lazy-brush

Lazy brush - smooth drawing using mouse or finger

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lazy-brush - smooth drawing with a mouse, finger or any pointing device
Demo drawing app
Example repository

This library provides the math required to implement a "lazy brush". It takes a radius and the {x,y} coordinates of a mouse/pointer and calculates the position of the brush. The brush will only move when the pointer is outside the "lazy area" of the brush. With this technique it's possible to freely draw smooth lines and curves with just a pointer or finger.
How it works
When the position of the pointer is updated, the distance to the brush is calculated. If this distance is larger than the defined radius, the brush will be moved by distance - radius pixels in the direction where the pointer is.
Usage
LazyBrush can be easily added in any canvas drawing scenario. It acts like a "proxy" between user input and drawing.
const lazy = new LazyBrush({
  radius: 30,
  enabled: true,
  initialPoint: { x: 0, y: 0 }
}) // default

lazy.update({ x: 50, y: 0 })
console.log(lazy.getBrushCoordinates()) // { x: 0, y: 0 }

lazy.update({ x: 200, y: 50 })
console.log(lazy.getBrushCoordinates()) // { x: 100, y: 0 }

Use the LazyBrush.update() function to update the pointer position. That would be when the mouse is moved. The function returns a boolean to indicate whether any of the values (brush or pointer) have changed. This can be used to prevent unneccessary canvas redrawing. If you need to know if the position of the brush was changed, you can get that boolean via LazyBrush.brushHasMoved().
To get the coordinates , use LazyBrush.getBrushCoordinates() or LazyBrush.getPointerCoordinates(). This will return an object with x and y properties.
The functions getBrush() and getPointer() will return a LazyPoint, which has some functions like getDistanceTo, getAngleTo or equalsTo.

Performance

For performance reasons it's best to decouple calculations and canvas rendering from mousemove/touchmove events: One way is to store the current coordinates in a variable. Then, using an animation loop (typically requestAnimationFrame), call the LazyBrush.update() function on every frame with the latest coordinates from the variable.
The library will only do calculations if the pointer or brush values changed.