1 简介

1.1 EAGL(Embedded Apple Graphics Library)

    与Android系统使用EGL连接OpenGL ES与原生窗口进行surface输出类似，iOS则用EAGL将CAEAGLLayer作为OpenGL ES输出目标。

    与 Android EGL 不同的是，iOS EAGL 不会让应用直接向 BackendFrameBuffer 和 FrontFrameBuffer 进行绘制，也不会让app直接控制双缓冲区的交换，系统自己保留了操作权，以便可以随时使用 Core Animation 合成器来控制显示的最终外观。

    OpenGL ES 通过 CAEAGLLayer 与 Core Animation 连接，CAEAGLLayer 是一种特殊类型的 Core Animation 图层，它的内容来自 OpenGL ES 的 RenderBuffer，Core Animation 将 RenderBuffer 的内容与其他图层合成，并在屏幕上显示生成的图像。所以同一时刻可以有任意数量的层。Core Animation 合成器会联合这些层并在后帧缓存中产生最终的像素颜色，然后切换缓存。

    一句话就是，OpenGL ES可与CAEAGLLayer共享RenderBuffer，用EAGLContext将CAEGLLayer与RenderBuffer进行绑定，可实现OpenGL ES最终输出到CAEGLLayer上并显示出

来。

 

2 显示

2.1 软解

• FFmpeg软解后的输出是AVFrame，其像素数据在uint8_t* data[AV_NUM_DATA_POINTERS]所指向的内存中；
• 在软解码线程里解码后可以播放时，调用SDL_Vout中SDL_VoutOverlay中func_fill_frame方法，对AVFrame增加引用计数，将overlay的pixels[]也指向这块儿内存；
• 然后将AVFrame放到FrameQueue队列尾部，待video_refresh_thread线程消费render；
• 此后在video_refresh_thread线程里把AVFrame取出来，将pixels[]所指像素数据喂给OpenGL ES进行render；
• 循环以上步骤；

2.1.1 像素源

    因此，FFmpeg软解后的像素数据来源了然了，来自AVFrame的uint8_t* data[AV_NUM_DATA_POINTERS]。

    此处举例说明，不需要转换像素格式的填充：

static void overlay_fill(SDL_VoutOverlay *overlay, AVFrame *frame, int planes)
{
    overlay->planes = planes;

    for (int i = 0; i < AV_NUM_DATA_POINTERS; ++i) {
        overlay->pixels[i] = frame->data[i];
        overlay->pitches[i] = frame->linesize[i];
    }
}

    需要转换像素格式的，调用libyuv或sws_scale处理，再入FrameQueue队列。 

2.2 硬解

• videotoolbox解码后输出是CVPixelBuffer，为了与FFmpeg统一接口，硬解后将CVPixelBuffer由AVFrame的opaque指向；
• 然后，在硬解码线程入队FrameQueue尾部，待video_refresh_thread线程消费render；
• 随后的逻辑基本与FFmpeg软解一致，在video_refresh_thread线程里把AVFrame取出来，将pixels[]所指向的像素喂给OpenGL ES进行render；
• 具体差异在yuv420sp_vtb_uploadTexture方法里；
• 循环以上步骤；

2.2.1 像素源

    所以，videotoolbox硬解后的像素数据来源也清楚了，来自AVFrame的opaque，实际是CVPixelBuffer。

static int func_fill_frame(SDL_VoutOverlay *overlay, const AVFrame *frame)
{
    assert(frame->format == IJK_AV_PIX_FMT__VIDEO_TOOLBOX);

    CVBufferRef pixel_buffer = CVBufferRetain(frame->opaque);
    SDL_VoutOverlay_Opaque *opaque = overlay->opaque;
    if (opaque->pixel_buffer != NULL) {
        CVBufferRelease(opaque->pixel_buffer);
    }
    opaque->pixel_buffer = pixel_buffer;
    overlay->format = SDL_FCC__VTB;
    overlay->planes = 2;

