VR系列——Oculus Rift 开发者指南：四、高级渲染配置（四）

通过缩小视场来提高性能

除了减少中间渲染目标的像素数目外，还可以通过减少FOV的横跨像素来提高性能。

由于FOV的缩小，可能会使视野收缩从而导致沉浸感的降低。尽管如此，还是有两种方式可以用来缩小FOV、提高性能。最明显的就是填充率。对于一个在视网膜上的固定像素密度而言，较小的FOV具有较少的象素。首先，因为投影公式的性质，FOV的最外边缘在像素数量上而言是最多的。其次则因为当每一帧中有更少的可见对象时，动画会更少，状态的改变会更少,绘制的调用也会更少。

缩小由玩家设置的FOV是个非常痛苦的决定。虚拟现实的关键体验之一是沉浸在虚拟的世界中，这其中很大一部分取决于宽FOV。我们不推荐对这方面的降低。但是，如果你已经尽你所能牺牲了很多的分辨率，而应用程序还是没能以60Hz运行在用户的计算机上的话，那就只能使用这种方法了。

我们建议给玩家最大限度的FOV滑块来定义每只眼睛视场的四个边缘。

ovrFovPort defaultFovLeft = session->DefaultEyeFov[ovrEye_Left];
ovrFovPort defaultFovRight = session->DefaultEyeFov[ovrEye_Right];
float maxFovAngle = ...get value from game settings panel...;
float maxTanHalfFovAngle = tanf ( DegreeToRad ( 0.5f * maxFovAngle ) );

ovrFovPort newFovLeft  = FovPort::Min(defaultFovLeft, FovPort(maxTanHalfFovAngle));
ovrFovPort newFovRight = FovPort::Min(defaultFovRight, FovPort(maxTanHalfFovAngle));

// 创建渲染目标
Sizei recommenedTex0Size = ovr_GetFovTextureSize(session, ovrEye_Left  newFovLeft,
pixelsPerDisplayPixel);
Sizei recommenedTex1Size = ovr_GetFovTextureSize(session, ovrEye_Right, newFovRight, pixelsPerDisplayPixel);
...

// 初始化渲染信息
ovrFovPort eyeFov[2];
eyeFov[0] = newFovLeft;
eyeFov[1] = newFovRight;
...

//确定投影矩阵。
ovrMatrix4f projMatrixLeft = ovrMatrix4f_Projection(newFovLeft, znear, zfar, isRightHanded);
ovrMatrix4f projMatrixRight = ovrMatrix4f_Projection(newFovRight, znear, zfar, isRightHanded);

尝试非正方形的视场可能会很有趣。例如，显著夹紧上下范围（如70度FOV），同时保留完整的水平FOV从而呈现出“宽银幕”的感觉。

OculusWorldDemo允许你将FOV缩小到低于默认值。导航到设置菜单（按下Tab键），然后选择“最大FOV”（Max FOV），按向上和向下箭头来更改最大角度值。

原文如下

Improving Performance by Decreasing Field of View

In addition to reducing the number of pixels in the intermediate render target, you can increase performance by decreasing the FOV that the pixels are stretched across.

Depending on the reduction, this can result in tunnel vision which decreases the sense of immersion. Nevertheless, reducing the FOV increases performance in two ways. The most obvious is fillrate. For a fixed pixel density on the retin
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a, a lower FOV has fewer pixels. Because of the properties of projective math, the outermost edges of the FOV are the most expensive in terms of numbers of pixels. The second reason is that there are fewer objects visible in each frame which implies less animation, fewer state changes, and fewer draw calls.

Reducing the FOV set by the player is a very painful choice to make. One of the key experiences of virtual reality is being immersed in the simulated world, and a large part of that is the wide FOV. Losing that aspect is not a thing we would ever recommend happily. However, if you have already sacrificed as much resolution as you can, and the application is still not running at 60Hz on the user’s machine, this is an option of last resort.

We recommend giving players a Maximum FOV slider that defines the four edges of each eye’s FOV.

ovrFovPort defaultFovLeft = session->DefaultEyeFov[ovrEye_Left];
ovrFovPort defaultFovRight = session->DefaultEyeFov[ovrEye_Right];
float maxFovAngle = ...get value from game settings panel...;
float maxTanHalfFovAngle = tanf ( DegreeToRad ( 0.5f * maxFovAngle ) );
ovrFovPort newFovLeft = FovPort::Min(defaultFovLeft, FovPort(maxTanHalfFovAngle));
ovrFovPort newFovRight = FovPort::Min(defaultFovRight, FovPort(maxTanHalfFovAngle));
// Create render target.
Sizei recommenedTex0Size = ovr_GetFovTextureSize(session, ovrEye_Left newFovLeft, pixelsPerDisplayPixel);
Sizei recommenedTex1Size = ovr_GetFovTextureSize(session, ovrEye_Right, newFovRight, pixelsPerDisplayPixel);
...
// Initialize rendering info.
ovrFovPort eyeFov[2];
eyeFov[0] = newFovLeft;
eyeFov[1] = newFovRight;
...
// Determine projection matrices.
ovrMatrix4f projMatrixLeft = ovrMatrix4f_Projection(newFovLeft, znear, zfar, isRightHanded);
ovrMatrix4f projMatrixRight = ovrMatrix4f_Projection(newFovRight, znear, zfar, isRightHanded);

It might be interesting to experiment with non-square fields of view. For example, clamping the up and down ranges significantly (e.g. 70 degrees FOV) while retaining the full horizontal FOV for a ‘Cinemascope’ feel.

OculusWorldDemo allows you to experiment with reducing the FOV below the defaults. Navigate to the settings menu (press the Tab key) and select the “Max FOV” value. Pressing the up and down arrows to change the maximum angle in degrees.