Ncurses text inside box flickering on refresh

Question

I am trying to draw a box around some text that will be updated. In the place holder the text is static, but flickering is still observed. My code is below

 /*
 * @file main.cc
 * @author Tom Eaton
 * @date 09 Jan 2018
 * @brief Command line interface to show Hawkcell PCB data.
 */
#include <iostream>
#include <string>
#include <cstring>
#include <ncurses.h>

using namespace std;
/*
 * Prints string in the horizontal center of row specified.
 * @param row - Row that text should be printed on
 * @param mesg - Pointer to char array containing message
 */
void printHorizCenter(int row, char* mesg) {
        mvprintw(row, ((COLS - strlen(mesg))/2)-1, mesg);
}

/*
 * Reads version of PCB and updates char array with version
 * @param outVersion - Pointer to char array where version should be stored.
 * @param port - Reference to serial port
 */
void printHorizBlock(int row, int startCol, int endCol) {
        for(int i = startCol; i<=endCol; i++) {
                mvaddch(row, i, ACS_BLOCK);
        }
}

int main(int argc, char** argv) {
        //Initialise curses
        initscr();
        cbreak();
        echo();
        keypad(stdscr, 1);

        //Make character capture non blocking
        nodelay(stdscr, 1);

        //Define char arrays for each cell
        char cellOne[2] = {'1', '\n'};
        char cellTwo[2] = {'2', '\n'};
        char cellThree[2] = {'3', '\n'};
        char cellFour[2] = {'4', '\n'};

        //Setup curses colour schemes
        start_color();
        use_default_colors();
        curs_set(0);

        //Setup curses colour pairs
        init_pair(1, COLOR_RED, -1);
        init_pair(2, COLOR_WHITE, COLOR_RED);

        WINDOW *win = newwin(10, 20, 1, 0);


        //Main loop
        while (1) {
                if(getch() == 12) {
                        endwin();
                        cerr << "You pressed left and exited" << endl;
                        exit(1);
                }
                //Print top title bar
                attron(COLOR_PAIR(1));
                printHorizBlock(0, 0, (COLS/2) - (strlen("HawkCell")/2));
                attroff(COLOR_PAIR(1));
                attron(COLOR_PAIR(2));
                printHorizCenter(0, (char *)"HawkCell");
                attroff(COLOR_PAIR(2));
                attron(COLOR_PAIR(1));
                printHorizBlock(0, (COLS/2) + (strlen("HawkCell")/2) - 1, COLS);
                attroff(COLOR_PAIR(1));


                //Print voltage data
                mvprintw(3, 1, "Cell 1: %.3fV\n", 4.0f);
                mvprintw(4, 1, "Cell 2: %.3fV\n", 4.0f);
                mvprintw(5, 1, "Cell 3: %.3fV\n", 4.0f);
                mvprintw(6, 1, "Cell 4: %.3fV\n", 4.0f);
                mvprintw(7, 1, "Total: %.3fV\n", 4.0f);

                //Print bottom info bar
                attron(COLOR_PAIR(1));
                printHorizBlock(LINES-1, 0, COLS);
                attroff(COLOR_PAIR(1));
                attron(COLOR_PAIR(2));
                printHorizCenter(LINES-1, (char *)"ctrl+c: Exit, ctrl+l: Toggle logging ");
                attroff(COLOR_PAIR(2));

                //Call curses refresh function to update screen
                box(win, 0, 0);
                //touchwin(win);
                wrefresh(win);
                wrefresh(stdscr);
        }

        getch();
        endwin();
        return 0;
}

The problem is that there is flickering when the screen is redrawn, shown in the gif below (Photo sensitivity warning)Gif showing problem. As you can see it only happens on the lines containing text, which makes me think that the whole line is being drawn when writing the text, not just the specific characters.

I have tried using a various combination of wrefresh(), touchwin(), erase() and refresh() but I couldn't find any that did not produce flickering.

How do I stop this flickering?

score 0 · Accepted Answer · answered Jan 09 '18 at 23:27

The problem is that your updates to stdscr and win alternate in updating the current screen (curscr), and since they write in the same area, they are doing a lot of repainting.

You could improve this by making a subwindow of win, and using that window for writing the text.

Something like

WINDOW *txt = derwin(win, 8, 18, 1, 1);

... modify all of the calls using stdscr:

mvwprintw(txt, 7, 1, "Total: %.3fV\n", 4.0f);

//Print bottom info bar
wattron(txt, COLOR_PAIR(1));

and using wgetch(txt) rather than getch() to get characters. The box only needs to be refreshed once (before the loop), and the wgetch() refreshes the window anyway: remove the last wrefresh and move the other right after creating win and calling box:

WINDOW *win = newwin(10, 20, 1, 0);
box(win, 0, 0);
wrefresh(win);

Ncurses text inside box flickering on refresh

1 Answers1