CODEWITHSUNDEEP
sdl-2
Parham
Parham

Reputation: 21

changing alpha value in SDL_SetRenderDrawColor doesn't effect anything. SDL2

This code shows just a simple window with a color:

#include<SDL.h>                     
SDL_Window* g_pWindow = 0;
SDL_Renderer* g_pRenderer = 0;
int main(int argc, char* args[])
{
    if (SDL_Init(SDL_INIT_EVERYTHING) >= 0)
    {
        g_pWindow = SDL_CreateWindow("Chapter 1: Setting up SDL",
            SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
            640, 480,
            SDL_WINDOW_SHOWN);
        if (g_pWindow != 0)
        {
            g_pRenderer = SDL_CreateRenderer(g_pWindow, -1, 0);
        }
    }
    else
    {
        return 1; // sdl could not initialize
    }
    SDL_SetRenderDrawColor(g_pRenderer, 80, 80, 253, 0);
    // clear the window to black
    SDL_RenderClear(g_pRenderer);
    // show the window
    SDL_RenderPresent(g_pRenderer);
    // set a delay before quitting
    SDL_Delay(2000);
    // clean up SDL
    SDL_Quit();
    return 0;
}

I'm testing to see what happens when I change the alpha factor in SDL_SetRenderDrawColor(g_pRenderer, 80, 80, 253, 0). When I change the alpha value from 0 to 255 it doesn't effect anything.

What's the problem here?

Upvotes: 1

Views: 3775

Answers (2)

Wakeel Furqan Ahmed
Wakeel Furqan Ahmed

Reputation: 103

Writing this line will make alpha values have effect on transparency:

SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);

No need to rewrite every time & for every effert, just write once before using transparency

see documentation on more blend modes: https://wiki.libsdl.org/SDL2/SDL_BlendMode

Upvotes: 0

keltar
keltar

Reputation: 18409

First of all, you haven't enabled blending (e.g. SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);).

But anyway, it makes no sense for clear operation to use blending, and I bet SDL_RenderClear ignores it.

If you want fullscreen blend, you should draw fullscreen rectangle with SDL_RenderFillRect.

Upvotes: 4

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