Unity RGBA16 + Dither

游戏开发中有些场合，ETC或者说PVRTC压缩质量不满足的情况下，RGBA32（原图）对美术而言肯定可以满足的，但是RGBA32是不管是对内存占用内存太厉害. RGBA16/RGB16会减少内存的占用，但是Unity中并没有处理色阶的问题，看起来会使这样：

之前有文章提到过使用对图片预先进行Dither（抖动）处理之后，再使用RGBA16压缩，就会消除或者说减少色阶问题，不过作者使用的是TexturePacker进行的处理，而对于不采用TexturePacker进行UI Atlas打包的Unity项目就要换种思路实现。

Image Dither

图像抖动的基本原理是误差扩散，也就是将减少色阶后的误差扩散到周边像素，减少相邻像素点整体的误差，从而使肉眼在观察图片时没有那么明显的色阶问题，如下图：

更多详细描述可以阅读：http://www.tannerhelland.com/4660/dithering-eleven-algorithms-source-code/，里面对几种dither算法了详细描述，并给出了处理后的效果。具体的实现可以参考github：
https://github.com/minhhh/unity-texture-dither。对比了几种效果，我最后采用Floyd–Steinberg抖动算法。

Unity 图片预处理

具体实现中，可以在Unity导入图片时通过unity AssetPostprocessor对需要进行压缩的图片进行Dither处理，具体代码：

using UnityEngine;
using UnityEditor;
using System.Collections;

class TextureModifier : AssetPostprocessor
{
    void OnPreprocessTexture()
    {
        var importer = (assetImporter as TextureImporter);

        importer.textureType = TextureImporterType.GUI;

        if (assetPath.EndsWith ("Dither.png")) {
            importer.textureFormat = TextureImporterFormat.RGBA32;
        }
    }

    // Floyd–Steinberg dithering实现参考：https://en.wikipedia.org/wiki/Floyd%E2%80%93Steinberg_dithering
    void OnPostprocessTexture (Texture2D texture)
    {
        if (!assetPath.EndsWith ("Dither.png")) {
            return;
        }

        var texw = texture.width;
        var texh = texture.height;

        var pixels = texture.GetPixels ();
        var offs = 0;

        var k1Per15 = 1.0f / 15.0f;
        var k1Per16 = 1.0f / 16.0f;
        var k3Per16 = 3.0f / 16.0f;
        var k5Per16 = 5.0f / 16.0f;
        var k7Per16 = 7.0f / 16.0f;

        for (var y = 0; y < texh; y++) {
            for (var x = 0; x < texw; x++) {
                float a = pixels [offs].a;
                float r = pixels [offs].r;
                float g = pixels [offs].g;
                float b = pixels [offs].b;

                var a2 = Mathf.Clamp01 (Mathf.Floor (a * 16) * k1Per15);
                var r2 = Mathf.Clamp01 (Mathf.Floor (r * 16) * k1Per15);
                var g2 = Mathf.Clamp01 (Mathf.Floor (g * 16) * k1Per15);
                var b2 = Mathf.Clamp01 (Mathf.Floor (b * 16) * k1Per15);

                var ae = a - a2;
                var re = r - r2;
                var ge = g - g2;
                var be = b - b2;

                pixels [offs].a = a2;
                pixels [offs].r = r2;
                pixels [offs].g = g2;
                pixels [offs].b = b2;

                var n1 = offs + 1;   // (x+1,y)
                var n2 = offs + texw - 1; // (x-1 , y+1)
                var n3 = offs + texw;  // (x, y+1)
                var n4 = offs + texw + 1; // (x+1 , y+1)

                if (x < texw - 1) {
                    pixels [n1].a += ae * k7Per16;
                    pixels [n1].r += re * k7Per16;
                    pixels [n1].g += ge * k7Per16;
                    pixels [n1].b += be * k7Per16;
                }

                if (y < texh - 1) {
                    pixels [n3].a += ae * k5Per16;
                    pixels [n3].r += re * k5Per16;
                    pixels [n3].g += ge * k5Per16;
                    pixels [n3].b += be * k5Per16;

                    if (x > 0) {
                        pixels [n2].a += ae * k3Per16;
                        pixels [n2].r += re * k3Per16;
                        pixels [n2].g += ge * k3Per16;
                        pixels [n2].b += be * k3Per16;
                    }

                    if (x < texw - 1) {
                        pixels [n4].a += ae * k1Per16;
                        pixels [n4].r += re * k1Per16;
                        pixels [n4].g += ge * k1Per16;
                        pixels [n4].b += be * k1Per16;
                    }
                }

                offs++;
            }
        }

        texture.SetPixels (pixels);
        EditorUtility.CompressTexture (texture, TextureFormat.RGBA4444, TextureCompressionQuality.Best);
    }
}

效果对比