看看UGUI教程之SpritePacker打包图集。我觉得我们有必要对比一下NGUI的图集，NGUI在打包图集的时候图集的默认格式是RGBA32，也就是支持带透明通道的图片，这样一张1024的图集也就是4M内存。为了优化图集，我们可以选择把带透明通道图片 和 不带透明通道的图片分开打图集，这样可以减少内存的占用量。

然而着一切的一切在NGUI上都需要手动操作，而SpritePacker则全自动完成。Sprite上的Packing Tag 同一标识的图片UGUI会把相同图片格式的图片打包成同一图集。如下图所示，MomoAtals和RUORUOAtlas就是Packing Tag的标识符，那么此时根据这两个标识符SpritePacker将打出两个图集出来。 因为MomoAtlas这些图片中，一部分是RGBA32格式，还有一部分是ETC 4bits格式，那么MomoAtlas将被在分成两个图集，就是尾缀带Group的。

       打包Sprite Packer有两个打包模式，如下图所示分别是DefaultPackerPolicy和TightPackerPolicy。

DefaultPackerPolicy：是默认的打包方式，也是矩形打包方式。他会把所有的小图按照矩形的方式来排列，如果宽高不一样的图片，它们会自动补起。

TightPackerPolicy：是紧密打包方式，也就是尽可能的把图片都打包在图集上，这种方式要比DefaultPackerPolicy打包的图片更多一些，也就是更省空间。

       根据图集的布局可以清晰的看到TightPackerPolicy图集更加紧密。

DefaultPackerPolicy模式打包是unity所推荐的，理论上所有图集都可以使用DefaultPackerPolicy来完成打包。还有一个特性就是可以让图集中某几张图片单独采取DefaultPackerPolicy或者TightPackerPolicy的方式。

如下图所示，比如当前打包图集是DefaultPackerPolicy 那么小图中[TIGHT]开头的就表示单独这张图采用TightPackerPolicy打包模式。

      如下图所示，比如当前打包图集是TightPackerPolicy 那么小图中[RECT]开头的就表示单独这张图采用DefaultPackerPolicy打包模式。

       Unity只提供了这两种图集打包方法。假如我想自定义打包方式咋办?比如我想设置图片打包格式，或者图集大小等等怎么办？把如下代码放在Editor文件夹下， 在代码里面就可以设置图集的属性了。

using System;
using System.Linq;
using UnityEngine;
using UnityEditor;
using UnityEditor.Sprites;
using System.Collections.Generic;
 
// DefaultPackerPolicy will pack rectangles no matter what Sprite mesh type is unless their packing tag contains \"[TIGHT]\".
class DefaultPackerPolicySample : UnityEditor.Sprites.IPackerPolicy
{
    protected class Entry
    {
        public Sprite            sprite;
        public AtlasSettings     settings;
        public string            atlasName;
        public SpritePackingMode packingMode;
    }
    
    public virtual int GetVersion() { return 1; }
    
    protected virtual string TagPrefix { get { return \"[TIGHT]\"; } }
    protected virtual bool AllowTightWhenTagged { get { return true; } }
    
    public void OnGroupAtlases(BuildTarget target, PackerJob job, int[] textureImporterInstanceIDs)
    {
        List<Entry> entries = new List<Entry>();
        
        foreach (int instanceID in textureImporterInstanceIDs)
        {
            TextureImporter ti = EditorUtility.InstanceIDToObject(instanceID) as TextureImporter;
            
            TextureImportInstructions ins = new TextureImportInstructions();
            ti.ReadTextureImportInstructions(ins, target);
            
            TextureImporterSettings tis = new TextureImporterSettings();
            ti.ReadTextureSettings(tis);
            
            Sprite[] sprites = AssetDatabase.LoadAllAssetRepresentationsAtPath(ti.assetPath).Select(x => x as Sprite).Where(x => x != null).ToArray();
            foreach (Sprite sprite in sprites)
            {
                                //在这里设置每个图集的参数
                Entry entry = new Entry();
                entry.sprite = sprite;
                entry.settings.format = ins.desiredFormat;
                entry.settings.usageMode = ins.usageMode;
                entry.settings.colorSpace = ins.colorSpace;
                entry.settings.compressionQuality = ins.compressionQuality;
                entry.settings.filterMode = Enum.IsDefined(typeof(FilterMode), ti.filterMode) ? ti.filterMode : FilterMode.Bilinear;
                entry.settings.maxWidth = 1024;
                entry.settings.maxHeight = 1024;
                entry.atlasName = ParseAtlasName(ti.spritePackingTag);
                entry.packingMode = GetPackingMode(ti.spritePackingTag, tis.spriteMeshType);
                
                entries.Add(entry);
            }
            
            Resources.UnloadAsset(ti);
        }
        
        // First split sprites into groups based on atlas name
        var atlasGroups =
            from e in entries
                group e by e.atlasName;
        foreach (var atlasGroup in atlasGroups)
        {
            int page = 0;
            // Then split those groups into smaller groups based on texture settings
            var settingsGroups =
                from t in atlasGroup
                    group t by t.settings;
            foreach (var settingsGroup in settingsGroups)
            {
                string atlasName = atlasGroup.Key;
                if (settingsGroups.Count() > 1)
                    atlasName += string.Format(\" (Group {0})\", page);
                
                job.AddAtlas(atlasName, settingsGroup.Key);
                foreach (Entry entry in settingsGroup)
                {
                    job.AssignToAtlas(atlasName, entry.sprite, entry.packingMode, SpritePackingRotation.None);
                }
                
                ++page;
            }
        }
    }
    
    protected bool IsTagPrefixed(string packingTag)
    {
        packingTag = packingTag.Trim();
        if (packingTag.Length < TagPrefix.Length)
            return false;
        return (packingTag.Substring(0, TagPrefix.Length) == TagPrefix);
    }
    
    private string ParseAtlasName(string packingTag)
    {
        string name = packingTag.Trim();
        if (IsTagPrefixed(name))
            name = name.Substring(TagPrefix.Length).Trim();
        return (name.Length == 0) ? \"(unnamed)\" : name;
    }
    
    private SpritePackingMode GetPackingMode(string packingTag, SpriteMeshType meshType)
    {
        if (meshType == SpriteMeshType.Tight)
            if (IsTagPrefixed(packingTag) == AllowTightWhenTagged)
                return SpritePackingMode.Tight;
        return SpritePackingMode.Rectangle;
    }
}