OK, we've drawn a square — next, a circle.
You can check if a point is in a circle by seeing if its distance from the circle's center is smaller than the circle's radius. In GLSL we measure distance using the length() function:
float d = length(p2 - p1)
Much more concise than it would be in JavaScript!
All that's left to do is compare that distance to the radius and you should be able to draw that circle out to the screen. Good luck :)
{"bannedTokens":[],"prefix":"","suffix":"","question":"uniform vec2 iResolution;\nuniform float iGlobalTime;\n\n//\n// Let's draw a circle in the center of the screen!\n//\n// Its center should be at uv == (0, 0), and its `radius`\n// should match the value passed into `inCircle`.\n//\nbool inCircle(vec2 uv, float radius) {\n return false;\n}\n\n// If it's starting to look like there's a lot going on in\n// main(), don't worry! You don't need to change it, it's\n// just doing some heavy lifting for you. Leaving it in\n// here in case it's helpful for you :)\nvoid main() {\n vec2 uv = 2.0 * gl_FragCoord.xy / iResolution.xy - 1.0;\n float radius = (sin(iGlobalTime * 0.25) * 0.5 + 0.5) * 0.5 + 0.3;\n if (inCircle(uv, radius)) {\n gl_FragColor = vec4(1, 0.6, 0.5, 1);\n } else {\n gl_FragColor = vec4(0.5, 0.8, 1, 1);\n }\n}\n","solution":"uniform vec2 iResolution;\nuniform float iGlobalTime;\n\nbool inCircle(vec2 uv, float radius) {\n return length(uv) < radius;\n}\n\nvoid main() {\n vec2 uv = 2.0 * gl_FragCoord.xy / iResolution.xy - 1.0;\n float radius = (sin(iGlobalTime * 0.25) * 0.5 + 0.5) * 0.5 + 0.3;\n if (inCircle(uv, radius)) {\n gl_FragColor = vec4(1, 0.6, 0.5, 1);\n } else {\n gl_FragColor = vec4(0.5, 0.8, 1, 1);\n }\n}\n","name":"06-circle"}