Soft shadow control for Unity UI Toolkit

Shadow.cs

/* MIT License

Copyright (c) 2022 David Tattersall 

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE. */


using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UIElements;

class Shadow : VisualElement
{

    private Vertex[] k_Vertices;
    public int shadowCornerRadius { get; set; }
    public float shadowScale { get; set; }
    public int shadowOffsetX { get; set; }
    public int shadowOffsetY { get; set; }
    public int shadowCornerSubdivisions => 3;

    public new class UxmlFactory : UxmlFactory<Shadow, UxmlTraits> { }

    public new class UxmlTraits : VisualElement.UxmlTraits
    {

        public override IEnumerable<UxmlChildElementDescription> uxmlChildElementsDescription
        {
            get { yield break; }
        }

        // Rounded corner radius. Increase to make the shadow "fluffier"
        UxmlIntAttributeDescription radiusAttr =
            new UxmlIntAttributeDescription { name = "shadow-corner-radius", defaultValue = 10 };

        // Scale. Increase to make the shadow extend farther away from the element.
        UxmlFloatAttributeDescription scaleAttr =
            new UxmlFloatAttributeDescription { name = "shadow-scale", defaultValue = 1.1f };

        // Offsets. Tweak to have e.g. a shadow below and to the right of an element.
        UxmlIntAttributeDescription offsetXAttr =
            new UxmlIntAttributeDescription { name = "shadow-offset-x", defaultValue = 0 };

        UxmlIntAttributeDescription offsetYAttr =
            new UxmlIntAttributeDescription { name = "shadow-offset-y", defaultValue = 0 };

        // Buggy right now - always set to 3.
        /*UxmlIntAttributeDescription subdivisionsAttr =
            new UxmlIntAttributeDescription { name="shadow-corner-subdivisions", defaultValue = 3};*/

        public override void Init(VisualElement ve, IUxmlAttributes bag, CreationContext cc)
        {
            base.Init(ve, bag, cc);
            var shadow = ve as Shadow;

            shadow.shadowCornerRadius = radiusAttr.GetValueFromBag(bag, cc);
            shadow.shadowScale = scaleAttr.GetValueFromBag(bag, cc);
            shadow.shadowOffsetX = offsetXAttr.GetValueFromBag(bag, cc);
            shadow.shadowOffsetY = offsetYAttr.GetValueFromBag(bag, cc);
            //shadow.shadowCornerSubdivisions = subdivisionsAttr.GetValueFromBag(bag, cc);
        }
    }

    public Shadow()
    {
        generateVisualContent += OnGenerateVisualContent;
    }

    private void OnGenerateVisualContent(MeshGenerationContext ctx)
    {

        Rect r = contentRect;

        float left = 0;
        float right = r.width;
        float top = 0;
        float bottom = r.height;
        float halfSpread = (shadowCornerRadius / 2f);
        int curveSubdivisions = this.shadowCornerSubdivisions;
        int totalVertices = 12 + ((curveSubdivisions - 1) * 4);

        /*

        4/5/6/7 = inset rectangle (rect-shadowInsetAmount)
        0/1/2/3/8/9/10/11 = outset rectangle (rect+shadowSpread)

            1        2     12 => 12+(subdivisions-1)
           \|         /
       10 - 5========6 - 11
            |        |
            |        |
            |        |
            |        |
        9 - 4========7 - 8
           /          \
            0        3     (12+subdivisions-1)+1 => 12 + 2*(subdivisions-1) + 1

        */


        // Outside edge
        k_Vertices = new Vertex[totalVertices];

        k_Vertices[0].position = new Vector3(left + halfSpread, bottom + halfSpread, Vertex.nearZ);
        k_Vertices[1].position = new Vector3(left + halfSpread, top - halfSpread, Vertex.nearZ);
        k_Vertices[2].position = new Vector3(right - halfSpread, top - halfSpread, Vertex.nearZ);
        k_Vertices[3].position = new Vector3(right - halfSpread, bottom + halfSpread, Vertex.nearZ);
        k_Vertices[0].tint = Color.clear;
        k_Vertices[1].tint = Color.clear;
        k_Vertices[2].tint = Color.clear;
        k_Vertices[3].tint = Color.clear;

        k_Vertices[8].position = new Vector3(right + halfSpread, bottom - halfSpread, Vertex.nearZ);
        k_Vertices[9].position = new Vector3(left - halfSpread, bottom - halfSpread, Vertex.nearZ);
        k_Vertices[10].position = new Vector3(left - halfSpread, top + halfSpread, Vertex.nearZ);
        k_Vertices[11].position = new Vector3(right + halfSpread, top + halfSpread, Vertex.nearZ);
        k_Vertices[8].tint = Color.clear;
        k_Vertices[9].tint = Color.clear;
        k_Vertices[10].tint = Color.clear;
        k_Vertices[11].tint = Color.clear;

