How does Matrix.postScale( sx, sy, px, py) work?

How does Matrix.postScale(sx, sy, px, py) work?

Understanding Matrix.postScale()

The Matrix.postScale(sx, sy, px, py) method in Canvas 2D API is a powerful tool for scaling graphics. It allows you to resize objects along the x and y axes, with the ability to define a pivot point for the scaling operation. This method is particularly useful for creating zoom effects, resizing images, and manipulating graphical elements.

Parameters

• sx: Scaling factor along the x-axis. Values greater than 1 enlarge the object, values less than 1 shrink it, and 1 maintains the original size.
• sy: Scaling factor along the y-axis. Similar to sx, it controls the vertical resizing.
• px: x-coordinate of the pivot point for scaling. This is the point around which the scaling will be applied. If omitted, the default is (0,0).
• py: y-coordinate of the pivot point for scaling. Similar to px, it defines the vertical position of the pivot point.

How it Works

1. Translation: The matrix is first translated by (-px, -py). This moves the pivot point to the origin (0, 0).
2. Scaling: The matrix is then scaled by (sx, sy), effectively resizing the object around the origin.
3. Inverse Translation: Finally, the matrix is translated back by (px, py), repositioning the scaled object back to its original location relative to the original pivot point.

Example

const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');

// Draw a rectangle
ctx.fillRect(50, 50, 100, 50);

// Apply postScale with a pivot at the center
ctx.translate(100, 75);
ctx.postScale(2, 1, 0, 0);
ctx.translate(-100, -75);

// Draw the scaled rectangle (now twice as wide)
ctx.fillRect(50, 50, 100, 50); 

// Output: A rectangle at (50, 50) with dimensions 100x50, and a scaled rectangle also at (50, 50) with dimensions 200x50, indicating the scaling effect around the center pivot point.

Comparison

Conclusion

Matrix.postScale(sx, sy, px, py) is a flexible tool for manipulating the size of graphical objects in Canvas 2D. By understanding its parameters and how it operates, you can effectively achieve various scaling effects, including zooming, resizing, and object manipulation with a specific pivot point for more precise control.