Unity图片优化-Dither算法（RGBA16优化）

在unity开发过程中，游戏图片占用了很大一部分的手机内存。所以在游戏开发中，对图片的优化也至关重要。

在Unity中常用的的图片格式有RGBA32，RGBA16，ETC，PVRTC等。这里我们主要讨论带透明通道的RGBA32和RGBA16这两种格式的图片在Unity占用的内存空间已经优化方案。

我们知道，RGBA32是R,G,B,A四个通道每个通道用8位来表示，RGBA16则是用4位。所以RGBA32能够带来更好的显示效果。同时也会带来更大的内存消耗。下面给两张RGBA16和RGBA32的对比图。

RGBA32

RGBA32.png

RGBA16

RGBA16.png

从上面两个图可以看得出，RGBA32能够带来更好的显示效果。而RGBA16在有些地方的色阶太明显，导致显示效果不尽人意。由此keijiro(Github地址)写了一个dither算法来消除这种色阶，以达到高于RGBA16低于RGBA32的显示效果。

下图为Dither优化之后的RGBA16

RGBA16-Dither.png

通过对比能明显看出优化后的RGBA16能够消除色阶，如果不是放大特意看，和RGBA32几乎差别不同。通过这种方式将内存降低一半确认带来更好的效果。

万事总会存在利弊，这个算法在消除了色阶的同事，带来的是更多的噪点。所以这个方法不适用于图片需要放大来显示的。总体来说，该方案在一定程度上还是能够带来很好的效果。
最后把算法的核心代码贴出来 。

public class TextureImportSetting : AssetPostprocessor {

    string[] assetTexturePath = new string[]{"Assets/Resources/TextureVN/"}; //放置需要优化的路径
    void OnPreprocessTexture(){
        foreach (var str in assetTexturePath)
        {
            if (this.assetPath.StartsWith(str) )
            {
                TextureImporter textureImporter = this.assetImporter as TextureImporter;
                textureImporter.textureType = TextureImporterType.Advanced;
                textureImporter.npotScale = TextureImporterNPOTScale.ToNearest;
                textureImporter.mipmapEnabled = false;
                textureImporter.isReadable = false;
                if ( textureImporter.DoesSourceTextureHaveAlpha())
                {
                    textureImporter.textureFormat = TextureImporterFormat.RGBA32;
                }
            }
        }
    }
    public static void OnPostprocessRGB16 (Texture2D texture)
    {
        
        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;
                var n2 = offs + texw - 1;
                var n3 = offs + texw;
                var n4 = offs + texw + 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);
    }
    void OnPostprocessTexture(Texture2D texture){

        foreach (var str in assetTexturePath)
        {
            if (this.assetPath.StartsWith(str) )
            {
                TextureImporter textureImporter = this.assetImporter as TextureImporter;
                if ( textureImporter.DoesSourceTextureHaveAlpha())
                {
                    OnPostprocessRGB16(texture);
                }
            }
        }
    }
}