How do I reduce input lag in an NCurses C Application

Question

I am getting major amounts of input lag when I run my application.

More details: When I press 'w', 'a', 's', 'd' (My assigned input keys) the object moves however it continues to move for an extended period of time after the key has been released. The source code is below however small parts of the code have been cut out to shorten the questions however if the source code below does not compile I have all of the code up on github. https://github.com/TreeStain/DodgeLinuxGame.git Thankyou for your time. -Tristan

dodge.c:

#define ASPECT_RATIO_X 2
#define ASPECT_RATIO_Y 1
#define FRAMES_PER_SECOND 60

#include <ncurses.h>
#include "object.h"
#include "render.h"

int main()
{
    initscr();
    cbreak();
    noecho();
    nodelay(stdscr, 1);

    object objs[1];

    object colObj; colObj.x = 10; colObj.y = 6;
                   colObj.w = 2;  colObj.h = 2;
                   colObj.sprite = '*';
                   colObj.ySpeed = 1;
                   colObj.xSpeed = 1;

    objs[0] = colObj;

    //halfdelay(1);

    while (1)
    {
        char in = getch();
        if (in == 'w')
            objs[0].y -= objs[0].ySpeed * ASPECT_RATIO_Y;
        if (in == 's')
            objs[0].y += objs[0].ySpeed * ASPECT_RATIO_Y;
        if (in == 'a')
            objs[0].x -= objs[0].xSpeed * ASPECT_RATIO_X;
        if (in == 'd')
            objs[0].x += objs[0].xSpeed * ASPECT_RATIO_X;
        render(objs, 1);
        napms(FRAMES_PER_SECOND);
    }

    getch();

    endwin();
    return 0;
 }

render.h:

void render(object obj[], int objectNum);

void render(object obj[], int objectNum)            //Takes array of objects and prints them to screen
 {
    int x, y, i, scrWidth, scrHeight;
    getmaxyx(stdscr, scrHeight, scrWidth);          //Get terminal height and width

    for (y = 0; y < scrHeight; y++)
    {
        for (x = 0; x < scrWidth; x++)
        {
            mvprintw(y, x, " ");
        }
    }

    for (i = 0; i < objectNum; i++)
    {
        int xprint = 0, yprint = 0;
        for (yprint = obj[i].y; yprint < obj[i].y + (obj[i].h * ASPECT_RATIO_Y); yprint++)
        {
            for (xprint = obj[i].x; xprint < obj[i].x + (obj[i].w * ASPECT_RATIO_X); xprint++)
                mvprintw(yprint, xprint, "%c", obj[i].sprite);
        }
    }
    refresh();
}

object.h:

typedef struct
{
    int x, y, w, h, ySpeed, xSpeed;
    char sprite;
}object;

P.S. please feel free to critique my methods and code as I am fairly new at programming and can take all the criticism I can get.

You might want to profile your code. Look here for tools that you could use: http://softwarerecs.stackexchange.com/questions/9992/linux-c-profiler — Plouff, Mar 25 '16 at 07:52
It's probably not the solution to your problem. But I would replace the multiple call to `mvprintw(y, x, " ");` by one call with the appropriate number of " " if possible. https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html might help — Plouff, Mar 25 '16 at 07:57

jrsmolley · Accepted Answer · 2016-03-25T08:17:32.180

I believe the reason is because getch() will only release one input-character at a time (even if there are many queued up in the input stream) so if they queue up faster than you 'remove' them from the stream, the loop will continue until the queue is emptied even after you release the key. Also, you'll want to go (1000 / FRAMES_PER_SECOND) to get your desired delay-time in milliseconds (this creates 60 frames per second).

Try this in your while loop instead.

while (1)
    {
        char in;
        /* We are ready for a new frame. Keep calling getch() until we hear a keypress */
        while( (in = getch()) == ERR) {}

        if (in == 'w')
            objs[0].y -= objs[0].ySpeed * ASPECT_RATIO_Y;
        if (in == 's')
            objs[0].y += objs[0].ySpeed * ASPECT_RATIO_Y;
        if (in == 'a')
            objs[0].x -= objs[0].xSpeed * ASPECT_RATIO_X;
        if (in == 'd')
            objs[0].x += objs[0].xSpeed * ASPECT_RATIO_X;
        render(objs, 1);

        /* Clear out any other characters that have been buffered */
        while(getch() != ERR) {}

        napms(1000 / FRAMES_PER_SECOND);
    }

From the top of your loop: while( (in = getch()) == ERR) {} will call getch() rapidly until a keypress is detected. If a keypress isn't detected, getch() will return ERR. What while(getch() != ERR) {} does is keep calling getch() until all buffered input characters are removed from the queue, then getch() returns ERR and moves on. Then the loop should sleep ~17ms and repeat. These lines should force the loop to only 'count' one keypress every ~17ms, and no more often than that.

See: http://linux.die.net/man/3/getch

Thanks this solved my problem and gave me a better understanding of how C and NCurses work. — Tristan Arthur, Mar 25 '16 at 10:03

n. m. could be an AI · Answer 2 · 2016-03-25T08:44:18.310

Ncurses does not detect key presses and key releases separately. You cannot move an object while a key is being held, and stop immediately after it is released.

The phenomenon you observe results from a ximbination of two factors: an auto-repeating keyboard, and a buffering keyboard driver. That is, the user holds a key, this generates a large amount of key events, and they are buffered by the driver and given to your application as it asks for key presses.

Neither the driver nor keyboard auto-repeat feature are under control of your application. The only thing you can hope to achieve is to process key events faster than they come out of the keyboard. If you want to do this, you have to get rid of napms in your main loop and process key presses as they come, between frame repaints. There are many ways to do that but the most straightforward is to use the timeout function.

 timeout (timeToRefresh);
 ch = getch();
 if (ch == ERR) refresh();
 else processKey(ch);

You need to calculate timeToRefresh each time using a real time clock.

Thank you this helped a lot in my understanding. – Tristan Arthur Mar 25 '16 at 10:01 — Tristan Arthur, Mar 25 '16 at 10:01

How do I reduce input lag in an NCurses C Application

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