test sdl relative mouse input with mouse warping

sdl-warptest.c

// gcc -Wall -o warptest `sdl2-config --cflags` sdl-warptest.c `sdl2-config --libs`

#include <stdio.h>
#include <SDL.h>

// set to 1 to use SDL_GetMouseState() and SDL_WarpMouseInWindow() for custom relative mode
// 0 uses SDL2s relative mode, while enforcing SDL2's internal warping with SDL_HINT_MOUSE_RELATIVE_MODE_WARP
#define DO_WARP_YOURSELF 0

static FILE* outfile = NULL;

static void UpdateWinTitle(int accumX, int accumY, SDL_Window* win)
{
	char buf[128];
	snprintf(buf, 128, "Accumulated: X: %d Y: %d\n", accumX, accumY);
	SDL_SetWindowTitle(win, buf);
}

static void my_SDL_LogOutputFunction(void* userdata,
                           int             category,
                           SDL_LogPriority priority,
                           const char*     message)
{
	fprintf(outfile, "#SDL: %s\n", message);
	fflush(outfile);
}

int main(int argc, char** argv)
{
	outfile = stdout; // if you can't use stdout on windows, you could fopen() a text file here for logging

	SDL_version ver;
	SDL_GetVersion(&ver);
	fprintf(outfile, "SDL Version: %d.%d.%d\n", (int)ver.major, (int)ver.minor, (int)ver.patch);
	
	SDL_LogSetOutputFunction(my_SDL_LogOutputFunction, NULL);
	
	SDL_Init(SDL_INIT_VIDEO);
	SDL_Window* win = SDL_CreateWindow("Move your mouse!", 100, 100, 640, 480, SDL_WINDOW_SHOWN | SDL_WINDOW_RESIZABLE);
	//SDL_SetWindowGrab(win, 1);
	SDL_Surface* winSurf = SDL_GetWindowSurface(win);
	Uint32 grey = SDL_MapRGB(winSurf->format, 127, 127, 127);
	
	// accum* holds the sum of all relative movements and is shown in window title
	int accumX = 0, accumY = 0;
	
#if DO_WARP_YOURSELF
	SDL_WarpMouseInWindow(win, 320, 240);
#else
	SDL_SetHint(SDL_HINT_MOUSE_RELATIVE_MODE_WARP, "1");
	SDL_SetRelativeMouseMode(1);
#endif	

	int keepRunning = 1;
	while(keepRunning) {
		winSurf = SDL_GetWindowSurface(win);
		SDL_FillRect(winSurf, NULL, grey);
		SDL_UpdateWindowSurface(win);
		SDL_Delay(10);
		
		SDL_Event ev;
		while(SDL_PollEvent(&ev)) {
			switch(ev.type) {
				case SDL_KEYDOWN:
					if(ev.key.keysym.sym == SDLK_q || ev.key.keysym.sym == SDLK_ESCAPE) {
						fprintf(outfile, "You pressed Q  or Esc - bye!\n");
						keepRunning = 0;
					}
					break;
				case SDL_MOUSEMOTION:
					if(ev.motion.xrel > 0)
						fprintf(outfile, "> Mouse motion: rel: %d %d pos: %d %d\n", (int)ev.motion.xrel, (int)ev.motion.yrel, (int)ev.motion.x, (int)ev.motion.y);
					else
						fprintf(outfile, "<<< Mouse motion: rel: %d %d pos: %d %d\n", (int)ev.motion.xrel, (int)ev.motion.yrel, (int)ev.motion.x, (int)ev.motion.y);
					fflush(outfile);
#if ! DO_WARP_YOURSELF
					accumX += ev.motion.xrel;
					accumY += ev.motion.yrel;
					UpdateWinTitle(accumX, accumY, win);
#endif
					break;
				
			}
		}
#if DO_WARP_YOURSELF
		int x,y, w,h;
		SDL_GetMouseState(&x, &y);
		SDL_GetWindowSize(win, &w, &h);
		x -= w/2;
		y -= h/2;
		SDL_WarpMouseInWindow(win, w/2, h/2);
		fprintf(outfile, "rel Mouse Position: %d %d\n", x, y);
		if(x != 0 || y != 0) {
			accumX += x;
			accumY += y;
			UpdateWinTitle(accumX, accumY, win);
		}
#endif
		fprintf(outfile, "### end of frame ###\n");
		fflush(outfile);
	}

	SDL_DestroyWindow(win);
	SDL_Quit();
	return 0;
}

SDL_mouse.c

// in src/events/SDL_mouse.c added some SDL_Log() calls to SDL_PrivateSendMouseMotion():

