Centered Circle Mask

Create a mask area centered on the canvas using distance().

Overview

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

Learning Objectives

• Understand how the distance() function calculates the distance between two points.
• Learn how to use the screen center as a reference point to calculate the distance of each fragment to the center.
• Master using distance and a threshold (e.g., combined with step() or smoothstep()) to create a circular mask.
• Be able to apply a circular mask to show or hide content in specific areas.

Prerequisites

• simple-circle
• step-function-mask

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

• Clamp t into [0,1] when needed.