public abstract class UIBasicSprite : UIWidget
{
    ...
    
    public enum FillDirection
	{
		Horizontal,
		Vertical,
		Radial90,
		Radial180,
		Radial360,
		Diagonal,
	}

    ...
    
    /// <summary>
	/// Filled sprite fill function.
	/// </summary>

	void FilledFill (BetterList<Vector3> verts, BetterList<Vector2> uvs, BetterList<Color32> cols)
	{
	    ...
        
		if (mFillAmount < 1f)
		{
            ...

            if (mFillDirection == FillDirection.Diagonal)
            {
				if (DiagonalCut(mTempPos, mTempUVs, mFillAmount, mInvert, 0))
				{
					for (int i = 0; i < 4; ++i)
					{
						verts.Add(mTempPos[i]);
						uvs.Add(mTempUVs[i]);
						cols.Add(c);
                    }
                }

				if (mFillAmount > 0.5f)
				{
					mTempPos[0] = new Vector2(v.x, v.y);
					mTempPos[1] = new Vector2(v.x, v.w);
					mTempPos[2] = new Vector2(v.z, v.w);
					mTempPos[3] = new Vector2(v.z, v.y);

					mTempUVs[0] = new Vector2(u.x, u.y);
					mTempUVs[1] = new Vector2(u.x, u.w);
					mTempUVs[2] = new Vector2(u.z, u.w);
					mTempUVs[3] = new Vector2(u.z, u.y);

					if (DiagonalCut(mTempPos, mTempUVs, mFillAmount, mInvert, 1))
					{
						for (int i = 0; i < 4; ++i)
						{
							verts.Add(mTempPos[i]);
							uvs.Add(mTempUVs[i]);
							cols.Add(c);
						}
					}
				}

				return;
			}
		}
        
        ...
	}
    
	/// <summary>
	/// Diagonal filled instead
	/// </summary>

	bool DiagonalCut(Vector2[] xy, Vector2[] uv, float fill, bool invert, int corner)
	{
		// Nothing to fill
		if (fill < 0.001f)
		{
			return false;
		}

        // Nothing to adjust
        if (!invert && fill > 0.999f)
        {
            return true;
        }

		DiagonalCut(xy, invert, fill, corner);
		DiagonalCut(uv, invert, fill, corner);
		return true;
	}

	static void DiagonalCut(Vector2[] xy,  bool invert, float fill, int corner)
    {
		if (corner == 0)
		{
			var plusPoint5Fill = fill / 0.5f;

			if (invert == false)
			{
				xy[0].y = Mathf.Lerp(xy[1].y, xy[3].y, plusPoint5Fill);
				xy[2].x = Mathf.Lerp(xy[1].x, xy[3].x, plusPoint5Fill);
				xy[3] = (xy[0] + xy[2]) / 2;
			}
			else
			{
				xy[0].x = Mathf.Lerp(xy[3].x, xy[1].x, plusPoint5Fill);
				xy[2].y = Mathf.Lerp(xy[3].y, xy[1].y, plusPoint5Fill);
				xy[1] = (xy[0] + xy[2]) / 2;
			}
		}
		else
		{
			var minusPoint5Fill = (fill - 0.5f) / 0.5f;

			if (invert == false)
			{
				xy[1] = xy[2];
				xy[2].y = Mathf.Lerp(xy[2].y, xy[0].y, minusPoint5Fill);
				xy[3].x = Mathf.Lerp(xy[0].x, xy[2].x, minusPoint5Fill);
			}
			else
			{
				xy[3] = xy[2];
				xy[1].y = Mathf.Lerp(xy[0].y, xy[2].y, minusPoint5Fill);
				xy[2].x = Mathf.Lerp(xy[3].x, xy[0].x, minusPoint5Fill);
			}
		}
	}
}