Identifying Perspective in Images

Last lesson we talked about perspective in general terms and looked at the relationship between the vanishing point of a shape and the horizon and also discussed how the camera can affect the perspective of the image. This lesson we are going to examine some images and find the horizon and vanishing point for various elements in the image. Knowing this information is critical to understanding if the cutout element will “fit” into the perspective of the target background of the composite.

It should be noted that you may not be able to map the vanishing point and horizon for all images. Macro images for example are captured so close to the subject and have such a narrow depth of field, that the vanishing point is irrelevant. But we seldom composite a macro image into another background.

Two other key points to understand. If you are compositing a model into your background and you are not going to use a full body image where the feet are visible, the perspective of the model may not matter. But when the feet are included in the composite, then the angle of the model and the model’s original horizon line becomes important. The original model’s horizon line and the target background horizon line must match. This applies to models as well as other subjects with feet.

Animals where the original image was captured with their feet on the ground plane and there is a horizon (either visible or not) will also need to match their horizon lines in the target composition. And like models, if the feet are not visible, it doesn’t matter. If I want to use this springbok in a composite, there is no need to worry about my perspective.

The second consideration involves clouds when doing a sky replacement. Clouds actually behave by the rules of perspective as well. Too often you see compositors switch out a sky with perspective for one without perspective. Clouds should be smaller at the horizon. With other elements we can change their gamma, blur, lighten and desaturated them to simulate the distance. But clouds have no structure so their size relative to each other is the only measure of perspective that can be used. If you replace a cloudy sky, determine if you need to wrap your clouds into perspective. Notice in these two examples how much smaller the clouds are in the distance.

So now we know that the horizon line is important, but how do we find it? The best way to find the horizon is to draw lines, leading lines, along the paths of the verticals within the image to a point in the distance where they would converge at the vanishing point. Remember from the first lesson. The vanishing points end at the horizon.

Here is a single vanishing point example. By drawing lines from the top and bottom of the tires out to infinity we find the place where the lines cross, or vanish to a single point. The vertical line that intersects this point is the horizon of this image.

Here is another example of a two-point perspective. Again, by drawing lines along the verticals within the image, you can find the horizon.

Not all images will allow you to easily draw these vertical lines. There is a tool available call “The Perspective Tool” that is an extension to Photoshop. I will put the links in the appendix of this article if you are interested as well the links to a couple of YouTube videos where you can see a demonstration. This tool actually finds the horizon for you on images where you have very little information available. You only have to select a couple of short vertical lines within the image and tool does the rest. Below are a few cropped example of images where I found the horizon when I had very little information available to use the method described above.

The horizon line in the first image is just at their foreheads. Because these are full model images, if I were to use these in a composite, I would need to place the models in the target scene with their heads about at the horizon in order for the perspective to look correct. Similarly, the horizon line is just above the zebra’s head. When placed in a new scene, I would need to place him in the scene with his head at the horizon line of the new background. We will learn how to scale his size to maintain that perspective in another article. The final image the model was kneeling. The horizon line is way above her head. To match perspective, she would need to be placed in a new background where the horizon line is similarly spaced above her head.