Sinewave

sinewave.c

#include "SDL.h"
#include "SDL_gfxPrimitives.h"
#include "SDL_framerate.h"

#include <stdbool.h>
#include <math.h>
#include <stdio.h>

double derp = 0;

void
draw_spiral( SDL_Surface *surface, int x1, int y1, int x2, int y2 ) {
    int plot_x, plot_y;
    int cent_x, cent_y;
    int i = 400;

    cent_x = x1 + ((x2 - x1) / 2);
    cent_y = y1 + ((y2 - y1) / 2);

    derp += 0.8;

    while( i-- ) {
        plot_x = cent_x + sin(i) * (i / 3);
        plot_y = cent_y + cos(i) * (i / 3);
        int C = fabs(sin((derp + i) / 20)) * 255;
        filledEllipseRGBA(surface,
            plot_x, plot_y,
            i / 2, i / 2,
            C, C, C, 255);
    }
}

#define WINDOW_WIDTH 700
#define WINDOW_HEIGHT 700

int
main( int argc, char **argv ) {
    int i;
    SDL_Init( SDL_INIT_VIDEO );
    SDL_Surface *screen = SDL_SetVideoMode( WINDOW_WIDTH, WINDOW_HEIGHT, 0, SDL_HWSURFACE | SDL_DOUBLEBUF );
    SDL_WM_SetCaption( "Moar Circles", 0 );

    SDL_Event event;
    FPSmanager framerate;
    bool is_running = true;

    SDL_initFramerate(&framerate);
    SDL_setFramerate(&framerate, 30);

    while( is_running ) {
        if( SDL_PollEvent(&event) ) {
            if( event.type == SDL_QUIT || event.type == SDL_KEYDOWN ) {
                is_running = false;
            }
        }

        SDL_FillRect( screen, NULL, SDL_MapRGB(screen->format, 255, 255, 255) );

        draw_spiral(screen, 0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);

        SDL_Flip( screen );

        SDL_framerateDelay(&framerate);
    }

    SDL_Quit();

    return 0;
}

Draws a spiral of ellipses, and colours them using a SIN() + animation.

It is also useful for getting visual knowledge of other mathematical functions, e.g. logarithms: