scanline based rendering with SDL2

vd.c

#include <SDL2/SDL.h>
#include<math.h>
enum {FALSE ,TRUE};
enum {W=600,H=400};

int _SCROLLX = 0;
int _SCROLLY = 0;

SDL_Surface* screen;

float t = 0;
SDL_Surface* myimg = NULL;
void _HBLANK(int sc) {
	_SCROLLX = -t*70 + 0.1*cos(t)*sc + sin(tan(t-sc*30))*-4 + cos(t/2+(float)sc/4.0)*3+1.5;
	_SCROLLY = t*100 + sin(-t*4+(float)sc/16.0)*20;
}

void _VBLANK(void) {
	SDL_BlitSurface(myimg, NULL, screen, NULL);
	t += 0.01;
}

void _INIT(void) {
	myimg = SDL_LoadBMP("myimg.bmp");
	if (!myimg) {
		printf("give me some bitmap file @ myimg.bmp\n");
		exit(1234);
	}
}

int imod(int x, int y) { 
	int m = x % y;
	return m < 0 ? y+m : m;
}

int main() {
	SDL_Init(SDL_INIT_VIDEO);

	SDL_Window* win;
	SDL_Renderer* ren;
	SDL_SetHint(SDL_HINT_RENDER_VSYNC, "1");
	SDL_CreateWindowAndRenderer(W,H,0,&win,&ren);
	screen = SDL_CreateRGBSurfaceWithFormat(0,W,H,8*4, SDL_PIXELFORMAT_RGBA32);
	SDL_Texture* tex = SDL_CreateTexture(ren, SDL_PIXELFORMAT_RGBA32, SDL_TEXTUREACCESS_STREAMING, W, H);

	_INIT();

	int quit = FALSE;
	while (!quit) {
		SDL_Event ev;
		while (SDL_PollEvent(&ev)) {
			if (ev.type == SDL_QUIT||(ev.type==SDL_KEYDOWN&&ev.key.keysym.sym==SDLK_ESCAPE)) quit = TRUE;
		}

/////////////
		
		_VBLANK();
      
		SDL_SetRenderDrawColor(ren, 255, 0, 0, 255);
		SDL_RenderClear(ren);
		
		/*void* pix; int pitch;
		SDL_LockTexture(tex, NULL, &pix, &pitch);
		memset(pix, 0, pitch*H);
		SDL_UnlockTexture(tex);*/

		for (int i = 0; i < H; i++) {
			int scrx = imod(-_SCROLLX, W), scry = imod(-_SCROLLY, H);
			void* pix = screen->pixels;
			SDL_Rect dstrc = {0, i, W, 1};
			int y = (i+scry) % H;
			if (scrx == 0 && scry == 0) {
				SDL_UpdateTexture(tex, &dstrc, screen->pixels + y*screen->pitch, screen->pitch);
			} else {
				int w0 = W-scrx;
				dstrc.w = w0;
				SDL_UpdateTexture(tex, &dstrc, screen->pixels + y*screen->pitch + scrx*4, screen->pitch);
				dstrc.x = w0;
				int w1 = W-w0;
				dstrc.w = w1;
				SDL_UpdateTexture(tex, &dstrc, screen->pixels + y*screen->pitch, screen->pitch);
			}

			_HBLANK(i);
		}
		SDL_RenderCopy(ren, tex, NULL,NULL);

		SDL_RenderPresent(ren);
	}

	SDL_Quit();
}