SDL：全屏半透明背景

Question

我正在尝试编写一个具有覆盖整个屏幕的半透明背景的程序。经过一些研究，似乎 SDL 将是可行的方法。

我已经编写了代码来创建一个全屏窗口，其背景的 alpha 等于 100（255 中的 100），但由于某种原因，它只是绘制纯色。我做错了什么？

// Initialise SDL
if (SDL_Init(SDL_INIT_EVERYTHING) != 0) {
        this->throwSDLError("SDL_Init Error");
}

// Create the window and renderer
if (SDL_CreateWindowAndRenderer(0, 0, SDL_WINDOW_FULLSCREEN_DESKTOP, &(this->window), &(this->renderer)) != 0) {
        this->throwSDLError("Could not create the window and renderer");
}

// Set the blend mode to specify how the alpha channel is used
if (SDL_SetRenderDrawBlendMode(this->renderer, SDL_BLENDMODE_BLEND) != 0) {
        this->throwSDLError("Could not set render draw blend mode");
}

// Set the colour to draw
if (SDL_SetRenderDrawColor(this->renderer, 200, 200, 200, 100) != 0) {
        this->throwSDLError("Could not set the drawing colour");
}

// Clear the screen using the colour
if (SDL_RenderClear(this->renderer) != 0) {
        this->throwSDLError("Could not render the screen");
}

// Present the rendered screen
SDL_RenderPresent(this->renderer);

Answer 1

在 Windows 上，您可以通过使用SetLayeredWindowAttributes无边框 SDL 窗口的背景色色度键来创建透明窗口。

代码：

// SDL window with transparent background v1.2
#include <SDL.h>
#include <SDL_syswm.h>
#include <Windows.h>

// Makes a window transparent by setting a transparency color.
bool MakeWindowTransparent(SDL_Window* window, COLORREF colorKey) {
    // Get window handle (/sf/answers/1688270181/)
    SDL_SysWMinfo wmInfo;
    SDL_VERSION(&wmInfo.version);  // Initialize wmInfo
    SDL_GetWindowWMInfo(window, &wmInfo);
    HWND hWnd = wmInfo.info.win.window;

    // Change window type to layered (/sf/answers/277915291/)
    SetWindowLong(hWnd, GWL_EXSTYLE, GetWindowLong(hWnd, GWL_EXSTYLE) | WS_EX_LAYERED);

    // Set transparency color
    return SetLayeredWindowAttributes(hWnd, colorKey, 0, LWA_COLORKEY);
}

int main(int argc, char** argv) {
    // Get resolution of primary monitor
    int desktopWidth = GetSystemMetrics(SM_CXSCREEN);
    int desktopHeight = GetSystemMetrics(SM_CYSCREEN);

    SDL_Window* window = SDL_CreateWindow("SDL Transparent Window",
        SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
        desktopWidth, desktopHeight, SDL_WINDOW_BORDERLESS);
    SDL_Renderer* renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);

    // Set background color to magenta and clear screen
    SDL_SetRenderDrawColor(renderer, 255, 0, 255, 255);
    SDL_RenderClear(renderer);

    // Draw blue square in top-left corner
    SDL_Rect rect1 = {0, 0, 100, 100};
    SDL_SetRenderDrawColor(renderer, 0, 0, 255, 255);
    SDL_RenderFillRect(renderer, &rect1);

    // Draw red square in center of the screen
    SDL_Rect rect2 = {(desktopWidth-100)/2, (desktopHeight-100)/2, 100, 100};
    SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255);
    SDL_RenderFillRect(renderer, &rect2);

    // Add window transparency (Magenta will be see-through)
    MakeWindowTransparent(window, RGB(255, 0, 255));

    // Render the square to the screen
    SDL_RenderPresent(renderer);

    // Loop until user quits
    bool quit = false;
    SDL_Event event;
    while (!quit) {
       while (SDL_PollEvent(&event) != 0) {
           if (event.type == SDL_QUIT) {
               quit = true;
           }
       }
    }

    SDL_DestroyWindow(window);
    SDL_Quit();
    return 0;
}

结果：

解释：

首先，创建一个覆盖整个桌面的无边框窗口。选择纯色掩蔽颜色并将其用作背景。（就我而言，我使用了洋红色）。然后，您可以使用 Win32 API 函数抠出您的遮罩颜色SetLayeredWindowAttributes。

具有这种颜色的窗口的任何部分都将完全透明。程序后面的其他窗口可以正常交互。默认情况下，其他应用程序可以移动到无边框窗口的顶部。

如果您希望 SDL 窗口始终位于其他窗口之上，则可以SDL_WINDOW_ALWAYS_ON_TOP在创建窗口时设置该标志。

也可以看看

• 堆栈溢出：在 C++ Win32 中创建透明窗口
• Stack Overflow：如何检测当前屏幕分辨率？
• 微软：分层窗口