ARKit & OpenGL ES

如果你想了解ATRKit的基础知识，请访问ARKit & OpenGL ES - ARKit原理篇
如果你想了解更多关于OpenGL ES的知识，请移步至OpenGL ES相关文章目录
本文所用的代码在https://github.com/SquarePants1991/OpenGLESLearn.git的ARKit分支中。

本文所用OpenGL基础代码来自OpenGL ES系列，具备渲染几何体，纹理等基础功能，实现细节将不赘述。

集成ARKit的关键代码都在ARGLBaseViewController中。我们来看一下它的代码。

处理ARFrame

- (void)session:(ARSession *)session didUpdateFrame:(ARFrame *)frame {    // 同步YUV信息到 yTexture 和 uvTexture    CVPixelBufferRef pixelBuffer = frame.capturedImage;    GLsizei imageWidth = (GLsizei)CVPixelBufferGetWidthOfPlane(pixelBuffer, 0);    GLsizei imageHeight = (GLsizei)CVPixelBufferGetHeightOfPlane(pixelBuffer, 0);    void * baseAddress = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 0);    glBindTexture(GL_TEXTURE_2D, self.yTexture);    glTexImage2D(GL_TEXTURE_2D, 0, GL_LUMINANCE, imageWidth, imageHeight, 0, GL_LUMINANCE, GL_UNSIGNED_BYTE, baseAddress);    glBindTexture(GL_TEXTURE_2D, 0);    imageWidth = (GLsizei)CVPixelBufferGetWidthOfPlane(pixelBuffer, 1);    imageHeight = (GLsizei)CVPixelBufferGetHeightOfPlane(pixelBuffer, 1);    void *laAddress = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 1);    glBindTexture(GL_TEXTURE_2D, self.uvTexture);    glTexImage2D(GL_TEXTURE_2D, 0, GL_LUMINANCE_ALPHA, imageWidth, imageHeight, 0, GL_LUMINANCE_ALPHA, GL_UNSIGNED_BYTE, laAddress);    glBindTexture(GL_TEXTURE_2D, 0);    self.videoPlane.yuv_yTexture = self.yTexture;    self.videoPlane.yuv_uvTexture = self.uvTexture;    [self setupViewport: CGSizeMake(imageHeight, imageWidth)];    // 同步摄像机    matrix_float4x4 cameraMatrix = matrix_invert([frame.camera transform]);    GLKMatrix4 newCameraMatrix = GLKMatrix4Identity;    for (int col = 0; col < 4; ++col) {        for (int row = 0; row < 4; ++row) {            newCameraMatrix.m[col * 4 + row] = cameraMatrix.columns[col][row];        }    }    self.cameraMatrix = newCameraMatrix;    GLKVector3 forward = GLKVector3Make(-self.cameraMatrix.m13, -self.cameraMatrix.m23, -self.cameraMatrix.m33);    GLKMatrix4 rotationMatrix = GLKMatrix4MakeRotation(M_PI / 2, forward.x, forward.y, forward.z);    self.cameraMatrix = GLKMatrix4Multiply(rotationMatrix, newCameraMatrix);}

上面的代码展示了如何处理ARKit捕捉的ARFrame，ARFrame的capturedImage存储了摄像头捕捉的图片信息，类型是CVPixelBufferRef。默认情况下，图片信息的格式是YUV，通过两个Plane来存储，也可以理解为两张图片。一张格式是Y（Luminance），保存了明度信息，另一张是UV（Chrominance、Chroma），保存了色度和浓度。我们需要把这两张图分别绑定到不同的纹理上，然后在Shader中利用算法将YUV转换成RGB。下面是处理纹理的Fragment Shader，利用公式进行颜色转换。

precision highp float;varying vec3 fragNormal;varying vec2 fragUV;uniform float elapsedTime;uniform mat4 normalMatrix;uniform sampler2D yMap;uniform sampler2D uvMap;void main(void) {    vec4 Y_planeColor = texture2D(yMap, fragUV);    vec4 CbCr_planeColor = texture2D(uvMap, fragUV);    float Cb, Cr, Y;    float R ,G, B;    Y = Y_planeColor.r * 255.0;    Cb = CbCr_planeColor.r * 255.0 - 128.0;    Cr = CbCr_planeColor.a * 255.0 - 128.0;    R = 1.402 * Cr + Y;    G = -0.344 * Cb - 0.714 * Cr + Y;    B = 1.772 * Cb + Y;    vec4 videoColor = vec4(R / 255.0, G / 255.0, B / 255.0, 1.0);    gl_FragColor = videoColor;}

