AspectFitRawImage.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

[RequireComponent(typeof(RawImage))]
[ExecuteInEditMode]
public class AspectFitRawImage : MonoBehaviour
{
    protected RawImage m_rawImage;
    protected Vector2 m_rectSize = Vector2.one;

    /// <summary>
    /// Awake is called when the script instance is being loaded.
    /// </summary>
    void Awake()
    {
        GetRectSize();                              // Get the view rect size
        m_rawImage = GetComponent<RawImage>();      // Get the image
        DoAspectFit();        
    }

    protected void GetRectSize() {
		RectTransform tf = transform as RectTransform;

		m_rectSize = tf.sizeDelta;
		if(m_rectSize.x == 0 || m_rectSize.y == 0) {
			m_rectSize = (transform.parent as RectTransform).sizeDelta;
		}

        if(m_rectSize.x <= 0 || m_rectSize.y <= 0) {    // Fail back logic 
            m_rectSize = Vector2.one;
        }
	}



    protected virtual void DoAspectFit() {
		if(m_rawImage == null) {
            Debug.Log("AspectFitRawImage: m_rawImage is null");
            return;
        }
        Texture texture = m_rawImage.texture;
        if(texture == null) {
            Debug.Log("AspectFitRawImage: texture is null");
            return;
        }

        float w = texture.width;
        float h = texture.height;
        if(w == h) {
			m_rawImage.uvRect = new Rect(0, 0, 1, 1);
            return;     // nothing to do
        }

		float textureRatio = w/h;
		float viewRatio = m_rectSize.x/m_rectSize.y;

        // ken: some debug value
		// Debug.Log("rectSize=" + m_rectSize);
        // Debug.Log("w=" + w + " h=" + h);
        // Debug.Log("textureRatio=" + textureRatio + " viewRatio=" + viewRatio);


		if(viewRatio == textureRatio) {
			if(m_rawImage != null) {
				m_rawImage.uvRect = new Rect(0, 0, 1, 1);
			}
		}


		if(viewRatio > textureRatio) {	// middle potion of the image 
			float potionRatio =  textureRatio / viewRatio;	// (diff - 1) * textureRatio;
			float offset = (1 - potionRatio) * 0.5f;
			m_rawImage.uvRect = new Rect(0, offset, 1, potionRatio);
		} else {
			float potionRatio =  viewRatio / textureRatio;	// (diff - 1) * textureRatio;
			float offset = (1 - potionRatio) * 0.5f;
			m_rawImage.uvRect = new Rect(offset, 0, potionRatio, 1);
		}
	}
}