Browsers supporting HTML5 allow you to draw on the browser’s screen without preparing an image file before. Drawing on a canvas is done using the wonderful Javascript language. If you want to draw a 2-dimensional image on canvas element ‘cnv’, get the drawing context using:

var ctx = cnv.getContext("2d")

And use that context to draw everything using the standard functions:

moveTo, lineTo, arc, fillRect, etc.

Learn more about drawing here.

You can use the functions to create more complicated shapes easily thanks to transform functions:

## Transformations: Linear Algebra Made Easy

The origin(0,0) of the canvas is defined to be the top-left pixel of the canvas element. And the coordinates are given in number of pixels. This can change by using transforms.

The transformation functions are:

- scale(x,y): this will make every shape x times wider and y time taller.
- translate(x,y): now coordinate (0,0) will be shifted x units to the right, and y units down.
- rotate(angle): will rotete the axes by given angle in radians.
- transform(a,b,c,d,e,f): If you want to use a transformation matrix
- setTransfrom(a,b,c,d,e,f): reset all transform, and perform transform(a,b,c,d,e,f).

a,b,c,d,e,f are values in the matrix:

The values a,b,c,d are used for rotating and scaling. e,f for translating.

Other useful methods of the context are:

- ctx.save() – to save current transform in a stack (Last In First Out).
- ctx.restore() – to retrieve the trnasform from the top of the stack.

## An Example – The Koch Snowflake

The algorithm for drawing the Koch Snowflake can be found in the post Drawing The Koch Snowflake Fractal With GIMP.

Here’s an example in Javascript:

function drawSide(ctx, len){ if (len > 1) { var thirdOfLen = len / 3.; var rotationAngles = [0, -Math.PI / 3, 2 * Math.PI / 3., -Math.PI / 3]; rotationAngles.forEach(function(val){ if (val != 0){ ctx.translate(thirdOfLen, 0); ctx.rotate(val); } ctx.save(); drawSide(ctx, thirdOfLen); ctx.restore(); }); } else { ctx.moveTo(0,0); ctx.lineTo(len,0); //ctx.stroke(); } } ctx.translate(startX, startY); for (i=0; i<3; i++){ ctx.save(); drawSide(ctx, sideLength); ctx.restore(); ctx.translate(sideLength,0); ctx.rotate(2*Math.PI/3); } ctx.stroke(); }

Warning: using ctx.stroke() after every little line you draw might make your code inefficient, and slow down your browser.

Transformation functions are also part of the Processing language.

Reblogged this on SutoCom Solutions.