The alpha threshold specifies the minimum alpha a pixel must have for the event to considered a "hit" on the Image.
Alpha values less than the threshold will cause raycast events to pass through the Image. An value of 1 would cause only fully opaque pixels to register raycast events on the Image. The alpha tested is retrieved from the image sprite only, while the alpha of the Image Graphic.color is disregarded.
alphaHitTestMinimumThreshold defaults to 0; all raycast events inside the Image rectangle are considered a hit. In order for greater than 0 to values to work, the sprite used by the Image must have readable pixels. This can be achieved by enabling Read/Write enabled in the advanced Texture Import Settings for the sprite and disabling atlassing for the sprite.
using UnityEngine; using System.Collections; using UnityEngine.UI; // Required when Using UI elements.
public class ExampleClass : MonoBehaviour {
public Image theButton;
// Use this for initialization void Start () { theButton.alphaHitTestMinimumThreshold = 0.5f; } }