visibility_off Sketchover #6 (Lemaori & Aurel.)

Broadcasts Design

It's time for a new Sketchover Tutorial! This week we'll be talking about specular reflection through an effective and easy technique to render a chrome finish.

Many thanks to Lemaori for supplying with the powerful base sketch of this tutorial!

1 / Reflection of light theory

Before rendering, let's review how reflection works and how to apply it in a 2D drawing.

A chrome finish, just like a mirror, relies on three main parameters:

- The surrounding environment

- The orientation and geometry of the reflective surface

- The point of view of the viewer

Representing the "surrounding environment" reflecting through countless complex surfaces can be time consuming and tricky. Thus, it is commonly accepted to render chrome using a simplification of a typical outdoor environment:

- A cold colored blue sky turning lighter as it gets closer to the horizon

- A horizon line (we'll see in the fourth step why it's helpful to reinforce the contrast)

- A warm colored ground, with some "foreground to background" horizontal transitions (using gradient, textures, ...)

The "orientation and geometry of the reflective surface" as well the "point of view of the viewer" can be treated together. Feel free to use a mirror or the selfie mode of your phone to run the following experiments.

Basic configuration: Let's say we are standing in the center of our typical outdoor environment. The mirror is held vertically in front of us and we're not being reflected by the mirror.

Vertical translation: Sliding the mirror up and down does not have much effect, and the reflection remains consistent with the basic configuration.

Horizontal translation: Sliding the mirror back and forth has a visible effect on the scale of elements in the foreground. However, as the horizon is typically 2.9 miles (4.7 km) away on Earth, short distance translations barely affect the reflection of the more distant background.

Rotation: Now we are getting into the interesting part of specular reflection! The angle of incidence (the angle at which the light hits the mirror) and the angle of reflection (the angle at which light bounces off the mirror) have for median a normal to the mirror. This means that for every degree the physical mirror rotates, the reflection shifts by 2 degrees! For that reason, it is important to keep in mind that even if two surfaces are in the same area and seem to be sharing a similar orientation, this slight difference may be important enough to justify two very different reflections!

Convex surface: When a surface is featuring a positive curvature, a bigger portion of the environment is reflected in the mirror. Thus, the reflection appears vertically compressed.

Slightly concave surface: When a surface is featuring a slight negative curvature, a smaller portion of the environment is reflected in the mirror. Thus, the reflection appears vertically stretched. This happens when the normals to the mirror's surface cross behind the viewer's eye.

Concave surface: When a surface features a curvature where the normals cross between the viewer and the mirror, the reflection appears upside down.

Compound surface: Now that we've seen the most typical reflections, it is possible to combine them. In that example the 2 flat surfaces feature a limited change of sky and ground reflections while the tight convex fillet concentrate most of the transition.

We're now good to go, let's get back to the refined sketch...

2 / Horizon line setup

In a flat environment, the horizon line is typically at the height of the viewer's eyes. As this car is supposed to be low and viewed from a straight side angle, the horizon could not be set at a much higher level.

3 / Definition of surfaces orientation

Time to apply the knowledge from step 1. Feel free to sketch sections of the car to apprehend the way surfaces are oriented. Note that at this point I've not been using any soft brush. This car is just a complex mirror reflecting a sharp break between a blue sky and a brownish ground.

4 / Graphic treatment of horizon line

A contrasted horizon line will not only help reinforce the graphic break between sky and ground, it also gives information about the nature of the surfaces it is going through (flat surface, tight radius,...). It is very important to remember that horizon lines don't randomly disappear, they just carry through, sometimes in very subtle ways.

5 / Drop shadow

Drop shadow of the car over the ground, reflected when applicable.

6 / Enhancement through gradients

Ready for some gradients and/or textures to add extra information about the surfaces facing down...

... and the one facing up.

7 / End result

Voilà! After some additional refinements here is the final rendering.

As usual, feel free to post questions, feedback and suggestions in the comments below.

Also, if you need help and feedback on a sketch OR if you want to help and give feedback, here is the place to go. You may be next week's "Sketchover"!




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