Cairo linear gradient with Win32 double buffering

I'm trying to make Cairo work in a Win32 window. The idea is just to make it render flicker-free.

If I create a Cairo surface directly with the window's HDC, then the window will flicker on resizing. That's normal, and expected. The usual solution is to create a compatible device context and render to a bitmap, then blit that bitmap to the window HDC.

The problem is that the same drawing code that worked when using the window's HDC doesn't work on the double-buffer device context. I just get a black square rather than a gradient circle.

Here's a small, functional example. If you comment out the #define DOUBLE_BUFFER line, then it will draw the Cairo rendering directly to the window's HDC. Otherwise, it will draw to the created one.

#include <windows.h>
#include <stdlib.h>
#include <string.h>
#include <tchar.h>
#include <cairo.h>
#include <cairo-win32.h>

// Global variables

// The main window class name.
static TCHAR szWindowClass[] = _T("CairoTestApp");

// The string that appears in the application's title bar.
static TCHAR szTitle[] = _T("Cairo Test Application");

HINSTANCE hInst;

// Forward declarations of functions included in this code module:
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int main(int argc, const char *argv)
{
    HINSTANCE hInstance = GetModuleHandle(NULL);

    WNDCLASSEX wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);
    wcex.style          = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra     = 0;
    wcex.cbWndExtra     = 0;
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_APPLICATION));
    wcex.hCursor        = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName   = NULL;
    wcex.lpszClassName  = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_APPLICATION));

    if (!RegisterClassEx(&wcex))
    {
        MessageBox(NULL, _T("Call to RegisterClassEx failed!"), szTitle, NULL);

        return 1;
    }

    hInst = hInstance; // Store instance handle in our global variable

    // The parameters to CreateWindow explained:
    // szWindowClass: the name of the application
    // szTitle: the text that appears in the title bar
    // WS_OVERLAPPEDWINDOW: the type of window to create
    // CW_USEDEFAULT, CW_USEDEFAULT: initial position (x, y)
    // 500, 100: initial size (width, length)
    // NULL: the parent of this window
    // NULL: this application does not have a menu bar
    // hInstance: the first parameter from WinMain
    // NULL: not used in this application
    HWND hWnd = CreateWindow(
       开发者_运维技巧 szWindowClass,
        szTitle,
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT,
        256, 256,
        NULL,
        NULL,
        hInstance,
        NULL
        );

    if (!hWnd)
    {
        MessageBox(NULL, _T("Call to CreateWindow failed!"), szTitle, NULL);

        return 1;
    }

    // The parameters to ShowWindow explained:
    // hWnd: the value returned from CreateWindow
    // nCmdShow: the fourth parameter from WinMain
    ShowWindow(hWnd, SW_SHOWNORMAL);
    UpdateWindow(hWnd);

    // Main message loop:
    MSG msg;
    while(true)
    {
        if(PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
        {
            if (msg.message == WM_QUIT)
            {
                break;
            }
            else
            {
                TranslateMessage(&msg);
                DispatchMessage(&msg);
            }
        }
    }

//  DestroyWindow(hWnd);
    UnregisterClass(szWindowClass, hInstance);

    return 0;
}

void gradientExample( cairo_t* cr ) {
    cairo_pattern_t *pat;

    pat = cairo_pattern_create_linear (0.0, 0.0,  0.0, 256.0);
    cairo_pattern_add_color_stop_rgba (pat, 1, 0, 0, 0, 1);
    cairo_pattern_add_color_stop_rgba (pat, 0, 1, 1, 1, 1);
    cairo_rectangle (cr, 0, 0, 256, 256);
    cairo_set_source (cr, pat);
    cairo_fill (cr);
    cairo_pattern_destroy (pat);

    pat = cairo_pattern_create_radial (115.2, 102.4, 25.6,
        102.4,  102.4, 128.0);
    cairo_pattern_add_color_stop_rgba (pat, 0, 1, 1, 1, 1);
    cairo_pattern_add_color_stop_rgba (pat, 1, 0, 0, 0, 1);
    cairo_set_source (cr, pat);
    cairo_arc (cr, 128.0, 128.0, 76.8, 0, 2 * 3.14159);
    cairo_fill (cr);
    cairo_pattern_destroy (pat);
}

#define DOUBLE_BUFFER

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    PAINTSTRUCT ps;
    HDC hdc;
    TCHAR greeting[] = _T("Hello, World!");

    switch (message)
    {
    case WM_PAINT:
        hdc = BeginPaint(hWnd, &ps);

        {
            HDC newDC = CreateCompatibleDC(hdc);
            RECT theRect;
            GetClientRect(hWnd, &theRect);
            int width, height;
            width = theRect.right - theRect.left;
            height = theRect.bottom - theRect.top;
            HBITMAP theBmp = CreateCompatibleBitmap(newDC, width, height);
            HGDIOBJ oldBmp = SelectObject(newDC, theBmp);

            //Test some text.
#ifdef DOUBLE_BUFFER
            TextOut(newDC, 5, 5, greeting, _tcslen(greeting));
#else
            TextOut(hdc, 5, 5, greeting, _tcslen(greeting));
#endif

            {
#ifdef DOUBLE_BUFFER
                cairo_surface_t *surface = cairo_win32_surface_create(newDC);
#else
                cairo_surface_t *surface = cairo_win32_surface_create(hdc);
#endif
                cairo_t *cr = cairo_create(surface);

                // Draw on the cairo context.
                cairo_set_source_rgb(cr, 1, 1, 1);
                cairo_paint(cr);

                gradientExample( cr );
                cairo_surface_finish(surface);

                // Cleanup.
                cairo_destroy(cr);
                cairo_surface_destroy(surface);
            }

#ifdef DOUBLE_BUFFER
            BitBlt(hdc, 0, 0, width, height, newDC, theRect.left, theRect.top, SRCCOPY);
#endif

            SelectObject(newDC, oldBmp);
            DeleteDC(newDC);
        }

        EndPaint(hWnd, &ps);
        break;
    case WM_ERASEBKGND:
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    case WM_KEYDOWN:                                // Is A Key Being Held Down?
        {
            if(wParam == VK_ESCAPE)
            {
                PostMessage(hWnd, WM_CLOSE, 0, 0);
            }
        }
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
        break;
    }

    return 0;
}

Try calling CreateCompatibleBitmap with the original device context (hdc) as parameter instead. Otherwise I believe you'll get one based on the default monochrome dummy bitmap of the supposedly-compatible DC you just created.