Raycaster with bilinear filtering

#include <stdlib.h>
#include <math.h>
#include <stdio.h>
#include <time.h>
#include <sys/time.h>
#include <SDL.h>
#include <SDL_ttf.h>

int scw = 1280;
int sch = 1024;

int mapw = 512;
int maph = 512;

char* my_itoa(unsigned int val)
{
    static char buf[11];
    short i=10;
    for(;val&&i;--i,val/=10)buf[i]="0123456789"[val%10];
    return &buf[i+1];
}

double mygettime()
{
  struct timeval tv;
  if(gettimeofday(&tv, 0) < 0)
  {
    perror("oops");
  }
  return (double)tv.tv_sec + (0.000001 * (double)tv.tv_usec);
}

void filldata (int *hei)
{
    int p = 0;
    int x,y;
    for (y=0; y<maph; y++)
    {
        for (x=0; x<mapw; x++)
        {
            double x1 = (x-mapw/2)*0.3;
            double y1 = (y-maph/2)*0.3;
//            col[p] = 3*(cos(x*0.2) + sin (y*0.3)) + 88;
            hei[p] = 1000*sin(sqrt(x1*x1+y1*y1))/(1+sqrt(x1*x1+y1*y1));

            p++;
        }
    }
}

void fillpalette (SDL_Surface *screen)
{
    SDL_Color colors[256];
    int i;
//     Fill colors with color information
    for(i=0;i<=256;i++)
    {
        colors[i].r=i;
        colors[i].g=i/2;
        colors[i].b=i/2;
    }
    SDL_SetPalette(screen, SDL_LOGPAL|SDL_PHYSPAL, colors, 0, 256);
}

void checkevents(double *ang, int *x, int *y, int *k)
{
    SDL_Event event;
    if (SDL_PollEvent(&event))
    {
        switch (event.type) {
        case SDL_QUIT:
            exit(0);
        }
    }
    Uint8 *keystate = SDL_GetKeyState (NULL);
    if (keystate[SDLK_LEFT]) *ang += 0.02;
    if (keystate[SDLK_RIGHT]) *ang -= 0.02;
    if (keystate[SDLK_UP]) *k -= 1;
    if (keystate[SDLK_DOWN]) *k += 1;
    if (keystate[SDLK_a] || keystate[SDLK_d] || keystate[SDLK_w] || keystate[SDLK_s])
    {
        int xd, yd;
        xd = 0;
        yd = 0;
        if (keystate[SDLK_a]) xd = -3;
        if (keystate[SDLK_d]) xd = 3;
        if (keystate[SDLK_w]) yd = 3;
        if (keystate[SDLK_s]) yd = -3;

        *x += yd*cos(*ang) + xd*sin(*ang);
        *y += -yd*sin(*ang) + xd*cos(*ang);
    }
}

void blacken_screen (SDL_Surface *screen)
{
    SDL_Rect rect;
    rect.x = 0;
    rect.y = 0;
    rect.w = scw;
    rect.h = sch;

    SDL_FillRect (screen, &rect, 0);
}

double get_filtered_val (int *val, double xf, double yf)
{
    int x = xf;
    int y = yf;
    double diffx = xf - x;
    double diffy = yf - y;
    double oppx = 1 - diffx;
    double oppy = 1 - diffy;

    if ((x==511) || (y==511)) return val[mapw*x+y];

    return (val[x*mapw+y]*oppx + val[(x+1)*mapw+y]*diffx)*oppy + \
        (val[x*mapw+y+1]*oppx + val[(x+1)*mapw+y+1]*diffx)*diffy;
}

double uint8_get_filtered_val (Uint8 *val, double xf, double yf)
{
    int x = xf;
    int y = yf;
    double diffx = xf - x;
    double diffy = yf - y;
    double oppx = 1 - diffx;
    double oppy = 1 - diffy;

    if ((x==511) || (y==511)) return val[mapw*x+y];

    return (val[x*mapw+y]*oppx + val[(x+1)*mapw+y]*diffx)*oppy + \
        (val[x*mapw+y+1]*oppx + val[(x+1)*mapw+y+1]*diffx)*diffy;
}

int main ()
{
    if ( SDL_Init(SDL_INIT_VIDEO) < 0 )
    {
        fprintf(stderr, "Cannot initialize SDL: %s\n", SDL_GetError());
        exit(1);
    }

    if (TTF_Init()==-1)
    {
        printf ("Error Init TTF\n");
        exit(1);
    }

