To make part of a window transparent while leaving the rest opaque, you’ll need to use SDL’s blending features or a related rendering library. SDL itself doesn’t directly support partial window transparency in the same way as full window opacity. Instead, you’ll need to render your window’s contents and use an alpha channel for specific regions.
Using SDL Textures for Partial Transparency
You can achieve this by drawing a texture with an alpha channel over the transparent region of your window. Here’s an example:
#include <SDL.h>
#include <iostream>
// Create a semi-transparent rectangle
// on the window
void DrawTransparentRect(
SDL_Renderer* renderer
) {
SDL_SetRenderDrawBlendMode(renderer,
SDL_BLENDMODE_BLEND);
SDL_SetRenderDrawColor(renderer,
255, 0, 0, 128); // RGBA (50% transparency)
SDL_Rect rect = {100, 100, 200, 150};
SDL_RenderFillRect(renderer, &rect);
}
int main() {
if (SDL_Init(SDL_INIT_VIDEO) < 0) {
std::cerr << "SDL Init Error: "
<< SDL_GetError() << "\n";
return -1;
}
SDL_Window* window = SDL_CreateWindow(
"Partial Transparency",
SDL_WINDOWPOS_CENTERED,
SDL_WINDOWPOS_CENTERED,
400, 300,
SDL_WINDOW_SHOWN
);
if (!window) {
std::cerr << "Window Creation Error: "
<< SDL_GetError() << "\n";
SDL_Quit();
return -1;
}
SDL_Renderer* renderer =
SDL_CreateRenderer(window, -1, 0);
SDL_SetRenderDrawColor(renderer,
0, 0, 0, 255); // Black background
SDL_RenderClear(renderer);
DrawTransparentRect(renderer);
SDL_RenderPresent(renderer);
SDL_Delay(3000);
SDL_DestroyRenderer(renderer);
SDL_DestroyWindow(window);
SDL_Quit();
return 0;
}A red semi-transparent rectangle appears over a black background.This approach allows for fine-grained control over transparency but requires handling all rendering manually.
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Window Opacity
Discover how to use SDL2 functions for controlling and retrieving window transparency settings.
