com.unity.uiextensions/Scripts/Effects/Gradient.cs

/// Credit Breyer
/// Sourced from - http://forum.unity3d.com/threads/scripts-useful-4-6-scripts-collection.264161/#post-1780095

using System.Collections.Generic;

namespace UnityEngine.UI.Extensions
{
    [AddComponentMenu("UI/Effects/Extensions/Gradient")]
    public class Gradient : BaseMeshEffect
    {
        public GradientMode gradientMode = GradientMode.Global;
        public GradientDir gradientDir = GradientDir.Vertical;
        public bool overwriteAllColor = false;
        public Color vertex1 = Color.white;
        public Color vertex2 = Color.black;
        private Graphic targetGraphic;

        protected override void Start()
        {
            targetGraphic = GetComponent<Graphic>();
        }

        public override void ModifyMesh(Mesh mesh)
        {
            if (!IsActive() || mesh.vertexCount == 0)
            {
                return;
            }

            Vector3[] vertexList = mesh.vertices;
            Color[] vertexListColors = mesh.colors;
            int count = mesh.vertexCount;
            Vector3 uiVertex = vertexList[0];
            Color uiVertexColor = vertexListColors[0];
            if (gradientMode == GradientMode.Global)
            {
                if (gradientDir == GradientDir.DiagonalLeftToRight || gradientDir == GradientDir.DiagonalRightToLeft)
                {
#if UNITY_EDITOR
                    Debug.LogWarning("Diagonal dir is not supported in Global mode");
#endif
                    gradientDir = GradientDir.Vertical;
                }
                float bottomY = gradientDir == GradientDir.Vertical ? vertexList[vertexList.Length - 1].y : vertexList[vertexList.Length - 1].x;
                float topY = gradientDir == GradientDir.Vertical ? vertexList[0].y : vertexList[0].x;

                float uiElementHeight = topY - bottomY;

                for (int i = 0; i < count; i++)
                {
                    uiVertex = vertexList[i];
                    uiVertexColor = vertexListColors[i];
                    if (!overwriteAllColor && uiVertexColor != targetGraphic.color)
                        continue;
                    uiVertexColor *= Color.Lerp(vertex2, vertex1, ((gradientDir == GradientDir.Vertical ? uiVertex.y : uiVertex.x) - bottomY) / uiElementHeight);
                    vertexListColors[i] = uiVertexColor;
                }
            }
            else
            {
                for (int i = 0; i < count; i++)
                {
                    uiVertex = vertexList[i];
                    uiVertexColor = vertexListColors[i];
                    if (!overwriteAllColor && !CompareCarefully(uiVertexColor, targetGraphic.color))
                        continue;
                    switch (gradientDir)
                    {
                        case GradientDir.Vertical:
                            uiVertexColor *= (i % 4 == 0 || (i - 1) % 4 == 0) ? vertex1 : vertex2;
                            break;
                        case GradientDir.Horizontal:
                            uiVertexColor *= (i % 4 == 0 || (i - 3) % 4 == 0) ? vertex1 : vertex2;
                            break;
                        case GradientDir.DiagonalLeftToRight:
                            uiVertexColor *= (i % 4 == 0) ? vertex1 : ((i - 2) % 4 == 0 ? vertex2 : Color.Lerp(vertex2, vertex1, 0.5f));
                            break;
                        case GradientDir.DiagonalRightToLeft:
                            uiVertexColor *= ((i - 1) % 4 == 0) ? vertex1 : ((i - 3) % 4 == 0 ? vertex2 : Color.Lerp(vertex2, vertex1, 0.5f));
                            break;

                    }
                    vertexListColors[i] = uiVertexColor;
                }
            }
            mesh.colors = vertexListColors;
        }
        private bool CompareCarefully(Color col1, Color col2)
        {
            if (Mathf.Abs(col1.r - col2.r) < 0.003f && Mathf.Abs(col1.g - col2.g) < 0.003f && Mathf.Abs(col1.b - col2.b) < 0.003f && Mathf.Abs(col1.a - col2.a) < 0.003f)
                return true;
            return false;
        }
    }

    public enum GradientMode
    {
        Global,
        Local
    }

    public enum GradientDir
    {
        Vertical,
        Horizontal,
        DiagonalLeftToRight,
        DiagonalRightToLeft
        //Free
    }
    //enum color mode Additive, Multiply, Overwrite
}