Sharing OpenGL context with Skia

[Ray Tawil]

Hi,

I'm trying to use Skia with a GPU accelerated backend. I figured that I have to implement GrGLInterface so that I can share the OpenGL context with Skia. Does anyone have an example/tutorial of a similar use case that they can share?

Thanks,

Ray

[Jim Van Verth]

You might want to look at example/SkiaSDLExample.cpp, which integrates Skia with an SDL backend. Lines 186-213 show you how to set up an SkSurface that uses the currently bound framebuffer and then you can get an SkCanvas from that.

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[Brian Salomon]

Hi Ray, unless you're doing something complicated just pass (GrBackendContext)(nullptr) to GrContext::Create() when your GL context is bound and Skia will build its own GrGLInterface internally.

Brian

On Wed, Jul 19, 2017 at 12:13 PM 'Ray Tawil' via skia-discuss <skia-d...@googlegroups.com> wrote:

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[Jim Van Verth]

We should probably fix the example -- currently it's explicitly using GrGLCreateNativeInterface() to create a default interface and passing it into GrContext::Create().

[Ray Tawil]

Thanks Brian & Jim. I have a follow up question in that case. It could be a stupid question but this is my first time doing a project involved with graphics and OpenGL. If I pass a nullptr and go for the default option of letting Skia build its own GrGLInterface internally, how is Skia going to figure out how to communicate with the GPU and what version of OpenGL is used, etc. 

And just for context, I want to integrate Skia with an internal framework that I can't discuss publicly unfortunately, for the purpose of supporting SVG rendering.

[李昀陞]

Thank you for the Ray's question.

I also have the same problems for sharing the OpenGL context with Skia.

I really want to know the correct example code on SkiaSDLExample.cpp.

Thanks.

Ray Tawil於 2017年7月20日星期四 UTC+8上午12時13分30秒寫道：

[Jim Van Verth]

I'm not sure what additional questions you may have. SkiaSDLExample.cpp is correct, except that you don't have to explicitly create the GrGLInterface. So instead of

// setup GrContext

sk_sp<const GrGLInterface> interface(GrGLCreateNativeInterface());

// setup contexts

sk_sp<GrContext> grContext(GrContext::Create(kOpenGL_GrBackend,

                                             (GrBackendContext)interface.get()));

you can simply do

// setup GrContext

sk_sp<GrContext> grContext(GrContext::Create(kOpenGL_GrBackend,

                                             (GrBackendContext) nullptr));

But both will work.

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[李昀陞]

Hi Jim, thank you for your reply. 

When I try to create the OpenGL context, it's failed for SkASSERT(grContext);

What;s the current error message? Thanks. Here is the code snippet:

// setup GrContext
    sk_sp<const GrGLInterface> interface(GrGLCreateNativeInterface());


 // setup contexts

    sk_sp<GrContext> grContext(GrContext::Create(kOpenGL_GrBackend, (GrBackendContext) nullptr));
 //sk_sp<GrContext> grContext(GrContext::Create(kOpenGL_GrBackend, (GrBackendContext)interface.get()));
    SkASSERT(grContext);


 // Wrap the frame buffer object attached to the screen in a Skia render target so Skia can
    // render to it
    GrGLint buffer;
    GR_GL_GetIntegerv(interface.get(), GR_GL_FRAMEBUFFER_BINDING, &buffer);
    GrGLFramebufferInfo info;
    info.fFBOID = (GrGLuint) buffer;
    GrBackendRenderTarget target(new_width, new_height, kMsaaSampleCount, kStencilBits, kSkia8888_GrPixelConfig, info);