Oculus VR SDK实现-Oculus针对双眼显示的交换链设计
目录
一、创建交换链
二、创建DepthBuffer和FrameBuffer
三、基于交换链对Buffer的使用
一、创建交换链
首先我们关注一下ovrFramebuffer这个结构体：
typedef struct {
int Width;
int Height;
int Multisamples;
int TextureSwapChainLength;
int TextureSwapChainIndex;
ovrTextureSwapChain * ColorTextureSwapChain;
GLuint * DepthBuffers;
GLuint * FrameBuffers;
} ovrFramebuffer;
这个结构体描述了oculus的Framebuffer包含的信息：
framebuffer的宽、高、采样率，交换链长度，当前正在使用的交换链索引，交换链地址指针。以及一个DepthBuffer和FrameBuffer的指针。
然后看下创建交换链的代码实现：
//创建一个Texture，类型是2D，格式是colorFormat
frameBuffer->ColorTextureSwapChain = vrapi_CreateTextureSwapChain(
VRAPI_TEXTURE_TYPE_2D, colorFormat, width, height, 1, true);
这个接口从参数上来看是创建了一个交换链，并将交换链句柄传递出来，交给frameBuffer->ColorTextureSwapChain.
根据汇编代码，glTexStorage2D和glTexStorage3D推断这里使用的是不可变纹理
即在加载数据之前指定纹理的格式和大小。这样可以预先进行所有一致性和内存检查
一旦纹理不可变，它的格式和大小就不会再变化，但应用仍然可以通过使用glTexSubImage2D
,glTexSubImage3D,glGennerateMipMap或者渲染到纹理加载图像数据
即创建了两个不可变纹理，并把存储这两个纹理地址的数组转换成ovrTextureSwapChain传递出去。
用图形表示，就是如下：
至于这些texture的格式和大小：
由http://blog.csdn.net/d_uanrock/article/details/48209533中对SuggestedEyeResolutionWidth以及SuggestedEyeResolutionHeight
的分析：
其中的ovrHmdInfo是VR 头盔（眼镜）相关的一个结构体，包含，分辨率，刷新率，默认分辨率，和水平垂直视野角度。
显示一幅分辨率为2560*2440的图像的时候，每个像素在眼睛中所占用的视角约为0.06度。展现完整360度就需要6000个像素（360/0.06=6000），展现90度的视角就需要占用其中的1/4。分辨率为1536*1536的图像能够产生1：1的图像。但是对于偏离中心的像素，需要提供大密度投影。考虑到不需要大密度投影的情况和更好的表现，这里提供了默认值1024*1024.
可以知道程序中默认的texture 对应Buffer的大小为1024x1024
接口vrapi_GetTextureSwapChainLength就是用于获取交换链长度，也就是交换链个数，这里是2
二、创建DepthBuffer和FrameBuffer
后面创建DepthBuffer和FrameBuffer的时候都用到了这个长度。
frameBuffer->DepthBuffers = (GLuint *) malloc(
frameBuffer->TextureSwapChainLength * sizeof(GLuint));
frameBuffer->FrameBuffers = (GLuint *) malloc(
frameBuffer->TextureSwapChainLength * sizeof(GLuint));
可以看到，这里只分配了2*sizeof(GLuint)的很小的空间，因为和前面的textureSwapChain一样，这里的的指针指向的也只是一个存储地址的数组。
分配了存储地址的数组之后，就开始通过glGenRenderbuffers实际为DepthBuffer和FrameBuffer分配空间：
//创建渲染缓冲区buffer，12章，P233，glGenRenderbuffers作用是分配n个渲染缓冲区对象名称
//这里分配了1个渲染缓冲区名称存储到frameBuffer->DepthBuffers[i]中，名称是不为0的无符号整数
// Create multisampled depth buffer.
GL(glGenRenderbuffers(1, &frameBuffer->DepthBuffers[i]));
//指定渲染缓冲区，第一个值必须指定为GL_RENDERBUFFER
GL(
glBindRenderbuffer(GL_RENDERBUFFER,
frameBuffer->DepthBuffers[i]));
GL(
glRenderbufferStorageMultisampleEXT(GL_RENDERBUFFER,
multisamples, GL_DEPTH_COMPONENT24, width, height));
GL(glBindRenderbuffer(GL_RENDERBUFFER, 0));
//创建帧缓冲区buffer
// Create the frame buffer.
