为什么绘制到隐藏的HDC与绘制到GetDC返回的HDC (HWND)时的结果不同？

Question

我有两种方法，paintDoubleBuffered和paint。他们都应该在屏幕上画这个图像：

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该图像由大约12个较小的图像组成，每个大小为256x256平铺在一起。

我的标准绘画方法和预期的一样。下面是：

void MainWindow::paint(HWND hwnd) {

    HDC hdc{GetDC(hwnd)};
    paint(hwnd, hdc);
    ReleaseDC(hwnd, hdc);

}

void MainWindow::paint(HWND hwnd, HDC hdc) {
    constexpr INT img_width{ MDNR_Map::pannel_width };
    constexpr INT img_height{ MDNR_Map::pannel_height };

    const INT width{ GetDeviceCaps(hdc, HORZRES) };
    const INT height{ GetDeviceCaps(hdc, VERTRES) };

    const INT num_width_pannels{ (width / img_width) + 1 };
    const INT num_height_pannels{ (height / img_height) + 1 };

    Gdiplus::Graphics g(hdc);

    g.SetCompositingMode(CompositingMode::CompositingModeSourceCopy);

    g.SetInterpolationMode(InterpolationMode::InterpolationModeNearestNeighbor);

    for (INT y = 0; y < num_height_pannels; y++) {
        for (INT x = 0; x < num_width_pannels; x++) {

            Location_t get_loaction(x + map_location.x, y + map_location.y, map_location.layer);
            const IMG_t v{ mdnr_map.get(get_loaction) };

            const Point drawPoint((INT)(img_width * x), (INT)(img_height * y));

            Status stat{ g.DrawImage(v, drawPoint) };

            if (stat != Status::Ok)
            {
                throw std::runtime_error(":(");
            }


        }
    }
}

这个油漆方法的问题是，mdnr_map.get是一个io绑定调用，可能需要几微秒。因为我需要叫它12次，它会导致闪烁。

为了解决这个问题，我尝试编写一个双缓冲的油漆方法，如下所示：

void MainWindow::paintDoubleBuffered(HWND hwnd) {

    // Get DC for window
    HDC hdc{ GetDC(hwnd) };

    const INT win_width{ GetDeviceCaps(hdc, HORZRES) };
    const INT win_height{ GetDeviceCaps(hdc, VERTRES) };

    // Create an off-screen DC for double-buffering
    HDC hdcMem{ CreateCompatibleDC(hdc) };

    HBITMAP hbmMem{ CreateCompatibleBitmap(hdc, win_width, win_height) };

    HANDLE hOld{ SelectObject(hdcMem, hbmMem) };

    // Draw into hdcMem here
    paint(hwnd, hdcMem);

    // Transfer the off-screen DC to the screen
    BitBlt(hdc, 0, 0, win_width, win_height, hdcMem, 0, 0, SRCCOPY);

    // Free-up the off-screen DC
    SelectObject(hdcMem, hOld);

    DeleteObject(hbmMem);
    DeleteDC(hdcMem);
}

但是，这样做不起作用，反而产生了这种令人憎恶的图像：

​

只要稍微戳一下，我就能发现，如果我把图像大小乘以1.5来改变我的双缓冲油漆方法，图像就不再那么乱了，但现在放大了1.5倍。

void MainWindow::paintDoubleBuffered(HWND hwnd) {

    // Get DC for window
    HDC hdc{ GetDC(hwnd) };

    const INT win_width{ GetDeviceCaps(hdc, HORZRES) };
    const INT win_height{ GetDeviceCaps(hdc, VERTRES) };

    // Create an off-screen DC for double-buffering
    HDC hdcMem{ CreateCompatibleDC(hdc) };

    HBITMAP hbmMem{ CreateCompatibleBitmap(hdc, win_width, win_height) };

    HANDLE hOld{ SelectObject(hdcMem, hbmMem) };

    // Draw into hdcMem here
    
    constexpr INT img_width{ MDNR_Map::pannel_width + 128 }; // MDNR_Map::pannel_width is 256
    constexpr INT img_height{ MDNR_Map::pannel_height + 128}; // MDNR_Map::pannel_height is 256

    const INT num_width_pannels{ (win_width / img_width) + 1 };
    const INT num_height_pannels{ (win_height / img_height) + 1 };

    Gdiplus::Graphics g(hdcMem);

    g.SetCompositingMode(CompositingMode::CompositingModeSourceCopy);

    g.SetInterpolationMode(InterpolationMode::InterpolationModeNearestNeighbor);

    for (INT y = 0; y < num_height_pannels; y++) {
        for (INT x = 0; x < num_width_pannels; x++) {

            Location_t get_loaction(x + map_location.x, y + map_location.y, map_location.layer);
            Gdiplus::Bitmap* pannel{ mdnr_map.get(get_loaction) };

            const Point drawPoint((INT)(img_width * x), (INT)(img_height * y));

            Status stat{ g.DrawImage(pannel, drawPoint) };
            if (stat != Status::Ok)
            {
                throw std::runtime_error(":(");
            }

        }
    }

    // Transfer the off-screen DC to the screen
    BitBlt(hdc, 0, 0, win_width, win_height, hdcMem, 0, 0, SRCCOPY);

    // Free-up the off-screen DC
    SelectObject(hdcMem, hOld);

    DeleteObject(hbmMem);
    DeleteDC(hdcMem);
}

​

​

我的问题是为什么CreateCompatibleBitmap返回的HDC与GetDC?返回的HDC产生不同的结果？

我试过了： BltBlt的所有光栅操作代码.我检查过临时的HDC和窗口的大小是一样的。我试过替换代码片段

const INT win_width{ GetDeviceCaps(hdc, HORZRES) };
const INT win_height{ GetDeviceCaps(hdc, VERTRES) };

使用

RECT rect;
GetWindowRect(hwnd, &rect);

const INT win_width{ rect.right - rect.left };
const INT win_height{ rect.bottom - rect.top };

我还在绘图之前调用了SetProcessDPIAware()。

根据@Paul的反馈，我重写了我的paintDoubleBuffered方法如下，注意，我在对象构造函数中调用了BufferedPaintInit：

void MainWindow::paintDoubleBuffered(HWND hwnd) {

    PAINTSTRUCT ps;
    HDC hdc{ BeginPaint(hwnd, &ps)};

    RECT sz;
    GetWindowRect(hwnd, &sz);

    BP_PAINTPARAMS paintParams = { 0 };

    paintParams.cbSize = sizeof(paintParams);
    paintParams.dwFlags = BPPF_ERASE;
    paintParams.pBlendFunction = NULL;
    paintParams.prcExclude = NULL;

    HDC hdcBuffer;

    HPAINTBUFFER hBufferedPaint = BeginBufferedPaint(hdc, &sz, BPBF_COMPATIBLEBITMAP, &paintParams, &hdcBuffer);

    if (hBufferedPaint && this->bufferedInitResult == Ok) {
        // Application specific painting code
        paint(hwnd, hdcBuffer);
        EndBufferedPaint(hBufferedPaint, TRUE);
    }
    else{
        paint(hwnd, hdc);
    }

    ReleaseDC(hwnd, hdc);
}

不幸的是，这不起作用，由此产生的屏幕如下所示：

​

Answer 1

问题最终不是在双重缓冲的方法中，而是在我的call方法中对Graphics::DrawImage(Image*, Gdiplus::Point)的调用中。更改为DrawImage(Image* image, INT x, INT y, INT width, INT height)修复了缩放问题。