    TTF_Font *font = TTF_OpenFont ("tahoma.ttf", 24);
    SDL_Color fontcolor;
    fontcolor.r = 255;
    fontcolor.g = 255;
    fontcolor.b = 255;

    SDL_Rect dest;
    dest.x = 20;
    dest.y = 20;
    SDL_Surface *textrend = TTF_RenderText_Solid (font, "FPS: ", fontcolor);

    SDL_Surface *screen;
    screen = SDL_SetVideoMode(scw, sch, 8, SDL_SWSURFACE);
    if ( screen == NULL )
    {// Если установить разрешение не удалось
        fprintf(stderr, "Cannot set resolution %ix%i: %s\n", scw, sch, SDL_GetError());
        exit(1);
    }

    fillpalette (screen);
    int hei[mapw*maph-1];
    //  int col[mapw*maph-1];
    filldata (hei);
//    SDL_Surface *text = SDL_LoadBMP ("./testtexture.bmp");
    SDL_Surface *text = SDL_LoadBMP ("./testtext3.bmp");
    if (!text) printf ("Error\n");
    Uint8 *col = text->pixels;

    int posx = 0;
    int posy = 0;

    double ang = 0;
    int horiz = -100;

    int k = 1;

    double dd = 2.0;
    double de = dd * 128;
    double x,y,z;

    double amax = M_PI/3.4;
    double a;
    int sx;

    int maxh;

    int bin1 = mapw*(maph-1);
    int bin2 = maph-1;

    double time1 = mygettime();
    double time2;
    int frames = 0;

    while (1)
    {
        SDL_LockSurface(screen);

        blacken_screen (screen);

        int startidx = ((posx*mapw) & bin1) + (posy&bin2);
        int starth = hei[startidx];
        int posz = 250+starth/2;
//        int posz = 400;

        double sinang = sin(ang);
        double cosang = cos(ang);

        for (sx=0;sx<scw;sx++)
        {
            double d;

            a = -amax + (2*amax)*sx/scw;
            //a = amax*sx/scw;
            double tana = tan (a);
            maxh = sch-1;
            for (d=0;d<=de;d+=dd)
            {
                double y1 = d*/*cos(amax)**/tana;
                double x1 = d;//*cos(amax);

                x = posx + x1*cosang+y1*sinang;
                y = posy - x1*sinang+y1*cosang;
                z = posz - k*d;//*sin(amax);

                int xdiv = x/mapw;
                int ydiv = y/maph;
                x = fabs (x - xdiv*mapw);
                y = fabs (y - ydiv*maph);
                float h = get_filtered_val (hei, x, y);

                int sy = sch/2 + 40*(z-h)/(d+1);
                if (sy<0) sy = 0;
                if (sy>sch-1) sy = sch-1;

                if (sy<maxh)
                {
                    int startpos = maxh;
                    maxh=sy;
                    int syi;

                    Uint8 color = uint8_get_filtered_val (col, x, y);
                    for (syi=startpos; syi>=sy; syi--)
                    {
                        *((Uint8*)screen->pixels+sx+scw*syi) = color;
                    }
                }
            }
        }

        SDL_UnlockSurface(screen);
        SDL_BlitSurface (textrend, NULL, screen, &dest);
        SDL_Flip (screen);
        checkevents (&ang, &posx, &posy, &k);

        frames++;

        time2 = mygettime() - time1;
        if (time2 > 1.0)
        {
            SDL_FreeSurface (textrend);
            char fpsstring[30] = "FPS: ";
            strcat (fpsstring, my_itoa (frames));
            textrend = TTF_RenderText_Solid (font, fpsstring, fontcolor);
            frames = 0;
            time1 = mygettime();
        }
    }

    return atexit(SDL_Quit);
}