浅谈Unity UI适配（二）

书接上回，继续讲CanvasScaler的第二种适配方式，ScaleWithScreenSize。

2.2 Scale With Screen Size —— 屏幕尺寸比例

这种缩放模式下的UI位置是根据屏幕的分辨率和设置的宽高比来调整UI的位置的，通常做屏幕UI自适应的时候都需要调整到这个缩放模式下。

先来看官方的算法：

Vector2 screenSize = new Vector2(Screen.width, Screen.height);
 
float scaleFactor = 0;
switch (m_ScreenMatchMode)
{
    case ScreenMatchMode.MatchWidthOrHeight:
    {
        // We take the log of the relative width and height before taking the average.
        // Then we transform it back in the original space.
        // the reason to transform in and out of logarithmic space is to have better behavior.
        // If one axis has twice resolution and the other has half, it should even out if widthOrHeight value is at 0.5.
        // In normal space the average would be (0.5 + 2) / 2 = 1.25
        // In logarithmic space the average is (-1 + 1) / 2 = 0
        float logWidth = Mathf.Log(screenSize.x / m_ReferenceResolution.x, kLogBase);
        float logHeight = Mathf.Log(screenSize.y / m_ReferenceResolution.y, kLogBase);
        float logWeightedAverage = Mathf.Lerp(logWidth, logHeight, m_MatchWidthOrHeight);
        scaleFactor = Mathf.Pow(kLogBase, logWeightedAverage);
        break;
    }
    case ScreenMatchMode.Expand:
    {
        scaleFactor = Mathf.Min(screenSize.x / m_ReferenceResolution.x, screenSize.y / m_ReferenceResolution.y);
        break;
    }
    case ScreenMatchMode.Shrink:
    {
        scaleFactor = Mathf.Max(screenSize.x / m_ReferenceResolution.x, screenSize.y / m_ReferenceResolution.y);
        break;
    }
}

下面，就详细的说一下Screen Match Mode缩放模式：

Expand(扩大)：将Canvas Size进行宽或高扩大

scaleFactor = Mathf.Min(screenSize.x / m_ReferenceResolution.x, screenSize.y / m_ReferenceResolution.y);

 意思是分别算出长宽 ，也就是Screen Size在Reference Resolution的比例。

举例来说，Reference Resolution为1280 X 720，Screen Size为800 X 600.

 套用ScaleFactor公式：

实际结果同我们计算的一致：

 

Shrink(收缩)：将Canvas Size进行宽或高收缩

根据官方代码，计算方式如下：

scaleFactor = Mathf.Max(screenSize.x / m_ReferenceResolution.x, screenSize.y / m_ReferenceResolution.y);

意思是分别算出长宽 ，也就是Screen Size在Reference Resolution的比例。

举例来说，Reference Resolution为1280 X 720，Screen Size为800 X 600.

 套用ScaleFactor公式：

Canvas Width: 800 / 0.8333 = 960
Canvas Height:600 / 0.8333 = 720

实际结果同我们计算的一致：

Match Width or Height：根据Width或Height进行混合缩放

根据官方代码，计算方式如下：

float logWidth = Mathf.Log(screenSize.x / m_ReferenceResolution.x, kLogBase);
float logHeight = Mathf.Log(screenSize.y / m_ReferenceResolution.y, kLogBase);
float logWeightedAverage = Mathf.Lerp(logWidth, logHeight, m_MatchWidthOrHeight);
scaleFactor = Mathf.Pow(kLogBase, logWeightedAverage);

分别对ScaleFactor Width、Height取对数后，再进行平均混合，那为什麽不直接使用March对Width、Height进行混合呢?? 让我们来比较一下：

假设Reference Resolution为400 X 300，Screen Size为200 X 600 大小关系是

Reference Resolution Width 是 Screen Size Width的2倍
Reference Resolution Height 是 Screen Size 的0.5倍

看起来会像下图：

 当March为0.5时，ScaleFactor应该要是 1 (拉平)：

ScaleFactor Width： 200/400=0.5
ScaleFactor Height：600/300=2

一般混合：

ScaleFactor = March * ScaleFactor Width + March * ScaleFactorHeight
ScaleFactor = 0.5 * 0.5 + 0.5 * 2 = 1.25

对数混合：

logWidth：log2(0.5) = -1
logHeight：log2(2) = 1
logWeightedAverage：0
ScaleFactor：20 = 1

scaleFactor一般混合为1.25，对数混合为1，结果很明显，使用对数混合能更完美的修正大小。

2-3、Constant Physical Size —— 恒定尺寸

 

属性说明：

1、Physical Unit单位

 2、Fallback Screen DPI：备用Dpi，当找不到设备Dpi时，使用此值
3、Default Sprite DPI：预设的图片Dpi

官方代码：

float currentDpi = Screen.dpi;
float dpi = (currentDpi == 0 ? m_FallbackScreenDPI : currentDpi);
float targetDPI = 1;
switch (m_PhysicalUnit)
{
    case Unit.Centimeters: targetDPI = 2.54f; break;
    case Unit.Millimeters: targetDPI = 25.4f; break;
    case Unit.Inches:      targetDPI =     1; break;
    case Unit.Points:      targetDPI =    72; break;
    case Unit.Picas:       targetDPI =     6; break;
}
 
SetScaleFactor(dpi / targetDPI);
SetReferencePixelsPerUnit(m_ReferencePixelsPerUnit * targetDPI / m_DefaultSpriteDPI);

 

结论

■ ScaleFactor 为 “目前硬件dpi” 占了 “目标单位” 的比例

■ ReferencePixelsPerUnit 要与目前的Dpi在运算求出新的值，再传入Canvas中求出大小，公式如下：

新的 Reference Pixels Per Unit = Reference Pixels Per Unit * Physical Unit / Default Sprite DPI

UI大小 = 原图大小(Pixels) / (Pixels Per Unit / 新的 Reference Pixels Per Unit)

参考资料：
【Unity3D-UGUI原理篇】（二）Canvas Scaler 缩放原理_Unity3D软件工程师。的技术博客_51CTO博客

Unity UI Performance tips - Sharing my findings - Unity Forum 

https://www.youtube.com/watch?v=be4gDI8RZ2U