How to properly connect skia to an existing opengl context?

[Andrew M]

Good day. I have a window with OWNDC and an Opengl context created. The glew library is also connected.

BOOL InitPixelFormat(HDC& _hDC, HWND& _hWnd)
{
    PIXELFORMATDESCRIPTOR pfd = 
    {
        sizeof(PIXELFORMATDESCRIPTOR),
         1,
         PFD_SUPPORT_OPENGL |
         PFD_DRAW_TO_WINDOW |
         PFD_DOUBLEBUFFER ,
         //PFD_SUPPORT_GDI,
         PFD_TYPE_RGBA,
         32,
         0, 0, 0, 0, 0, 0,
         0,
         0,
         0,
         0, 0, 0, 0,
         32,
         8,
         0,
         PFD_MAIN_PLANE,
         0,
         0, 0, 0
    };

    if (!(_hDC = GetDC(_hWnd)))
    {
        MessageBox(NULL, L"Can't Create A GL Device Context.", L"ERROR", MB_OK | MB_ICONEXCLAMATION);
        return FALSE;                
    }

    int pixelFormat = ChoosePixelFormat(_hDC, &pfd);
    if (!pixelFormat)
    {
        MessageBox(NULL, L"Can't Find A Suitable PixelFormat.", L"ERROR", MB_OK | MB_ICONEXCLAMATION);
        return FALSE;                
    }

    if ((!SetPixelFormat(_hDC, pixelFormat, &pfd)))
    {
        MessageBox(NULL, L"Can't Set The PixelFormat.", L"ERROR", MB_OK | MB_ICONEXCLAMATION);
        return FALSE;             
    }

    return TRUE;
}

BOOL InitOpenGL(HDC& _hDC, HGLRC& _hGLRC)
{
   
    _hGLRC = wglCreateContext(_hDC);

    if (!_hGLRC)
    {
        MessageBox(NULL, L"Can't Create A GL Rendering Context.", L"ERROR", MB_OK | MB_ICONEXCLAMATION); return FALSE;
    }
    if (!(wglMakeCurrent(_hDC, _hGLRC)))
    {
        MessageBox(NULL, L"Can't Activate The GL Rendering Context.", L"ERROR", MB_OK | MB_ICONEXCLAMATION);
        return FALSE;               
    }

    return TRUE;
}

I output the opengl rendering code in the OpenGLPaint function via a while loop:

    while (true)
    {
        /* check for messages */
        if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
        {
            /* handle or dispatch messages */
            if (msg.message == WM_QUIT)
            {
                break;
            }
            else if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
            {
                TranslateMessage(&msg);
                DispatchMessage(&msg);
            }
        }
        else
        {
             
            OpenGLPaint();

        }
    }

Here is the code of the OpenGLPaint() function: 

void OpenGLPaint()
{

glClearColor(1.0, 1.0, 1.0, 1.0);//white

glClearStencil(0);

glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

    //sk_sp<const GrGLInterface> intf = nullptr;
    sk_sp<const GrGLInterface> intf = GrGLMakeNativeInterface();
    if (intf->validate()) cout << "Good" << endl;
    sk_sp<GrDirectContext> context = GrDirectContext::MakeGL(intf);
    SkImageInfo info = SkImageInfo::MakeN32Premul(glWindowWidth, glWindowHeight);
    sk_sp<SkSurface> gpuSurface(SkSurface::MakeRenderTarget(context.get(),  SkBudgeted::kNo, info));
    if (!gpuSurface) cout << "gpu surface wasn't create properly" << endl;
    SkCanvas* gpuCanvas = gpuSurface->getCanvas();

    SkRect rect = SkRect::MakeXYWH(50, 50, 40, 60);
    SkPaint paint;
    paint.setStrokeWidth(20);
    paint.setColor(SK_ColorRED);
    gpuCanvas->drawRect(rect, paint);

glFlush();
SwapBuffers(hDC2);

Skia initializes successfully, but the rectangle is not output. What am I doing wrong? Opengl renders normally. I need to draw via skia in my opengl context, no other way.

I also tried initializing kia in a separate function, also doesn't work.

[Brian Salomon]

So if you're trying to connect Skia to the OpenGL's default framebuffer object (FBO ID 0) then you need to import that into Skia. SkSurface::MakeRenderTarget() creates a new texture and FBO to draw into. Instead use SkSurface::MakeFromBackendRenderTarget(). It should look something like:

GrGLFramebufferInfo fbInfo;
fbInfo.fFBOID = 0;
fbInfo.fFormat = GL_RGBA8;

GrBackendRenderTarget backendRT(glWindowWidth,
                                glWindowHeight,
                                /*sampelCount=*/1,
                                /*stencilBits=*/8,
                                fbInfo);

sk_sp<SkSurface> gpuSurface = SkSurface::MakeFromBackendRenderTarget(context.get(), backendRT,
                                                                     kBottomLeft_GrSurfaceOrigin,
                                                                     kRGBA_8888_SkColorType,

                                                                     /*colorSpace=*/nullptr,
                                                                     /*surfaceProps=*/nullptr);

Also, after you are finished drawing to SkSurface, before you swap buffers, you have to call surface->flush(). This tells Skia to flush all its work to OpenGL. You shouldn't need the glFlush().

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[Andrew M]

Thanks, it works. But only in the Opengl rendering function. If I create global variables:

sk_sp<const GrGLInterface> intf;
sk_sp<GrDirectContext> context;
sk_sp<SkSurface> gpu Surface;
SkCanvas* gpu Canvas;

and I initialize skia in a separate function, only one frame is displayed and then a white screen is displayed all the time. I'm racking my head over this and I can't figure out what the reason is.

четверг, 3 ноября 2022 г. в 20:51:37 UTC+8, Brian Salomon:

[Lin Wither]

hi, I met the same problem that only one frame is displayer. I finally found out that skia context should be reset for each frame to draw.

just add this line for drawing loop:

 

    context->resetContext();

[Jim Van Verth]

FYI, if you know the subset of GL state you're changing in your code then you can be more granular by passing in GrGLBackendState flags to resetContext().

To view this discussion on the web visit https://groups.google.com/d/msgid/skia-discuss/1aeb0fb8-ead1-4d77-8c23-1ec381451b3cn%40googlegroups.com.

--

Jim Van Verth | Software Engineer | Google.com