I made this little widget to help myself visualize the methods on the HTML5 canvas' 2d context. I was having a hard time drawing rotated tiles because keeping track of the transform matrix can get confusing when you don't understand how it works.
The x-axis is red and the y-axis is green. Clicking on one of the buttons will show you the code for that function.
You can imagine the rendering context as a plane that's superimposed over the canvas. When you are painting an image on a canvas, you have to manipulate the rendering context so that it faces the canvas in the right way.
There are three transformation operations that can be done: scale, rotate and translate. These operations always apply on the rendering context, not on the canvas itself. That means that the origin (0, 0) is in the upper-left corner only if the context hasn't moved.
You can use the scale operation to flip the context along one or both axes. The x-axis follows the horizontal edges of the canvas by default. But if the context is rotated by 90 degrees, suddenly the x-axis follows the vertical edges. That means that flipping accross the x-axis axis would result in a vertical flip.
Try drawing a 270-degree rotated tile in the middle of the canvas!
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