#include "SDL_log.h"
static int
SDL_PrivateSendMouseMotion(SDL_Window * window, SDL_MouseID mouseID, int relative, int x, int y)
{
    SDL_Mouse *mouse = SDL_GetMouse();
    int posted;
    int xrel;
    int yrel;

    if (mouseID == SDL_TOUCH_MOUSEID && !mouse->touch_mouse_events) {
        return 0;
    }

    if (mouseID != SDL_TOUCH_MOUSEID && mouse->relative_mode_warp) {
        int center_x = 0, center_y = 0;
        SDL_GetWindowSize(window, &center_x, &center_y);
        center_x /= 2;
        center_y /= 2;
        if (x == center_x && y == center_y) {
            SDL_Log("## Resetting mouse->last* to %d %d\n", center_x, center_y);
            mouse->last_x = center_x; // possible fix for the issue: move this ..
            mouse->last_y = center_y; // and this line before the if(x == center_x && y == center_y)
            return 0;
        }
        SDL_Log("## Warping mouse to %d %d\n", center_x, center_y);
        SDL_WarpMouseInWindow(window, center_x, center_y);
    }

    if (relative) {
        if (mouse->relative_mode) {
            x = GetScaledMouseDelta(mouse->relative_speed_scale, x, &mouse->scale_accum_x);
            y = GetScaledMouseDelta(mouse->relative_speed_scale, y, &mouse->scale_accum_y);
        } else {
            x = GetScaledMouseDelta(mouse->normal_speed_scale, x, &mouse->scale_accum_x);
            y = GetScaledMouseDelta(mouse->normal_speed_scale, y, &mouse->scale_accum_y);
        }
        xrel = x;
        yrel = y;
        x = (mouse->last_x + xrel);
        y = (mouse->last_y + yrel);
    } else {
        xrel = x - mouse->last_x;
        yrel = y - mouse->last_y;
    }

    SDL_Log("# x,y: %d/%d rel: %d/%d, last: %d/%d\n", x, y, xrel, yrel, mouse->last_x, mouse->last_y);
    SDL_Log("# relative: %d mouse->relative_mode: %d\n", relative, mouse->relative_mode);

    /* Drop events that don't change state */
    if (!xrel && !yrel) {
#ifdef DEBUG_MOUSE
        printf("Mouse event didn't change state - dropped!\n");
#endif
        return 0;
    }

    /* Ignore relative motion when first positioning the mouse */
    if (!mouse->has_position) {
        xrel = 0;
        yrel = 0;
        mouse->has_position = SDL_TRUE;
    }

    /* Ignore relative motion positioning the first touch */
    if (mouseID == SDL_TOUCH_MOUSEID && !mouse->buttonstate) {
        xrel = 0;
        yrel = 0;
    }

    /* Update internal mouse coordinates */
    if (!mouse->relative_mode) {
        mouse->x = x;
        mouse->y = y;
    } else {
        mouse->x += xrel;
        mouse->y += yrel;
    }

    /* make sure that the pointers find themselves inside the windows,
       unless we have the mouse captured. */
    if (window && ((window->flags & SDL_WINDOW_MOUSE_CAPTURE) == 0)) {
        int x_max = 0, y_max = 0;

        /* !!! FIXME: shouldn't this be (window) instead of (mouse->focus)? */
        SDL_GetWindowSize(mouse->focus, &x_max, &y_max);
        --x_max;
        --y_max;

        if (mouse->x > x_max) {
            mouse->x = x_max;
        }
        if (mouse->x < 0) {
            mouse->x = 0;
        }

        if (mouse->y > y_max) {
            mouse->y = y_max;
        }
        if (mouse->y < 0) {
            mouse->y = 0;
        }
    }

    mouse->xdelta += xrel;
    mouse->ydelta += yrel;

    /* Move the mouse cursor, if needed */
    if (mouse->cursor_shown && !mouse->relative_mode &&
        mouse->MoveCursor && mouse->cur_cursor) {
        mouse->MoveCursor(mouse->cur_cursor);
    }

    /* Post the event, if desired */
    posted = 0;
    if (SDL_GetEventState(SDL_MOUSEMOTION) == SDL_ENABLE) {
        SDL_Event event;
        event.motion.type = SDL_MOUSEMOTION;
        event.motion.windowID = mouse->focus ? mouse->focus->id : 0;
        event.motion.which = mouseID;
        event.motion.state = mouse->buttonstate;
        event.motion.x = mouse->x;
        event.motion.y = mouse->y;
        event.motion.xrel = xrel;
        event.motion.yrel = yrel;
        posted = (SDL_PushEvent(&event) > 0);
    }
    if (relative) {
        mouse->last_x = mouse->x;
        mouse->last_y = mouse->y;
    } else {
        /* Use unclamped values if we're getting events outside the window */
        mouse->last_x = x;
        mouse->last_y = y;
    }
    return posted;
}

// ...