C++ GDI+ 清除上一屏幕

Question

所以我试图在它旁边绘制我的光标位置，到目前为止它工作得很好。预计它有时会留下前几帧的 "trail"，所以我想知道如何清除它们？有快速的技巧吗？

这是一个快速 GIF：

这是我的代码：

void DrawCursorCoords(Gdiplus::Graphics &graphics, Gdiplus::Font &font, Gdiplus::Brush &brush)
{
    POINT cursorPos;
    GetCursorPos(&cursorPos);

    std::wstring x = std::to_wstring(cursorPos.x);
    std::wstring y = std::to_wstring(cursorPos.y);

    graphics.DrawString(x.c_str(), x.length(), &font, Gdiplus::PointF(cursorPos.x, cursorPos.y), &brush);
    graphics.DrawString(y.c_str(), y.length(), &font, Gdiplus::PointF(cursorPos.x, cursorPos.y + 50), &brush);
}


int main()
{
    // Start GDI+
    Gdiplus::GdiplusStartupInput gdiplusStartupInput;
    ULONG_PTR gdiplusToken;
    Gdiplus::GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);

    // Initialize graphics, brushes etc...
    HWND hWnd = GetDesktopWindow();
    HDC hdc = GetDC(hWnd);
    Gdiplus::Graphics * gfx = new Gdiplus::Graphics(hdc);
    Gdiplus::Pen * pen = new Gdiplus::Pen(Gdiplus::Color(255, 0, 255));
    Gdiplus::Font * cursorFont = new Gdiplus::Font(L"Consolas", 16);
    Gdiplus::SolidBrush * cursorBrush = new Gdiplus::SolidBrush(Gdiplus::Color(255, 0, 0, 150));
    Gdiplus::SolidBrush * clearBrush = new Gdiplus::SolidBrush(Gdiplus::Color::Transparent);

    while (!GetAsyncKeyState(VK_INSERT))
    {
        printf("Drawing!\n");

        DrawCursorCoords(*gfx, *cursorFont, *cursorBrush);

        // 1. Super slow + just fills the screen black
        //gfx->Clear(Gdiplus::Color::Transparent);

        // 2. Doesn't "flush" anything?
        //gfx->Flush();

        // 3. Super slow + does nothing
        //gfx->FillRectangle(clearBrush, Gdiplus::Rect(0, 0, GetSystemMetrics(SM_CXSCREEN), GetSystemMetrics(SM_CYSCREEN)));
    }

    printf("Stopped drawing!\n");

    delete gfx;
    delete pen;
    delete cursorFont;
    delete cursorBrush;
    delete clearBrush;
    ReleaseDC(hWnd, hdc);
    Gdiplus::GdiplusShutdown(gdiplusToken);

    return 0;
}

Answer 1

你不应该在桌面上画画。您应该只绘制自己的 window。如果您在桌面或任何其他人的 window 上绘制，它将在下一个绘制周期中被清除 window。这是不可能控制的。

你也不能在控制台上绘图。 Windows 控制台有它自己的 window 和它自己的不可访问的绘制例程。控制台确实提供了一些功能，例如 SetConsoleCursorPosition 允许在不同位置打印文本。

示例：

#include <Windows.h>
#include <iostream>

int main()
{
    HANDLE hout = GetStdHandle(STD_OUTPUT_HANDLE);
    while(true)
    {
        COORD coord = { 0, 0 };
        SetConsoleCursorPosition(hout, coord);
        std::cout << "Move the mouse and click a key\n";
        system("pause");

        POINT pt;
        GetCursorPos(&pt);
        HWND hwnd = GetConsoleWindow();

        RECT rc;
        GetClientRect(hwnd, &rc);
        ScreenToClient(hwnd, &pt);

        CONSOLE_SCREEN_BUFFER_INFO inf;
        GetConsoleScreenBufferInfo(hout, &inf);

        coord.X = (SHORT)MulDiv(pt.x, inf.srWindow.Right, rc.right);
        coord.Y = (SHORT)MulDiv(pt.y, inf.srWindow.Bottom, rc.bottom);

        SetConsoleCursorPosition(hout, coord);
        std::cout << "X";
    }
    return 0;
}

控制台的绘图选项有限。对于实际绘图，您必须使用 CreateWindow 创建一个 window，以及一个 window 消息循环来响应 WM_PAINT 进行绘图，以及处理其他 window 消息。

请参阅 Walkthrough: Creating Windows Desktop Applications 了解 windows 编程的介绍，或参阅 Windows 编程的介绍书籍。

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