SkSurface created by MakeFromBackendRenderTarget readpixels return empty data

[l1 shu]

Hi all, I make a SkSurface based on GPU, it all normal except for when I readpixels from the SkSurface,  I got a empty data

Here is the way I created SkSurface :

```

GrGLFramebufferInfo framebufferInfo = {};
framebufferInfo.fFBOID = 0;
framebufferInfo.fFormat = GL_RGBA8_OES;
GrBackendRenderTarget renderTarget(mWidth0, mHeight, 0, 0, framebufferInfo);

return SkSurface::MakeFromBackendRenderTarget(
   context, // the GrDirectContext made outside
   renderTarget,
   GrSurfaceOrigin::kBottomLeft_GrSurfaceOrigin,
   kRGBA_8888_SkColorType,
   SkColorSpace::MakeSRGB(),
   nullptr
);

```

Here is the way I used it

```

char* buf = (char*)malloc(mWidth * mHeight * 4);
memset(buf, 0, mWidth * mHeight * 4);
const SkImageInfo imageInfo = SkImageInfo::Make(
   mWidth,
   mHeight,
   kRGBA_8888_SkColorType,
   kPremul_SkAlphaType,
   SkColorSpace::MakeSRGB()
);

bool success = mSkSurface->readPixels(
   imageInfo,
   buf,
   mWidth * 4,
   0,
   0
);

free(buf);

```

the `success` is true, but the `buf` read from skSurface is empty

[l1 shu]

The reason is the readPixels shoud not using  after  swapBuffers.... 

[Brian Salomon]

Oh glad you figured it out. If you need that to work you can probably configure your windowing layer to preserve the backbuffer during a swap, but you'll incur a copy of the contents.

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[l1 shu]

Thanks for replying, Brian