    if (CVPixelBufferLockBaseAddress(pixel_buffer, 0) != kCVReturnSuccess) {
        overlay->pixels[0]  = NULL;
        overlay->pixels[1]  = NULL;
        overlay->pitches[0] = 0;
        overlay->pitches[1] = 0;
        overlay->w = 0;
        overlay->h = 0;
        CVBufferRelease(pixel_buffer);
        opaque->pixel_buffer = NULL;
        return -1;
    }
    overlay->pixels[0]  = NULL;
    overlay->pixels[1]  = NULL;
    overlay->pitches[0] = CVPixelBufferGetWidthOfPlane(pixel_buffer, 0);
    overlay->pitches[1] = CVPixelBufferGetWidthOfPlane(pixel_buffer, 1);
    CVPixelBufferUnlockBaseAddress(pixel_buffer, 0);

    overlay->is_private = 1;

    overlay->w = (int)frame->width;
    overlay->h = (int)frame->height;
    return 0;
}

2.3 render

• 首先让UIView的子类视频窗口的layerClass返回CAEAGLLayer；
• 设置CAEAGLLayer的属性，创建EAGLContext，并设置render的上下文；
• 创建FBO和RBO并绑定，再用所创建的EAGLContext调用renderbufferStorage，将GL_RENDERBUFFER与CAEAGLLayer进行绑定；
• 将RBO作为GL_COLOR_ATTACHMENT0附加到FBO上；
• 将EAGLContext上下文切为当前所创建的Context，并用OpenGL ES开始1帧的render工作；
• 然后，调用EAGLContext的presentRenderbuffer方法将RenderBuffer输出到CAEAGLLayer再显示；
• 最后，将把Context切为render之前的Context；
• 重复4~6步骤，继续render下一帧；

     让UIView子类返回CAEAGLLayer class：

+ (Class) layerClass
{
	return [CAEAGLLayer class];
}

    设置CAEAGLLayer属性为不透明，并创建EAGLContext，设置render的上下文：

- (BOOL)setupGL
{
    if (_didSetupGL)
        return YES;

    CAEAGLLayer *eaglLayer = (CAEAGLLayer*) self.layer;
    // 设置为不透明
    eaglLayer.opaque = YES;
    eaglLayer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys:
                                    [NSNumber numberWithBool:NO], kEAGLDrawablePropertyRetainedBacking,
                                    kEAGLColorFormatRGBA8, kEAGLDrawablePropertyColorFormat,
                                    nil];

    _scaleFactor = [[UIScreen mainScreen] scale];
    if (_scaleFactor < 0.1f)
        _scaleFactor = 1.0f;

    [eaglLayer setContentsScale:_scaleFactor];

    _context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
    if (_context == nil) {
        NSLog(@"failed to setup EAGLContext\n");
        return NO;
    }

    EAGLContext *prevContext = [EAGLContext currentContext];
    [EAGLContext setCurrentContext:_context];

    _didSetupGL = NO;
    if ([self setupEAGLContext:_context]) {
        NSLog(@"OK setup GL\n");
        _didSetupGL = YES;
    }

    [EAGLContext setCurrentContext:prevContext];
    return _didSetupGL;
}

     创建FBO和RBO并绑定，并将RBO以GL_COLOR_ATTACHMENT0附加到FBO上：

- (BOOL)setupEAGLContext:(EAGLContext *)context
{
    glGenFramebuffers(1, &_framebuffer);
    glGenRenderbuffers(1, &_renderbuffer);
    glBindFramebuffer(GL_FRAMEBUFFER, _framebuffer);
    glBindRenderbuffer(GL_RENDERBUFFER, _renderbuffer);
    [_context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)self.layer];
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &_backingWidth);
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &_backingHeight);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, _renderbuffer);

    GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
    if (status != GL_FRAMEBUFFER_COMPLETE) {
        NSLog(@"failed to make complete framebuffer object %x\n", status);
        return NO;
    }

    GLenum glError = glGetError();
    if (GL_NO_ERROR != glError) {
        NSLog(@"failed to setup GL %x\n", glError);
        return NO;
    }

    return YES;
}

    iOS端显示的主函数，是在iOS端SDL_Vout接口display_overlay中回调的： 

- (void)display: (SDL_VoutOverlay *) overlay
{
    if (_didSetupGL == NO)
        return;

    if ([self isApplicationActive] == NO)
        return;

    if (![self tryLockGLActive]) {
        if (0 == (_tryLockErrorCount % 100)) {
            NSLog(@"IJKSDLGLView:display: unable to tryLock GL active: %d\n", _tryLockErrorCount);
        }
        _tryLockErrorCount++;
        return;
    }