//创建framebuffer
GL(glGenFramebuffers(1, &frameBuffer->FrameBuffers[i]));
GL(glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer->FrameBuffers[i]));
GL(glFramebufferTexture2DMultisampleEXT(GL_FRAMEBUFFER,
GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, colorTexture,
0, multisamples));
GL(glFramebufferRenderbuffer(GL_FRAMEBUFFER,
GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER,
frameBuffer->DepthBuffers[i]));
GL(GLenum renderFramebufferStatus = glCheckFramebufferStatus( GL_FRAMEBUFFER ));
GL(glBindFramebuffer(GL_FRAMEBUFFER, 0));
if (renderFramebufferStatus != GL_FRAMEBUFFER_COMPLETE) {
ALOGE("Incomplete frame buffer object: %s",
EglFrameBufferStatusString(renderFramebufferStatus));
return false;
}
这里，为DepthBuffer和FrameBuffer分配了空间，并进行了相应的初始化。
这里要理解一下：
glFramebufferTexture2D和glFramebufferRenderbuffer
根据
http://www.songho.ca/opengl/gl_fbo.html
中的解释：
FBO provides
glFramebufferTexture2D()
to switch 2D texture objects, and
glFramebufferRenderbuffer()
to switch renderbuffer objects
说明，这里是将一个2Dtexture 对象和renderbuffer对象绑定到FBO上。
好了，到这里。再次梳理一下现在的数据结构：
注意，这里的FrameBuffer其实就是一个FBO了。其中包含的是一系列的附着点。
因此，这里在每一个FrameBuffer的GL_COLOR_ATTACHMENT0上绑定了一个texture，GL_DEPTH_ATTACHMENT上绑定了一个DepthBuffer
(可以再回头分析一下链接里的那篇文章)
因此现在的数据结构图就是下面这样的了：
三、基于交换链对Buffer的使用
了解完这一部分，我们现在来看下，程序是如何基于交换链对这些数据结构进行使用的：
ovrFramebuffer_Resolve(frameBuffer);
parms.Layers[VRAPI_FRAME_LAYER_TYPE_WORLD].Textures[eye].ColorTextureSwapChain =
frameBuffer->ColorTextureSwapChain;
parms.Layers[VRAPI_FRAME_LAYER_TYPE_WORLD].Textures[eye].TextureSwapChainIndex =
frameBuffer->TextureSwapChainIndex;
parms.Layers[VRAPI_FRAME_LAYER_TYPE_WORLD].Textures[eye].TexCoordsFromTanAngles =
renderer->TexCoordsTanAnglesMatrix; //纹理坐标
parms.Layers[VRAPI_FRAME_LAYER_TYPE_WORLD].Textures[eye].HeadPose =
updatedTracking.HeadPose;
ovrFramebuffer_Advance(frameBuffer);
typedef struct
{
// Layers composited in the time warp.
ovrFrameLayer Layers[VRAPI_FRAME_LAYER_TYPE_MAX];
int LayerCount;
// Combination of ovrFrameOption flags.
int WarpOptions;
// Which program to run with these layers.
ovrFrameProgram WarpProgram;
// Program-specific tuning values.
float ProgramParms[4];
// Application controlled frame index that uniquely identifies this particular frame.
// This must be the same frame index that was passed to vrapi_GetPredictedDisplayTime()
// when synthesis of this frame started.
long long FrameIndex;
// WarpSwap will not return until at least this many V-syncs have
// passed since the previous WarpSwap returned.
// Setting to 2 will reduce power consumption and may make animation
// more regular for applications that can't hold full frame rate.
int MinimumVsyncs;
// Latency Mode.
ovrExtraLatencyMode ExtraLatencyMode;
// Rotation from a joypad can be added on generated frames to reduce
// judder in FPS style experiences when the application framerate is
// lower than the V-sync rate.
// This will be applied to the view space distorted
// eye vectors before applying the rest of the time warp.
// This will only be added when the same ovrFrameParms is used for
// more than one V-sync.
ovrMatrix4f ExternalVelocity;
// jobject that will be updated before each eye for minimal
// latency with VRAPI_FRAME_PROGRAM_MASKED_PLANE_EXTERNAL.
// IMPORTANT: This should be a JNI weak reference to the object.
