SDL2：生成完全透明的纹理

Question

How using SDL_CreateTexture create transparent texture? By default I'm creating texure with such code:

SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888,SDL_TEXTUREACCESS_TARGET, x, y);

And then I'm paining on this texture with redirecting output to this texture. However at the end what I want to render this on screen any (nonupdated) pixel is black.

I have tried different ways with using of:

 SDL_RenderClear(_Renderer);

or even with drawing and on created texture with painting transparent Rect with different blending modes but all I had as a result was still nontransparent texture :/

   SDL_Rect rect={0,0,Width,Height};
   SDL_SetRenderDrawBlendMode(_Renderer,SDL_BLENDMODE_BLEND);
   SDL_SetRenderDrawColor(_Renderer,255,255,255,0);
   SDL_RenderFillRect(_Renderer,&rect);

To be more specific:

    //this->texDefault.get()->get() = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888,SDL_TEXTUREACCESS_TARGET, x, y);
    SDL_SetRenderTarget(_Renderer.get()->get(), this->texDefault.get()->get());
    SDL_SetRenderDrawBlendMode(this->_Renderer.get()->get(),SDL_BLENDMODE_NONE);
    SDL_SetRenderDrawColor(this->_Renderer.get()->get(),255,0,255,0);
    SDL_RenderClear(this->_Renderer.get()->get());
    //SDL_Rect rect={0,0,Width,Height};
    //SDL_SetRenderDrawColor(this->_Renderer.get()->get(),255,255,255,255);
    //SDL_RenderFillRect(this->_Renderer.get()->get(),&rect);
    //SDL_RenderClear(this->_Renderer.get()->get());
    //SDL_SetRenderDrawBlendMode(this->_Renderer.get()->get(),SDL_BLENDMODE_NONE);
    SDL_SetRenderTarget(_Renderer.get()->get(), NULL);
    SDL_Rect rect= {relTop+Top,relLeft+Left,Height,Width};
    SDL_SetRenderDrawBlendMode(this->_Renderer.get()->get(),SDL_BLENDMODE_BLEND);
    SDL_RenderCopy(this->_Renderer.get()->get(), this->texDefault->get(), NULL, &rect);

This code is always producing nontransparent Texture independenty what i will set for blending and alpha

The result is :

Maybe there is some other simple method to create transparent empty texture in SDL2 something like x/y-sized fully transparent png but loading having such image in file is little bit pointless :/

Answer 1

• First, you need to set renderer blend mode: SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND); .
• Second, you need to set texture blend mode: SDL_SetTextureBlendMode(textures[i], SDL_BLENDMODE_BLEND); .

Here is working example I created. You can use keys A and S to change alpha channel of third texture, which is invisible at start of the application.

#include <iostream>
#include <vector>
#include <SDL.h>

void fillTexture(SDL_Renderer *renderer, SDL_Texture *texture, int r, int g, int b, int a)
{
    SDL_SetRenderTarget(renderer, texture);
    SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_NONE);
    SDL_SetRenderDrawColor(renderer, r, g, b, a);
    SDL_RenderFillRect(renderer, NULL);
}

void prepareForRendering(SDL_Renderer *renderer)
{
    SDL_SetRenderTarget(renderer, NULL);
    SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);
    SDL_SetRenderDrawColor(renderer, 128, 128, 128, 255);
}

void checkSdlError()
{
    const char *sdlError = SDL_GetError();
    if(sdlError && *sdlError)
    {
        ::std::cout << "SDL ERROR: " << sdlError << ::std::endl;
    }
}

int main(int argc, char *argv[])
{
    SDL_Init(SDL_INIT_VIDEO | SDL_INIT_HAPTIC);

    SDL_Window *window = SDL_CreateWindow("SDL test",
        SDL_WINDOWPOS_CENTERED,
        SDL_WINDOWPOS_CENTERED,
        320, 240,
        SDL_WINDOW_OPENGL);
    SDL_Renderer *renderer = SDL_CreateRenderer(
        window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_TARGETTEXTURE);

    const int width = 50;
    const int height = 50;

    ::std::vector<SDL_Texture*> textures;

    SDL_Texture *redTexture = SDL_CreateTexture(renderer,
        SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, width, height);
    textures.push_back(redTexture);

    SDL_Texture *greenTexture = SDL_CreateTexture(renderer,
        SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, width, height);
    textures.push_back(greenTexture);

    SDL_Texture *purpleTexture = SDL_CreateTexture(renderer,
        SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, width, height);
    textures.push_back(purpleTexture);

    // Here is setting the blend mode for each and every used texture:
    for(int i = 0; i < textures.size(); ++i)
    {
        SDL_SetTextureBlendMode(textures[i], SDL_BLENDMODE_BLEND);
    }

    int purpleAlpha = 0;

    fillTexture(renderer, redTexture, 255, 0, 0, 255);
    fillTexture(renderer, greenTexture, 0, 255, 0, 128);
    fillTexture(renderer, purpleTexture, 255, 0, 255, purpleAlpha);

    prepareForRendering(renderer);

    bool running = true;
    while(running)
    {
        SDL_Rect rect;
        rect.w = width;
        rect.h = height;

        SDL_RenderClear(renderer);

        rect.x = 50;
        rect.y = 50;
        SDL_RenderCopy(renderer, redTexture, NULL, &rect);

        rect.x = 75;
        rect.y = 70;
        SDL_RenderCopy(renderer, greenTexture, NULL, &rect);

        rect.x = 75;
        rect.y = 30;
        SDL_RenderCopy(renderer, purpleTexture, NULL, &rect);

        SDL_RenderPresent(renderer);

        // Process events
        {
            SDL_Event event;
            while(SDL_PollEvent(&event) == 1)
            {
                if(event.type == SDL_QUIT)
                {
                    running = false;
                }
                else if(event.type == SDL_KEYDOWN)
                {
                    switch(event.key.keysym.sym)
                    {
                    case SDLK_ESCAPE:
                        running = false;
                        break;
                    case SDLK_a:
                        purpleAlpha = ::std::max(purpleAlpha - 32, 0);
                        fillTexture(renderer, purpleTexture, 255, 0, 255, purpleAlpha);
                        prepareForRendering(renderer);
                        ::std::cout << "Alpha: " << purpleAlpha << ::std::endl;
                        break;
                    case SDLK_s:
                        purpleAlpha = ::std::min(purpleAlpha + 32, 255);
                        fillTexture(renderer, purpleTexture, 255, 0, 255, purpleAlpha);
                        prepareForRendering(renderer);
                        ::std::cout << "Alpha: " << purpleAlpha << ::std::endl;
                        break;
                    }
                }
            }

            checkSdlError();
        }
    }

    for(int i = 0; i < textures.size(); ++i)
    {
        SDL_DestroyTexture(textures[i]);
    }
    textures.clear();

    SDL_DestroyRenderer(renderer);
    renderer = NULL;
    SDL_DestroyWindow(window);
    window = NULL;

    SDL_Quit();

    checkSdlError();

    return 0;
}

EDIT: Completely rewritten the answer, original one basically contained blend mode of renderer.