val rect = RectF(100.0f, 200.0f, 300.0f, 400.0f) val (left, top, right, bottom) = rect // left = 100.0f, top = 200.0f, right = 300.0f, bottom = 400.0f
You can do other operations with a
Rect
class, for instance, you can union two
Rects
together. This basically just includes the points from both
Rects
and returns a bigger
Rect
that contains both
Rects
inside it. There are also extension functions for this operation, but it is also possible without the extension:
val rect = RectF(100.0f, 200.0f, 300.0f, 400.0f)
val otherRect = RectF(50.0f, 400.0f, 150.0f, 500.0f)
rect.union(otherRect)
// rect = RectF(50.0, 200.0, 300.0, 500.0)
// alternatively:
val combinedRect = rect + otherRect
// or alternatively:
val combinedRect = rect or otherRect
// combinedRect = RectF(50.0, 200.0, 300.0, 500.0)
There are other operations you can perform on a
Rect
, such as :
and
,
xor
,
or
.
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
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.
Example of two different coordinate systems
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]) }
In the above example, the input point would be the touch event from Android, and the translation and rotation that we apply on the computeMatrix is the same translation and rotation we applied on the image when it was drawn. Then we create a float array which contains the original x and y point. We then call the mapPoints method with this array. It’ll then transform the values in place and when we query the array for the first and second values it’ll be the mapped coordinate, which is the point inside the image view.
You can see that the Android Graphics APIs contain loads of useful classes that you can leverage to do a lot of the calculations and mathematics for you. From Points to Rects to more complex calculation classes like Matrixclasses, we can see that there are many things we can use to help us draw graphics on the screen! Make sure to include KTX to have an even smoother experience when working with Android Graphics classes.
Have any questions or comments? Feel free to reach out and say hi to me on Twitter.