// The system will try to convert it into a global reference before
// calling SurfaceTexture->Update, which allows it to be safely
// freed by the application.
jobject SurfaceTextureObject;
// CPU/GPU performance parameters.
ovrPerformanceParms PerformanceParms;
// For handling HMD events and power level state changes.
ovrJava Java;
} ovrFrameParms;
从 http://blog.csdn.net/d_uanrock/article/details/48209533中对这一部分的分析结合实现代码。可以知道
左右眼都对应着有一组数据结构ovrFrameLayerTexture
typedef struct
{
// Because OpenGL ES does not support clampToBorder, it is the
// application's responsibility to make sure that all mip levels
// of the primary eye texture have a black border that will show
// up when time warp pushes the texture partially off screen.
//CLAMP_TO_BORDER causes OpenGL to only take the border color at the edge of the texture
//rather than the average of the border color and texture edge texels.
//This allows for a perfect border around the texture.
ovrTextureSwapChain * ColorTextureSwapChain;
// The depth texture is optional for positional time warp.
ovrTextureSwapChain * DepthTextureSwapChain;
// Index to the texture from the set that should be displayed.
int TextureSwapChainIndex;
// Points on the screen are mapped by a distortion correction
// function into ( TanX, TanY, -1, 1 ) vectors that are transformed
// by this matrix to get ( S, T, Q, _ ) vectors that are looked
// up with texture2dproj() to get texels.
ovrMatrix4f TexCoordsFromTanAngles;
// Only texels within this range should be drawn.
// This is a sub-rectangle of the [(0,0)-(1,1)] texture coordinate range.
ovrRectf TextureRect;
// The tracking state for which ModelViewMatrix is correct.
// It is ok to update the orientation for each eye, which
// can help minimize black edge pull-in, but the position
// must remain the same for both eyes, or the position would
// seem to judder "backwards in time" if a frame is dropped.
ovrRigidBodyPosef HeadPose;
} ovrFrameLayerTexture;
当处理左眼的时候，就更新左眼的这个数据结构的值，当处理右眼的时候，就处理右眼的这个数据结构的值。
结合上面的数据结构总图，可以知道，程序的设计师左眼用一组FrameBuffer，右眼用一组FrameBuffer。
1.计算要绘制的数据及左右眼视角矩阵
2.绑定交换链中的一个Framebuffer
static void ovrFramebuffer_SetCurrent(ovrFramebuffer * frameBuffer) {
GL(glBindFramebuffer(GL_FRAMEBUFFER,frameBuffer->FrameBuffers[frameBuffer->TextureSwapChainIndex]));
}
3.绘制图像到绑定的FrameBuffer
4.解绑FrameBuffer
static void ovrFramebuffer_Resolve(ovrFramebuffer * frameBuffer) {
// Discard the depth buffer, so the tiler won't need to write it back out to memory.
const GLenum depthAttachment[1] = { GL_DEPTH_ATTACHMENT };
glInvalidateFramebuffer(GL_FRAMEBUFFER, 1, depthAttachment);
// Flush this frame worth of commands.
glFlush();
}
parms.Layers[VRAPI_FRAME_LAYER_TYPE_WORLD].Textures[eye].ColorTextureSwapChain =
frameBuffer->ColorTextureSwapChain;
parms.Layers[VRAPI_FRAME_LAYER_TYPE_WORLD].Textures[eye].TextureSwapChainIndex =
frameBuffer->TextureSwapChainIndex;
parms.Layers[VRAPI_FRAME_LAYER_TYPE_WORLD].Textures[eye].TexCoordsFromTanAngles =
renderer->TexCoordsTanAnglesMatrix; //纹理坐标
parms.Layers[VRAPI_FRAME_LAYER_TYPE_WORLD].Textures[eye].HeadPose =
updatedTracking.HeadPose;
6.移动到下一帧要使用的交换链索引
static void ovrFramebuffer_Advance(ovrFramebuffer * frameBuffer) {
// Advance to the next texture from the set.
frameBuffer->TextureSwapChainIndex =
(frameBuffer->TextureSwapChainIndex + 1)
% frameBuffer->TextureSwapChainLength;
}
本篇主要将了Oculus是在OpenGL层面上对纹理交换链做了哪些数据结构的设计，如何基于交换链实现左右眼画面的显示的。