Unlock the Hidden Potential of Xamarin Android Scale Animations in Just 4 Easy Steps

As you've seen in The Translate Transform article, the translate transform can move a graphical object from one location to another. In contrast, the scale transform changes the size of the graphical object:

Elements that let you select horizontal and vertical scaling factors between 0 and 10. The BasicScalePage.xaml.cs code-behind file uses those values to call

Before displaying a rounded rectangle stroked with a dashed line and sized to fit some text in the upper-left corner of the canvas:

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The text, the width of the dashed line, the length of the dashes in that line, the rounding of the corners, and the 10-pixel margin between the left and top edges of the canvas and the rounded rectangle are all subject to the same scaling factors.

Anisotropic scaling causes the stroke width to become different for lines aligned with the horizontal and vertical axes. (This is also evident from the first image on this page.) If you don't want the stroke width to be affected by the scaling factors, set it to 0 and it will always be one pixel wide regardless of the

Scaling is relative to the upper-left corner of the canvas. This might be exactly what you want, but it might not be. Suppose you want to position the text and rectangle somewhere else on the canvas and you want to scale it relative to its center. In that case you can use the fourth version of the Scale method, which includes two additional parameters to specify the center of scaling:

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. This is a point relative to the upper-left corner of the canvas that is not affected by the scaling. All scaling occurs relative to that center.

Method are set to those values plus the width and height of the text, which is also the width and height of the rounded rectangle. This means that all scaling is relative to the center of that rectangle:

Elements in this program have a range of –10 to 10. As you can see, negative values of vertical scaling (such as on the Android screen in the center) cause objects to flip around the horizontal axis that passes through the center of scaling. Negative values of horizontal scaling (such as in the UWP screen on the right) cause objects to flip around the vertical axis that passes through the center of scaling.

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Now run the program again. You'll see that the rectangle and text are shifted so that the center is in the upper-left corner of the canvas. You can barely see it. The sliders don't work of course because now the program doesn't scale at all.

If you're familiar with this exercise in other graphics programming systems, you might think that's wrong, but it's not. Skia handles successive transform calls a little differently from what you might be familiar with.

Calls, the center of the rounded rectangle is still in the upper-left corner, but you can now scale it relative to the upper-left corner of the canvas, which is also the center of the rounded rectangle.

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Are 1. It's easy to convince yourself that the pivot point (px, py) is not transformed by these formulas. It remains in the same location relative to the canvas.

, the translation factors are not scaled. This process becomes somewhat clearer (albeit more mathematical) when the subject of transform matrices is introduced.

Properties are approximately 200. (Most of the actual values are a little less because the points of the stars are defined by a circle with a radius of 100 but only the top point is parallel with the horizontal or vertical axes.)

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The availability of this information implies that it should be possible to derive scale and translate factors suitable for scaling a path to the size of the canvas. The Anisotropic Scaling page demonstrates this with the 11-pointed star. An

Scale means that it's unequal in the horizontal and vertical directions, which means that the star won't retain its original aspect ratio. Here's the relevant code in the

Rectangle is obtained near the top of this code, and then used later with the width and height of the canvas in the

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Call but the star will be centered in the upper-right corner of the canvas. It needs to be shifted down and to the left. This is the job of the

Call, those values are effectively scaled by the scaling factors, so they move the center of the star to the center of the canvas:

Indicates the dimensions of the coordinates encoded in the path, and that's what the program uses to scale it. When the path is rendered with a particular stroke width, the rendered path is larger than the canvas.

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To fix this problem you need to compensate for that. One easy approach in this program is to add the following statement right before the

Rectangle by 1.5 units on all four sides. This is a reasonable solution only when the stroke join is rounded. A miter join can be longer and is difficult to calculate.

If you need to preserve the aspect ratio of the graphical objects, you'll want to use isotropic scaling. The Isotropic Scaling page demonstrates this for the 11-pointed star. Conceptually, the steps for displaying a graphical object in the center of the page with isotropic scaling are:

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The code also displays the star 10 more times, each time decreasing the scaling factor by 10% and progressively changing the color from red to blue:

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