't do anything.

2. You must have some idea of the view your going for. Is the camera up high in the air, or is it low to the ground? Maybe its at a normal persons height. Once you have decided you can start. The example below assumes that the man is always exactly 6 feet tall. If the camera is exactly 6' off the ground then the horizon will be level with the top of his head. No matter how far away he is from the camera, the top of his head will always be along the horizon. In the middle panel the camera has been moved down to 3' off the ground. The same rules apply...No matter how far from the camera the man is, the horizon will always line up with his waist. Finally, the camera is 1' off the ground. All the same rules apply.

Compositing pieces together like this takes a little guess work, but its not hard to get REALLY close just eye balling it.

3. Now we will choose a point on the horizon. Known as "Vanishing points." For the example below I've guessed the camera to be about 4' off the ground. If we draw two lines from the vanishing point to the top of his head and to the bottom of his feet. We now have a visual reference of how tall he should be as we move him away from the camera.

4. Below you can see that the man is still always 6' tall, yet by accurately scaling him over distance to the horizon we get a realistic sense of perspective.

5. The same rules for the man apply to every object you composite. If you add a tree to your scene, it is obviously going to be much larger than a man. So now you must take a guess at how big it is. As long as your in the ball park and scale it consistently like you've done with the man, it will look correct.

6. Now onto something a little tougher, but not much. So far we've only dealt with 1 point perspective. Now we will look at 2 point perspective. When drawing architecture by hand, 2 point perspective is the easiest and most common method. What this means is that were only taking into account 2 axis of a three dimensional object. With this method you will always have completely vertical walls in your drawing. Notice below that the sides of the box run strait up and down. In real life there are 3 points of perspective which I'll get to in a minute. That third point begins the drawing nightmare and that is why it is often neglected in architectural illustrations.

One very important thing to keep in mind is that if this was to be a photoreal drawing, we have no idea what camera lens we are depicting, and unfortunately there isn't any way to tell. The vanishing points I've chosen for this example were just randomly selected spots on the horizon. Because they are fairly close together we get a very steep perspective which simulates a wider angle camera lens. The further apart we make those points (they may go off your drawing area, deal with it), the narrower the angle of the camera lens.

7. If we use our vanishing points as a guide and always keep vertical elements, well...vertical, we can easily add more and more detail to our drawings and still have correct perspective. In the image below I've added doors, which are typically about 7' tall. Notice that I've placed the top of the doors right along the horizon, which means our camera is 7' off the ground.

8. If you know the rules you can just start dropping in other elements. Like the man is 6' tall, the camera is 7' off the ground, we leave a 1' gap between the top of his head and the horizon (relative to the mans size in frame) and tada!!! Perfect perspective every time.

9. Finally I'll touch on the evil 3 point perspective. In the above examples we've assumed that the camera is always pointing straight out to the horizon, but if we tilt the camera up or down we create a third vanishing point. If you stand at the bottom of a tall building, the top looks smaller than where you are at the bottom. If you follow the buildings vertical lines they will cross at a vanishing point somewhere up in the sky. So how do you decide where to place that point in your drawings? Nobody knows....Its a secret that has been guarded for centuries. No, actually I just don't know, and about this time in my life I discovered that computers can do it for you automatically. My suggestion would be if you were interested in trying to draw this by hand, that the narrower your perspective lines are for the 3rd vanishing point, the better, but its up to you.

Alright, I'm falling asleep just talking about this, but now that you understand how to draw a perspective, you can work backwards by taking your 3d model which has all the information you need built in, and just follow the vanishing points of your model.... So when you go to composite your background photo you'll know where the horizon should be. It may have occurred to you...What if you can't actually see the horizon in your background photo??? Just remember the rules of perspective. There may be other objects in the photo that will give you some clues. Otherwise, make an educated guess.