        // Inside rectangle
        k_Vertices[4].position = new Vector3(0 + halfSpread, r.height - halfSpread, Vertex.nearZ);
        k_Vertices[5].position = new Vector3(0 + halfSpread, 0 + halfSpread, Vertex.nearZ);
        k_Vertices[6].position = new Vector3(r.width - halfSpread, 0 + halfSpread, Vertex.nearZ);
        k_Vertices[7].position = new Vector3(r.width - halfSpread, r.height - halfSpread, Vertex.nearZ);
        k_Vertices[4].tint = resolvedStyle.color;
        k_Vertices[5].tint = resolvedStyle.color;
        k_Vertices[6].tint = resolvedStyle.color;
        k_Vertices[7].tint = resolvedStyle.color;

        // Top right corner
        for (int i = 0; i < curveSubdivisions - 1; i++)
        {
            int vertexId = 12 + i;
            float angle = (Mathf.PI * 0.5f / curveSubdivisions) + (Mathf.PI * 0.5f / curveSubdivisions) * i;

            k_Vertices[vertexId].position = new Vector3(r.width - halfSpread + Mathf.Sin(angle) * shadowCornerRadius, 0 + halfSpread + (-Mathf.Cos(angle) * shadowCornerRadius), Vertex.nearZ);
            k_Vertices[vertexId].tint = Color.clear;
        }

        // Bottom right corner
        for (int i = 0; i < curveSubdivisions - 1; i++)
        {
            int vertexId = 12 + i + (curveSubdivisions - 1);
            float angle = (Mathf.PI * 0.5f) + (Mathf.PI * 0.5f / curveSubdivisions) + (Mathf.PI * 0.5f / curveSubdivisions) * i;

            k_Vertices[vertexId].position = new Vector3(r.width - halfSpread + Mathf.Sin(angle) * shadowCornerRadius, r.height - halfSpread + (-Mathf.Cos(angle) * shadowCornerRadius), Vertex.nearZ);
            k_Vertices[vertexId].tint = Color.clear;
        }

        // Bottom left corner
        for (int i = 0; i < curveSubdivisions - 1; i++)
        {
            int vertexId = 12 + i + (curveSubdivisions - 1) * 2;
            float angle = (Mathf.PI) + (Mathf.PI * 0.5f / curveSubdivisions) + (Mathf.PI * 0.5f / curveSubdivisions) * i;

            k_Vertices[vertexId].position = new Vector3(0 + halfSpread + Mathf.Sin(angle) * shadowCornerRadius, r.height - halfSpread + (-Mathf.Cos(angle) * shadowCornerRadius), Vertex.nearZ);
            k_Vertices[vertexId].tint = Color.clear;
        }

        // Top left corner
        for (int i = 0; i < curveSubdivisions - 1; i++)
        {
            int vertexId = 12 + i + (curveSubdivisions - 1) * 3;
            float angle = (Mathf.PI * 1.5f) + (Mathf.PI * 0.5f / curveSubdivisions) + (Mathf.PI * 0.5f / curveSubdivisions) * i;

            k_Vertices[vertexId].position = new Vector3(0 + halfSpread + Mathf.Sin(angle) * shadowCornerRadius, 0 + halfSpread + (-Mathf.Cos(angle) * shadowCornerRadius), Vertex.nearZ);
            k_Vertices[vertexId].tint = Color.clear;
        }

        Vector3 dimensions = new Vector3(r.width, r.height, Vertex.nearZ);

        for (int i = 0; i < k_Vertices.Length; i++)
        {
            // Do not scale the inner rectangle
            Vector3 newPos = k_Vertices[i].position;
            newPos = newPos + new Vector3(shadowOffsetX, shadowOffsetY, 0);

            if (i >= 4 && i <= 7)
            {
                // Do nothing
            }
            else
            {
                newPos = ((newPos - (dimensions * 0.5f)) * shadowScale) + (dimensions * 0.5f);

            }

            // Scale verticles using scale factor
            k_Vertices[i].position = newPos;
        }

