How can I scale my UI elements to the lesser screen dimension in Unity 5?

Question

I want to create a custom mobile pad for my game. The mobile pad consists of two parts (both inside an ui canvas):

• The D-Pad, which will be anchored to the bottom-left side.
• The button group will be anchored to the bottom-right side.

I would like to resize the element on different resolutions, so that the element (in my case, the D-Pad is an image):

• Has a size of 20% of the minimum of the width and the height.
• Has pivot of x=0.5, y=0.5.
• Having anchor of left-bottom, its coordinates are (15%, 15%) wrt the minimum of the width and the height.

What I tried is to use an Aspect Ratio Filter in the D-Pad image with ratio=1, but it did not work as intended (the coordinates led the image to an unexpected place).

Note: I am a n00b to Unity. I don't know whether it is right but my Canvas Scaler scales by Constant Pixel Size

These images should illustrate what I want to achieve:

What should I do to make it work like in the pictures?

Edit I attached this behavior, based on answer, to handle the change by runtime:

[RequireComponent(typeof(UI.CanvasScaler))]
class CanvasSmartScaler : MonoBehavior {
    private CanvasScaler _scaler;
    void Start() {
        _scaler=GetComponent<CanvasScaler>();
    }
    void Update() {
        //OMITTED CODE: here I set the scale mode as in the Editor but right now i am in my mobile device and cannot remember the exact lines
        if (Screen.currentResolution.width < Screen.currentResolution.height) {
            _scaler.matchWidthOrHeight=0;
        } else {
            _scaler.matchWidthOrHeight=1;
        }
    }
}

And attached that behavior to the canvas. However, for my debug exe (just for debug purposes) it always picks the 1-case regardless how do I resize the window. What am I missing?

Answer 1

On the canvas scaler try :

UI Scale Mode = Scale With Screen Size Then give a reference resolution. I like to use the standard 1920 x 1080. Screen Match Mode = Match Width or Height with a Match value of 0.5.

Also you might want to checkout how to make your UI image into a 9 sliced type. Check this out.

You can get a reference to the canvas scaler.

using UnityEngine.UI;
CanvasScaler scaler;
GameObject canvasGameObject;

void Start()
{
 scaler = canvasGameObject.GetComponent<CanvasScaler>();
}

I'm writing this code directly off the top of my head so it might not work. Let me know.