    _tryLockErrorCount = 0;
    if (_context && !_didStopGL) {
        EAGLContext *prevContext = [EAGLContext currentContext];
        [EAGLContext setCurrentContext:_context];
        [self displayInternal:overlay];
        [EAGLContext setCurrentContext:prevContext];
    }

    [self unlockGLActive];
}

     具体执行render显示工作：

// NOTE: overlay could be NULl
- (void)displayInternal: (SDL_VoutOverlay *) overlay
{
    if (![self setupRenderer:overlay]) {
        if (!overlay && !_renderer) {
            NSLog(@"IJKSDLGLView: setupDisplay not ready\n");
        } else {
            NSLog(@"IJKSDLGLView: setupDisplay failed\n");
        }
        return;
    }

    [[self eaglLayer] setContentsScale:_scaleFactor];

    if (_isRenderBufferInvalidated) {
        NSLog(@"IJKSDLGLView: renderbufferStorage fromDrawable\n");
        _isRenderBufferInvalidated = NO;

        glBindRenderbuffer(GL_RENDERBUFFER, _renderbuffer);
        [_context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)self.layer];
        glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &_backingWidth);
        glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &_backingHeight);
        IJK_GLES2_Renderer_setGravity(_renderer, _rendererGravity, _backingWidth, _backingHeight);
    }

    glBindFramebuffer(GL_FRAMEBUFFER, _framebuffer);
    glViewport(0, 0, _backingWidth, _backingHeight);

    if (!IJK_GLES2_Renderer_renderOverlay(_renderer, overlay))
        ALOGE("[EGL] IJK_GLES2_render failed\n");

    glBindRenderbuffer(GL_RENDERBUFFER, _renderbuffer);
    [_context presentRenderbuffer:GL_RENDERBUFFER];

    int64_t current = (int64_t)SDL_GetTickHR();
    int64_t delta   = (current > _lastFrameTime) ? current - _lastFrameTime : 0;
    if (delta <= 0) {
        _lastFrameTime = current;
    } else if (delta >= 1000) {
        _fps = ((CGFloat)_frameCount) * 1000 / delta;
        _frameCount = 0;
        _lastFrameTime = current;
    } else {
        _frameCount++;
    }
}

    首次或overlay的format发生变更，则创建 IJK_GLES2_Renderer实例：

- (BOOL)setupRenderer: (SDL_VoutOverlay *) overlay
{
    if (overlay == nil)
        return _renderer != nil;

    if (!IJK_GLES2_Renderer_isValid(_renderer) ||
        !IJK_GLES2_Renderer_isFormat(_renderer, overlay->format)) {

        IJK_GLES2_Renderer_reset(_renderer);
        IJK_GLES2_Renderer_freeP(&_renderer);

        _renderer = IJK_GLES2_Renderer_create(overlay);
        if (!IJK_GLES2_Renderer_isValid(_renderer))
            return NO;

        if (!IJK_GLES2_Renderer_use(_renderer))
            return NO;

        IJK_GLES2_Renderer_setGravity(_renderer, _rendererGravity, _backingWidth, _backingHeight);
    }

    return YES;
}

3 小节

    最后，对iOS端FFmpeg软解和硬解做一个小节：

视频显示	相同点	异同点
FFmpeg	1.都是通过创建EAGLContext上下文将RenderBuffer与CAEAGLLayer绑定，与OpenGL ES共享RenderBuffer，再render； 2.显示主逻辑是统一的；	1.像素数据源不同，FFmpeg软解的输出保存在AVFrame的uint8_t* data[AV_NUM_DATA_POINTERS]，而videotoolbox的输出保存在AVFrame的opaque中，具体是CVPixelBuffer； ​​​​​​​ 2.在给OpenGL ES喂像素数据时不同，FFmpeg解码后直接输出像素数据，因此在func_uploadTexture喂数据时直接喂raw data即可，而videotoolbox则是CVPixelBuffer；
videotoolbox