        List<ushort> tris = new List<ushort>();
        tris.AddRange(new ushort[]{
            1,6,5,
            2,6,1,
            6,11,8,
            6,8,7,
            4,7,3,
            4,3,0,
            10,5,4,
            10,4,9,
            5,6,4,
            6,7,4,
        });

        for (ushort i = 0; i < curveSubdivisions; i++)
        {
            if (i == 0)
            {
                tris.AddRange(new ushort[] { 2, 12, 6 });
            }
            else if (i == curveSubdivisions - 1)
            {
                tris.AddRange(new ushort[] { (ushort)(12 + i - 1), 11, 6 });
            }
            else
            {
                tris.AddRange(new ushort[] { (ushort)(12 + i - 1), (ushort)(12 + i), 6 });
            }
        }
        for (ushort i = 0; i < curveSubdivisions; i++)
        {
            if (i == 0)
            {
                tris.AddRange(new ushort[] { 7, 8, 14 });
            }
            else if (i == curveSubdivisions - 1)
            {
                tris.AddRange(new ushort[] { (ushort)(12 + i - 1 + (curveSubdivisions - 1)), 3, 7 });
            }
            else
            {
                tris.AddRange(new ushort[] { (ushort)(12 + i - 1 + (curveSubdivisions - 1)), (ushort)(12 + i + (curveSubdivisions - 1)), 7 });
            }
        }
        for (ushort i = 0; i < curveSubdivisions; i++)
        {
            if (i == 0)
            {
                tris.AddRange(new ushort[] { 4, 0, 16 });
            }
            else if (i == curveSubdivisions - 1)
            {
                tris.AddRange(new ushort[] { (ushort)(12 + i - 1 + 2 * (curveSubdivisions - 1)), 9, 4 });
            }
            else
            {
                tris.AddRange(new ushort[] { (ushort)(12 + i - 1 + 2 * (curveSubdivisions - 1)), (ushort)(12 + i + (2 * (curveSubdivisions - 1))), 4 });
            }
        }
        for (ushort i = 0; i < curveSubdivisions; i++)
        {
            if (i == 0)
            {
                tris.AddRange(new ushort[] { 5, 10, 18 });
            }
            else if (i == curveSubdivisions - 1)
            {
                tris.AddRange(new ushort[] { (ushort)(12 + i - 1 + 3 * (curveSubdivisions - 1)), 1, 5 });
            }
            else
            {
                tris.AddRange(new ushort[] { (ushort)(12 + i - 1 + 3 * (curveSubdivisions - 1)), (ushort)(12 + i + 3 * (curveSubdivisions - 1)), 5 });
            }
        }

        MeshWriteData mwd = ctx.Allocate(k_Vertices.Length, tris.Count);
        mwd.SetAllVertices(k_Vertices);
        mwd.SetAllIndices(tris.ToArray());

    }
}

This is great thank you so much!
A feature I hope they will build into later UI toolkit versions.

I've made a minor change to the transparent color so it doens't darken: https://gist.github.com/MartinCox86/5566584602282cf49ff0736d17f7ac0f

private Color clearColor;
clearColor = new(resolvedStyle.color.r, resolvedStyle.color.g, resolvedStyle.color.b, 0);
//Used instead of Color.clear
k_Vertices[0].tint = clearColor;

It seems like that you need to have attribute [UnityEngine.Scripting.Preserve] over the UxmlFactory to have the editor recognize the component. (I am using Unity 2022.3.5.f1) For those unable to see the component inside the Ui Builder after importing the script, try to add [UnityEngine.Scripting.Preserve] above public new class UxmlFactory : UxmlFactory<Shadow, UxmlTraits> { }. In a short, replace

public new class UxmlFactory : UxmlFactory<Shadow, UxmlTraits> { } with

    [UnityEngine.Scripting.Preserve]
    public new class UxmlFactory : UxmlFactory<Shadow, UxmlTraits> { }

If you want, you can get rid of all your GC allocs (new ushort[], new Vertex[], new List<ushort>, ToArray). GC allocs can cause Garbage Collection spikes and create stutter.

In Unity 2021.x and 2022.x, consider using temporary NativeArrays to store your vertices and indices. Call the NativeArray constructor with Allocator.Temp. Then, when you're done, you can call SetAllVertices and SetAllIndices with a NativeSlice, which can be obtained from the NativeArray.

Starting from Unity 2023.x, a better alternative would be to use MeshGenerationContext.AllocateTempMesh and MeshGenerationContext.DrawMesh.

I was inspired by this Gist to create an "outer glow effect" custom control, which is also useful for creating drop shadows, so I want to thank you for the inspiration and head start 😀

@alexandret-unity Good suggestion. You can pass a the NativeArray directly to SetAllVertices method, there is no need to get a NativeSlice. I have tested it here:
https://gist.github.com/edap/304781e090392068f767c4901aef1449

For anyone who uses Unity 2023.2 and later versions, UxmlFactory and UxmlTrait is deprecated and Unity suggests using UxmlElementAttribute for custom control creation. I have made a minor change based on @edap version:
https://gist.github.com/PhamQuangHung-UIT/f06e9c0076ee8df1715256ffa7c8f11f