处理并绑定好纹理后，为了保证不同屏幕尺寸下，纹理不被非等比拉伸，所以对viewport进行重了新计算[self setupViewport: CGSizeMake(imageHeight, imageWidth)];。接下来将ARKit计算出来的摄像机的变换赋值给self.cameraMatrix。注意ARKit捕捉的图片需要旋转90度后才能正常显示，所以在设置Viewport时特意颠倒了宽和高，并在最后对摄像机进行了旋转。

VideoPlane

VideoPlane是为了显示视频编写的几何体，它能够接收两个纹理，Y和UV。

@interface VideoPlane : GLObject@property (assign, nonatomic) GLuint yuv_yTexture;@property (assign, nonatomic) GLuint yuv_uvTexture;- (instancetype)initWithGLContext:(GLContext *)context;- (void)update:(NSTimeInterval)timeSinceLastUpdate;- (void)draw:(GLContext *)glContext;@end...- (void)draw:(GLContext *)glContext {    [glContext setUniformMatrix4fv:@"modelMatrix" value:self.modelMatrix];    bool canInvert;    GLKMatrix4 normalMatrix = GLKMatrix4InvertAndTranspose(self.modelMatrix, &canInvert);    [glContext setUniformMatrix4fv:@"normalMatrix" value:canInvert ? normalMatrix : GLKMatrix4Identity];    [glContext bindTextureName:self.yuv_yTexture to:GL_TEXTURE0 uniformName:@"yMap"];    [glContext bindTextureName:self.yuv_uvTexture to:GL_TEXTURE1 uniformName:@"uvMap"];    [glContext drawTrianglesWithVAO:vao vertexCount:6];}

其他的功能很简单，就是绘制一个正方形，最终配合显示视频的Shader，渲染YUV格式的数据。

透视投影矩阵

在ARFrame可以获取渲染需要的纹理和摄像机矩阵，除了这些，和真实摄像头匹配的透视投影矩阵也是必须的。它能够让渲染出来的3D物体透视看起来很自然。

- (void)session:(ARSession *)session cameraDidChangeTrackingState:(ARCamera *)camera {    matrix_float4x4 projectionMatrix = [camera projectionMatrixWithViewportSize:self.viewport.size orientation:UIInterfaceOrientationPortrait zNear:0.1 zFar:1000];    GLKMatrix4 newWorldProjectionMatrix = GLKMatrix4Identity;    for (int col = 0; col < 4; ++col) {        for (int row = 0; row < 4; ++row) {           newWorldProjectionMatrix.m[col * 4 + row] = projectionMatrix.columns[col][row];        }    }    self.worldProjectionMatrix = newWorldProjectionMatrix;}

上面的代码演示了如何通过ARKit获取3D透视投影矩阵，有了透视投影矩阵和摄像机矩阵，就可以很方便的利用OpenGL渲染物体了。

- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect {    [super glkView:view drawInRect:rect];    [self.objects enumerateObjectsUsingBlock:^(GLObject *obj, NSUInteger idx, BOOL *stop) {        [obj.context active];        [obj.context setUniform1f:@"elapsedTime" value:(GLfloat)self.elapsedTime];        [obj.context setUniformMatrix4fv:@"projectionMatrix" value:self.worldProjectionMatrix];        [obj.context setUniformMatrix4fv:@"cameraMatrix" value:self.cameraMatrix];        [obj.context setUniform3fv:@"lightDirection" value:self.lightDirection];        [obj draw:obj.context];    }];}

本文主要介绍了OpenGL ES渲染ARKit的基本思路，没有对OpenGL ES技术细节描述太多。如果你有兴趣，可以直接clone Github上的代码深入了解。

原文链接：http://www.jianshu.com/p/380df8ae273f