SDL2: Fast Pixel Manipulation

Question

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

In every frame I have to manipulate about 100 - 1000 pixels. I have a multi-threaded approach here, which wont give me 30FPS (what I want). Currently I store a Pixel array in the RAM which contains all Pixels and have a SDL_Surface . When a pixel in the array changes, it gets changed in the Surface too and is then after all manipulation is done gets blitted to the screen. My current approach is too slow and I did a bit of thinking on how I could increase the Speed.

My current thoughts are:

• Use OpenGL to do the pixel manipulation directly on the GPU, which some forums tells me that this is way slower than my current approach as "this is not how a GPU works"
• Don't store a pixel array, store a BMP in RAM directly, manipulate that and then move it to an SDL_Surface or SDL_Texture

Are there any other approaches on how I could manipulate Pixels in a fast manner?

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 <algorithm>
#include <chrono>

void PrintFrameTiming(std::ostream& os = std::cout, float period = 2.0f)
{
    static unsigned int frames = 0;
    frames++;
    static auto start = std::chrono::steady_clock::now();
    auto end = std::chrono::steady_clock::now();

    float seconds = std::chrono::duration_cast< std::chrono::duration<float> >(end - start).count();
    if( seconds > period )
    {
        float spf = seconds / frames;
        os
            << frames << " frames in "
            << std::setprecision(1) << std::fixed << seconds << " seconds = "
            << std::setprecision(1) << std::fixed << 1.0f / spf << " FPS ("
            << std::setprecision(3) << std::fixed << spf * 1000.0f << " ms/frame)\n";
        frames = 0;
        start = end;
    }
}

int main( int, char** )
{
    SDL_Init( SDL_INIT_EVERYTHING );
    SDL_Window* window = SDL_CreateWindow( "SDL", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 600, 600, SDL_WINDOW_SHOWN );
    SDL_Renderer* renderer = SDL_CreateRenderer( window, -1, SDL_RENDERER_ACCELERATED );
    SDL_SetHint( SDL_HINT_RENDER_SCALE_QUALITY, "1" );
    
    // dump renderer info
    SDL_RendererInfo info;
    SDL_GetRendererInfo( renderer, &info );
    std::cout << "Renderer name: " << info.name << '\n';
    std::cout << "Texture formats: " << '\n';
    for( Uint32 i = 0; i < info.num_texture_formats; i++ )
    {
        std::cout << SDL_GetPixelFormatName( info.texture_formats[i] ) << '\n';
    }

    // create texture
    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 );

    bool useLocktexture = false;

    // main loop
    bool running = true;
    while( running )
    {
        SDL_SetRenderDrawColor( renderer, 0, 0, 0, SDL_ALPHA_OPAQUE );
        SDL_RenderClear( renderer );

        // handle events
        SDL_Event ev;
        while( SDL_PollEvent( &ev ) )
        {
            if( ( SDL_QUIT == ev.type ) ||
                ( SDL_KEYDOWN == ev.type && SDL_SCANCODE_ESCAPE == ev.key.keysym.scancode ) )
            {
                running = false;
                break;
            }

            if( SDL_KEYDOWN == ev.type && SDL_SCANCODE_L == ev.key.keysym.scancode )
            {
                useLocktexture = !useLocktexture;
                std::cout << "Using " << ( useLocktexture ? "SDL_LockTexture() + std::copy_n()" : "SDL_UpdateTexture()" ) << '\n';
            }
        }
        
        // 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 * y * 4 ) + 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
        }

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

        SDL_RenderCopy( renderer, texture, nullptr, nullptr );
        SDL_RenderPresent( renderer );
        
        PrintFrameTiming();
    }

    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).