XDrawRectangle绘制矩形缺失右下角像素，求排查解决方法

在Linux系统基于X11/Xlib.h实现XDrawRectangle的包装函数后，绘制矩形时右下角像素未显示。

X11版本信息 #

$ dpkg -l | grep libx11-dev
ii  libx11-dev:amd64 2:1.7.5-1ubuntu0.3 amd64 X11 client-side library (development headers)

XDrawRectangle函数定义 #

当前安装的X11/Xlib.h中该函数定义如下：

extern int XDrawRectangle(
Display*        /* display */,
Drawable        /* d */,
GC          /* gc */,
int         /* x */,
int         /* y */,
unsigned int    /* width */,
unsigned int    /* height */
);

我的代码实现 #

头文件声明 #

/* Draw a rectangle at (x,y) with width (w) and height (h) */
void gfx_rect( int x, int y, unsigned int w, unsigned int h );

函数实现 #

/* Draw a rectangle at (x,y) with width (w) and height (h) */
void gfx_rect( int x, int y, unsigned int w, unsigned int h ) {
    XDrawRectangle(gfx_display,gfx_window,gfx_gc,x,y,w,h);
}

调用代码 #

const int offset = 10;
int x = 0;
int y = 0;

gfx_rect(x+offset, y+offset, 20, 20);

绘制效果 #

矩形右下角像素缺失，呈现不完整的边框。

可复现代码及编译命令 #

编译命令 #

gcc main.c -o fontview -lX11 -lm

完整可复现代码 #

#include <X11/Xlib.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

/* Open a new graphics window. */
void gfx_open( int width, int height, const char *title );
/* Wait for the user to press a key or mouse button. */
char gfx_wait();

static Display *gfx_display=0;
static Window  gfx_window;
static GC      gfx_gc;

int main() {
    char c;

    // Open a new window for drawing.
    gfx_open(532,276,"Font Viewer");

    XDrawRectangle(gfx_display,gfx_window,gfx_gc,10,10,20,20);
    
    while(1) {
        // Wait for the user to press a character.
        c = gfx_wait();

        // Quit if it is the letter q.
        if(c=='q') break;
    }

    return 0;
}

/* Open a new graphics window. */
void gfx_open( int width, int height, const char *title ) {
    gfx_display = XOpenDisplay(0);
    if(!gfx_display) {
        fprintf(stderr,"gfx_open: unable to open the graphics window.\n");
        exit(1);
    }

    int blackColor = BlackPixel(gfx_display, DefaultScreen(gfx_display));
    int whiteColor = WhitePixel(gfx_display, DefaultScreen(gfx_display));

    gfx_window = XCreateSimpleWindow(gfx_display, DefaultRootWindow(gfx_display), 0, 0, width, height, 0, blackColor, blackColor);

    XSelectInput(gfx_display, gfx_window, StructureNotifyMask|KeyPressMask|ButtonPressMask);

    XMapWindow(gfx_display,gfx_window);

    gfx_gc = XCreateGC(gfx_display, gfx_window, 0, 0);

    XSetForeground(gfx_display, gfx_gc, whiteColor);

    // Wait for the MapNotify event
    for(;;) {
        XEvent e;
        XNextEvent(gfx_display, &e);
        if (e.type == MapNotify)
            break;
    }
}

/* Wait for the user to press a key or mouse button. */
char gfx_wait() {
    XEvent event;

    while(1) {
        XNextEvent(gfx_display,&event);

        if(event.type==KeyPress) {
            return XLookupKeysym(&event.xkey,0);
        } else if(event.type==ButtonPress) {
            return event.xbutton.button;
        }
    }
}

补充吐槽 #

• 有人居然只用10行代码就实现了？！

内容的提问来源于stack exchange，提问作者Mike