Matrix 🔢

A 3 by 3 Matrix that stores information which can be used to transform a canvas. A Matrix can store the following kind of transformation information: scale, skew, rotation, translation.

Below is an example of using a Matrix to transform a Bitmap that is drawn on a Canvas.

To use a Matrix when drawing, you can do the following:

val customMatrix = Matrix() // in onDraw()

customMatrix.postRotate(20.0f) canvas.withMatrix(customMatrix) {

drawBitmap(bitmap, null, rect, paint)

}

The above code will draw a bitmap on a canvas that is rotated at 20 degrees. There are a few other functions on a Matrix that we can use such as scaling, rotating and skewing. The great part about using a Matrix over doing everything yourself manually with Canvas transformations, is that the Matrix holds cumulative information about the transformations that are applied to it.

If you translate the Matrix , rotate, scale and translate again, the end values of the translation would be a bit different than the original values. If you were to do this yourself, you would need to calculate that manually if you were performing normal Canvas translate, scale functions.

preRotate vs proRotate vs setRotate

You might be wondering what postRotate means, considering the fact that there are other methods such as setRotate and preRotate on a Matrix . These three methods all do slightly different things:

setRotate — Completely resets the current Matrix and applies the rotation, thus losing any information that you may already have stored in your Matrix .

preRotate — The rotation will be applied before whatever the current Matrix contains.

postRotate — The rotation will be applied after whatever the current Matrix contains.

Perspective Drawing with Matrix

A Matrix object can also provide us with the ability to perform perspective drawing, which is not possible to achieve with just standard Canvas transformation APIs. The function that allows perspective drawing or skewing of a canvas is Matrix#setPolyToPoly() . This method does sound a bit confusing at first, but once you wrap your head around how it works, it is not so bad!

Here is an example bitmap that has been “skewed” using the setPolyToPoly method.

Bitmap drawn with `setPolyToPoly` on a Matrix

The setPolyToPoly method takes input ( src ) “points”, and maps them to the specified output ( dst ) “points”. I say “points” because they aren’t real point objects as we’ve just explored earlier in this post, but they are rather just values in a float array, which can be quite confusing.

You can see in the src array below, the first two values are representing the top left point of the image, the second two values represent the top right point and so on. These points can be in any order, but they must match with the corresponding point that you want it to map to, in the dst array.

val src = floatArrayOf(

0f, 0f, // top left point

width, 0f, // top right point

width, height, // bottom right point

0f, height // bottom left point

) val dst = floatArrayOf(

50f, -200f, // top left point

width, -200f, // top right point

width, height +200f, // bottom right point

0f, height // bottom left point

) val pointCount = 4 // number of points // the second and fourth arguments represent the index in the src and dest arrays from which the points should start being mapped from

newMatrix.setPolyToPoly(src, 0, dst, 0, pointCount) canvas.withMatrix(newMatrix) {

drawBitmap(bitmap, null, rect, paint)

}

In this example, the bottom right point, will be mapped from the point [width, height] to the point [width, height +200f] .

So you can see from the above example that a Matrix can do some pretty powerful and interesting stuff.

Tip: Use the Matrix class to work between different coordinate systems

If you have two different coordinate systems that you are dealing with on a single view, then leveraging a Matrix class can help you map between the two.

For instance, if you get a touch event from Android that is measured in the height and width of the size of the screen, but you would like to know what that point would be inside the image you are drawing on screen, within that coordinate system (ie the coordinate system of the image), you can use a Matrix to map between these two systems.

Two coordinate systems with the red point being the touch event within the larger coordinate system.

In order to get the point mapped inside the image drawn on screen, we can use the Matrix#mapPoints() method:

fun mapPoint(point: PointF): PointF {

computeMatrix.reset() // apply the same transformations on the matrix that are applied to the Image

computeMatrix.postTranslate(20f, 20f)

computeMatrix.postRotate(20f, x, y) // create float array with the points we want to map

val arrayPoint = floatArrayOf(point.x, point.y) // use the map points function to apply the same transformations that the matrix has, onto the input array of coordinates

computeMatrix.mapPoints(arrayPoint) // get the points out from the array, these will now be transformed by the matrix.

return PointF(arrayPoint[0], arrayPoint[1])

}