SDL2: Fast Pixel Manipulation

Question

I want to draw pixels on the monitor which change frequently after certain parameters. E.G. if a Red and Green Pixel collide, they would both vanish, etc.

In every f

Answer 1

SDL_CreateTexture() w/SDL_TEXTUREACCESS_STREAMING + SDL_UpdateTexture() seems to work well enough with the right pixel format.

On my system using the default renderer:

Renderer name: direct3d
Texture formats:
SDL_PIXELFORMAT_ARGB8888
SDL_PIXELFORMAT_YV12
SDL_PIXELFORMAT_IYUV

(though the opengl info is the same:)

Renderer name: opengl
Texture formats:
SDL_PIXELFORMAT_ARGB8888
SDL_PIXELFORMAT_YV12
SDL_PIXELFORMAT_IYUV

SDL_PIXELFORMAT_ARGB8888 gives me ~1ms/frame:

// g++ main.cpp `pkg-config --cflags --libs sdl2`
#include <SDL.h>
#include <iostream>
#include <iomanip>
#include <vector>
#include <cstring>

int main( int argc, char** argv )
{
    SDL_Init( SDL_INIT_EVERYTHING );

    SDL_Window* window = SDL_CreateWindow
        (
        "SDL2",
        SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
        600, 600,
        SDL_WINDOW_SHOWN
        );

    SDL_Renderer* renderer = SDL_CreateRenderer
        (
        window,
        -1,
        SDL_RENDERER_ACCELERATED
        );

    SDL_RendererInfo info;
    SDL_GetRendererInfo( renderer, &info );
    std::cout << "Renderer name: " << info.name << std::endl;
    std::cout << "Texture formats: " << std::endl;
    for( Uint32 i = 0; i < info.num_texture_formats; i++ )
    {
        std::cout << SDL_GetPixelFormatName( info.texture_formats[i] ) << std::endl;
    }

    const unsigned int texWidth = 1024;
    const unsigned int texHeight = 1024;
    SDL_Texture* texture = SDL_CreateTexture
        (
        renderer,
        SDL_PIXELFORMAT_ARGB8888,
        SDL_TEXTUREACCESS_STREAMING,
        texWidth, texHeight
        );

    std::vector< unsigned char > pixels( texWidth * texHeight * 4, 0 );

    SDL_Event event;
    bool running = true;
    bool useLocktexture = false;
    
    unsigned int frames = 0;
    Uint64 start = SDL_GetPerformanceCounter();

    while( running )
    {

        SDL_SetRenderDrawColor( renderer, 0, 0, 0, SDL_ALPHA_OPAQUE );
        SDL_RenderClear( renderer );

        while( SDL_PollEvent( &event ) )
        {
            if( ( SDL_QUIT == event.type ) ||
                ( SDL_KEYDOWN == event.type && SDL_SCANCODE_ESCAPE == event.key.keysym.scancode ) )
            {
                running = false;
                break;
            }
            if( SDL_KEYDOWN == event.type && SDL_SCANCODE_L == event.key.keysym.scancode )
            {
                useLocktexture = !useLocktexture;
                std::cout << "Using " << ( useLocktexture ? "SDL_LockTexture() + memcpy()" : "SDL_UpdateTexture()" ) << std::endl;
            }
        }
        
        // splat down some random pixels
        for( unsigned int i = 0; i < 1000; i++ )
        {
            const unsigned int x = rand() % texWidth;
            const unsigned int y = rand() % texHeight;

            const unsigned int offset = ( texWidth * 4 * y ) + x * 4;
            pixels[ offset + 0 ] = rand() % 256;        // b
            pixels[ offset + 1 ] = rand() % 256;        // g
            pixels[ offset + 2 ] = rand() % 256;        // r
            pixels[ offset + 3 ] = SDL_ALPHA_OPAQUE;    // a
        }

        if( useLocktexture )
        {
            unsigned char* lockedPixels = nullptr;
            int pitch = 0;
            SDL_LockTexture
                (
                texture,
                NULL,
                reinterpret_cast< void** >( &lockedPixels ),
                &pitch
                );
            std::memcpy( lockedPixels, pixels.data(), pixels.size() );
            SDL_UnlockTexture( texture );
        }
        else
        {
            SDL_UpdateTexture
                (
                texture,
                NULL,
                pixels.data(),
                texWidth * 4
                );
        }

        SDL_RenderCopy( renderer, texture, NULL, NULL );
        SDL_RenderPresent( renderer );
        
        frames++;
        const Uint64 end = SDL_GetPerformanceCounter();
        const static Uint64 freq = SDL_GetPerformanceFrequency();
        const double seconds = ( end - start ) / static_cast< double >( freq );
        if( seconds > 2.0 )
        {
            std::cout
                << frames << " frames in "
                << std::setprecision(1) << std::fixed << seconds << " seconds = "
                << std::setprecision(1) << std::fixed << frames / seconds << " FPS ("
                << std::setprecision(3) << std::fixed << ( seconds * 1000.0 ) / frames << " ms/frame)"
                << std::endl;
            start = end;
            frames = 0;
        }
    }

    SDL_DestroyRenderer( renderer );
    SDL_DestroyWindow( window );
    SDL_Quit();

    return 0;
}

Make sure you don't have have vsync enabled (forced in the driver, running a compositor, etc.) or else all your frame times will be ~16ms (or whatever your display refresh is set to).