SDL2: Generate fully transparent texture - c++

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.