Unity 分离贴图 alpha 通道实践

// 刷新图集缓存
UpdateAtlases(buildTarget);

// 找到所有要处理的项
FindAllEntries(buildTarget, m_spriteEntries, m_atlasEntries);

// 生成 alpha 纹理
GenerateAlphaTextures(m_atlasEntries);

// 保存纹理到文件
SaveTextureAssets(m_atlasEntries);

// 刷新资源
AssetDatabase.Refresh();

// 从文件中加载 alpha 纹理
ReloadTextures(m_atlasEntries);

// 修改所有 sprite 的 Render Data
WriteSpritesRenderData(m_atlasEntries);

// 禁用 SpritePacker 准备打包
EditorSettings.spritePackerMode = SpritePackerMode.Disabled;

// 临时渲染贴图
var rt = RenderTexture.GetTemporary(texWidth, texHeight,
0, RenderTextureFormat.ARGB32);
Graphics.SetRenderTarget(rt);
GL.Clear(true, true, Color.clear);
GL.PushMatrix();
GL.LoadOrtho();

foreach (var spriteEntry in atlasEntry.SpriteEntries)
{
    var sprite = spriteEntry.Sprite;
    var uvs = spriteEntry.Uvs;
    var atlasUvs = spriteEntry.AtlasUvs;

    // 将压缩前 sprite 的顶点信息渲染到临时贴图上
    mat.mainTexture = spriteEntry.Texture;
    mat.SetPass(0);
    GL.Begin(GL.TRIANGLES);
    var triangles = sprite.triangles;
    foreach (var index in triangles)
    {
        GL.TexCoord(uvs[index]);
        GL.Vertex(atlasUvs[index]);
    }

    GL.End();
}

GL.PopMatrix();

// 最终的 alpha 贴图
var finalTex = new Texture2D(texWidth, texHeight, TextureFormat.RGBA32, false);
finalTex.ReadPixels(new Rect(0, 0, texWidth, texHeight), 0, 0);

// 修改颜色
var colors = finalTex.GetPixels32();
var count = colors.Length;
var newColors = new Color32[count];
for (var i = 0; i < count; ++i)
{
    var a = colors.a;
    newColors = new Color32(a, a, a, 255);
}

finalTex.SetPixels32(newColors);
finalTex.Apply();

RenderTexture.ReleaseTemporary(rt);

// 支持多 page 图集
var hashCode = atlasEntry.Texture.GetHashCode();

// 导出 alpha 纹理
if (atlasEntry.NeedSeparateAlpha)
{
    var fileName = atlasEntry.Name + "_" + hashCode + "_alpha.png";
    var filePath = Path.Combine(path, fileName);
    File.WriteAllBytes(filePath, atlasEntry.AlphaTexture.EncodeToPNG());
    atlasEntry.AlphaTextureAssetPath = Path.Combine(assetPath, fileName);
}

var spr = spriteEntry.Sprite;
var so = new SerializedObject(spr);

// 获取散图属性
var rect = so.FindProperty("m_Rect").rectValue;
var pivot = so.FindProperty("m_Pivot").vector2Value;
var pixelsToUnits = so.FindProperty("m_PixelsToUnits").floatValue;
var tightRect = so.FindProperty("m_RD.textureRect").rectValue;
var originSettingsRaw = so.FindProperty("m_RD.settingsRaw").intValue;

// 散图(tight)在散图(full rect)中的位置和宽高
var tightOffset = new Vector2(tightRect.x, tightRect.y);
var tightWidth = tightRect.width;
var tightHeight = tightRect.height;

// 计算散图(full rect)在图集中的 rect 和 offset
var fullRectInAtlas = GetTextureFullRectInAtlas(atlasTexture,
    spriteEntry.Uvs, spriteEntry.AtlasUvs);
var fullRectOffsetInAtlas = new Vector2(fullRectInAtlas.x, fullRectInAtlas.y);

// 计算散图(tight)在图集中的 rect
var tightRectInAtlas = new Rect(fullRectInAtlas.x + tightOffset.x,
    fullRectInAtlas.y + tightOffset.y, tightWidth, tightHeight);

// 计算 uvTransform
// x: Pixels To Unit X
// y: 中心点在图集中的位置 X
// z: Pixels To Unit Y
// w: 中心点在图集中的位置 Y
var uvTransform = new Vector4(
    pixelsToUnits,
    rect.width * pivot.x + fullRectOffsetInAtlas.x,
    pixelsToUnits,
    rect.height * pivot.y + fullRectOffsetInAtlas.y);

// 计算 settings
// 0 位：packed。1 表示 packed，0 表示不 packed
// 1 位：SpritePackingMode。0 表示 tight，1 表示 rectangle
// 2-5 位：SpritePackingRotation。0 表示不旋转，1 表示水平翻转，2 表示竖直翻转，3 表示 180 度旋转，4 表示 90 度旋转
// 6 位：SpriteMeshType。0 表示 full rect，1 表示 tight
// 67 = SpriteMeshType(tight) + SpritePackingMode(rectangle) + packed
var settingsRaw = 67;

// 写入 RenderData
so.FindProperty("m_RD.texture").objectReferenceValue = atlasTexture;
so.FindProperty("m_RD.alphaTexture").objectReferenceValue = alphaTexture;
so.FindProperty("m_RD.textureRect").rectValue = tightRectInAtlas;
so.FindProperty("m_RD.textureRectOffset").vector2Value = tightOffset;
so.FindProperty("m_RD.atlasRectOffset").vector2Value = fullRectOffsetInAtlas;
so.FindProperty("m_RD.settingsRaw").intValue = settingsRaw;
so.FindProperty("m_RD.uvTransform").vector4Value = uvTransform;
so.ApplyModifiedProperties();

// 备份原数据，用于恢复
spriteEntry.OriginTextureRect = tightRect;
spriteEntry.OriginSettingsRaw = originSettingsRaw;

private static Rect GetTextureFullRectInAtlas(Texture2D atlasTexture, Vector2[] uvs, Vector2[] atlasUvs)
{
    var textureRect = new Rect();

    // 找到某一个 x/y 都不相等的点
    var index = 0;
    var count = uvs.Length;
    for (var i = 1; i < count; i++)
    {
        if (Math.Abs(uvs.x - uvs[0].x) > 1E-06 &&
            Math.Abs(uvs.y - uvs[0].y) > 1E-06)
        {
            index = i;
            break;
        }
    }

    // 计算散图在大图中的 texture rect
    var atlasWidth = atlasTexture.width;
    var atlasHeight = atlasTexture.height;
    textureRect.width = (atlasUvs[0].x - atlasUvs[index].x) / (uvs[0].x - uvs[index].x) * atlasWidth;
    textureRect.height = (atlasUvs[0].y - atlasUvs[index].y) / (uvs[0].y - uvs[index].y) * atlasHeight;
    textureRect.x = atlasUvs[0].x * atlasWidth - textureRect.width * uvs[0].x;
    textureRect.y = atlasUvs[0].y * atlasHeight - textureRect.height * uvs[0].y;

    return textureRect;
}

// 需要分离 alpha 通道的情况
if (TextureUtility.IsTransparent(settings.format))
{   
    settings.format = TextureUtility.TransparentToNoTransparentFormat(settings.format);        
}