2022-06-14 Canvas2d绘制动画不clear的话是不清屏的 底层触发多缓冲时走的buffer保留模式么？

1. webcanvas走的保留模式，不clear的话 上一帧的内容依然保留
2. nativecanvas是如何做的呢有两种方案。

增量绘制有两种方案

1. 系统 EGL_SWAP_BEHAVIOR_PRESERVED_BIT

设置了buffer保留模式

2. 自己搞FBO，先绘制到FBO再swapbuffer前，绘制到在屏，swapbuffer结束后，切回到FBO
   void AGHwSkSurfaceWrapAndroid::willCanvasSwap() {
   unbindPBufferFbo();
   drawBackBufferToFront();
   }
   void AGHwSkSurfaceWrapAndroid::didCanvasSwap() {
   bindPBufferFbo();
   }

skia创建surface的流程

GPU模式

MakeRenderTarget 离屏FBO
MakeFromBackendTexture 内建FBO并绑定给定的textureid
MakeFromBackendRenderTarget 使用已有FBO 默认是0
MakeFromAHardwareBuffer

GrBackendTexture
GrBackendRenderTarget

SK_API_HIDDEN static sk_sp<SkSurface> MakeFromBackendTexture(GrRecordingContext* context,
                                               const GrBackendTexture& backendTexture,
                                               GrSurfaceOrigin origin, int sampleCnt,
                                               SkColorType colorType,
                                               sk_sp<SkColorSpace> colorSpace,
                                               const SkSurfaceProps* surfaceProps,
                                               TextureReleaseProc textureReleaseProc = nullptr,
                                               ReleaseContext releaseContext = nullptr);

static sk_sp<SkSurface> MakeFromBackendRenderTarget(GrRecordingContext* context,
                                            const GrBackendRenderTarget& backendRenderTarget,
                                            GrSurfaceOrigin origin,
                                            SkColorType colorType,
                                            sk_sp<SkColorSpace> colorSpace,
                                            const SkSurfaceProps* surfaceProps,
                                            RenderTargetReleaseProc releaseProc = nullptr,
                                            ReleaseContext releaseContext = nullptr);

static sk_sp<SkSurface> MakeRenderTarget(GrRecordingContext* context, SkBudgeted budgeted,
                                         const SkImageInfo& imageInfo,
                                         int sampleCount, GrSurfaceOrigin surfaceOrigin,
                                         const SkSurfaceProps* surfaceProps,
                                         bool shouldCreateWithMips = false);

MakeFromBackendTexture 内建FBO并绑定给定的 textureid

 SkSurface::MakeFromBackendTexture -> GrSurfaceDrawContext::MakeFromBackendTexture -> GrProxyProvider::wrapRenderableBackendTexture -> GrResourceProvider::wrapRenderableBackendTexture-> GrGLGpu::onWrapRenderableBackendTexture ->  GrGLGpu::createRenderTargetObjects ->  GL_CALL(GenFramebuffers(1, &rtIDs->fRTFBOID)); 

MakeFromBackendRenderTarget 使用已有FBO

 onWrapBackendRenderTarget    GrBackendObjectOwnership::kBorrowed 不删除外部传入的FBO

MakeRenderTarget 离屏FBO。内建FBO并且不指定textureid

context->priv().caps()->getDefaultBackendFormat(colorType, GrRenderable::kYes);

MakeFromAHardwareBuffer

AHardwareBuffer* buffer = nullptr;
sk_sp<SkSurface> surface = SkSurface::MakeFromAHardwareBuffer(context, buffer, surfaceOrigin,
                                                              nullptr, nullptr);

hardwareBuffer eglGetNativeClientBufferANDROID eglCreateImageKHR glEGLImageTargetTexture2DOES 到纹理id。

MakeFromAHardwareBuffer -》 GrAHardwareBufferUtils::MakeBackendTexture -》 MakeFromBackendTexture

static GrBackendTexture make_gl_backend_texture(GrDirectContext* dContext, AHardwareBuffer* hardwareBuffer)
while (GL_NO_ERROR != glGetError()) {} //clear GL errors

离屏和FBO有什么区别呢？

可以用于fallbackPbSurface场景 ： 离屏Pbuffer有自己完整的context，可以创建1*1的 fallbackPbSurface 用于做fallbackPbSurface 逻辑。在销毁后异步调用makeCurrent场景。

FBO不需要切换glContext

        bool forceFallbackPbSurface = is_onscreen && canvas_config.forceFallbackPbSurface && isSurfaceTextureDestroyed();


bool AEGLWindowContext::makeCurrent(bool forceFallbackPbSurface) {}

    if (isSurfaceless) {
        fallbackPbSurface = EGL_NO_SURFACE;
    } else {
        // 创建fallback surface逻辑
        if (fallbackPbSurface == EGL_NO_SURFACE) {
            EGLint pbufAttribs[] = {EGL_WIDTH, 1, EGL_HEIGHT, 1, EGL_NONE};
            fallbackPbSurface = eglCreatePbufferSurface(eglDisplay, eglConfig, pbufAttribs);
        }
    }