如何在矩形内画圆而不显示正方形的突出部分

Question

I'm trying to draw a circle inside a square on canvas, but I don't want the part of the circle that sticks out of the square to get painted on screen. To explain myself better I put a couple of images.

This is what I get:

But this is what I want to get:

For this I have tried to use a mixture of "ctx.arc" for the curved part and "ctx.moveTo" + "ctx.lineTo" for the straight lines that coincide with the edges of the rectangle, but although I have not succeeded yet, I It is going to be a rather cumbersome code and that is why I ask to you, because perhaps there is an easier way to do it and I have not found it nor has it occurred to me yet.

To clarify even more, the images that I have put are examples, but what I need is a generic code since the circle could be of any size and have its center of position anywhere inside the square.

Thank you.

All the best.

Answer 1

You can use clip() to cut off parts of the circle. Then you don't need to calculate start points of the arc at all.

CODE SNIPPET

 let ctx = document.getElementById("canvas").getContext("2d"); // Clip a rectangular area ctx.rect(50, 50, 200, 200); ctx.clip(); // Draw red rectangle after clip() ctx.fillStyle = "red"; ctx.fillRect(0, 0, 150, 100); ctx.fill(); ctx.fillStyle = "blue"; ctx.beginPath(); ctx.arc(70, 70, 120, 0, Math.PI * 2, true); ctx.fill();

 <canvas id="canvas" width="400" height="400"><canvas>