Drawing Circles

Learn to draw basic shapes using distance functions and understand the pixel coordinate system and distance function applications.

Overview

• Use a distance field and a mask to shape the image.

Learning Objectives

• Understand the pixel coordinate system and how to center coordinates.
• Master the usage of the distance() function to calculate the distance from a point to the circle's center.
• Learn how to draw a circle based on distance and radius.
• Understand the application of step() or smoothstep() functions in edge handling.

Prerequisites

• uv-coordinates

Inputs

• vec2 u_resolution — Canvas size in pixels.
• float u_time — Time in seconds.

Key Concepts

• Distance to center builds a distance field.

vec2 p = vUv - 0.5;
float d = length(p);

• Convert distance into a mask.

float mask = 1.0 - smoothstep(r, r + aa, d);

How To Implement (Step-by-step)

• Center coordinates: p = vUv - 0.5.
• Compute distance: d = length(p).
• Build a mask with smoothstep or step.
• Mix foreground/background by the mask.

Self-check

• Does it compile without errors?
• Does the output match the goal?
• Are key values kept in [0,1]?

Common Mistakes

• Don’t use raw pixels without normalization.