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
RGBA16
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
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 。1
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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);
}
}
}
}
}
