我如何在 Xlib 中将 XCopyArea 与 colors 一起使用而不是蓝色？

Question

I'm trying to build a ray tracer in C++. To that end, I'd like to draw the final image to a window (instead of just outputting to PPM, which is what I'm currently doing). I have some code below that saves the color of each pixel to an array that matches the size of an image and then attempts to display that image using Xlib. This code snippet below allocates data to a 100 x 100 array image:

int width = 100;
int height = 100;

int data[width * height * 4];

for (int i = 0; i < width * height; i++){

  data[i] = 255;

}
XImage *image  = XCreateImage(d, DefaultVisual(d, 0), 24, ZPixmap, 0, (char*) data, 100, 100, 32, 0); 

Pixmap pm = XCreatePixmap(d, w, 100, 100, 24);

XPutImage(d, pm, gc, image, 0,0,0,0, 100, 100);

This outputs a nice blue square in my window. But, if I change the for loop in the above example to:

for (int i = 0; i < width * height; i+= 4){

  data[i] = 255;
  data[i + 1] = 255;
  data[i + 2] = 0;  
  data[i + 3] = 0;

}

I still get blue with some black mixed in, but I expected yellow. What am I doing wrong here?

Full code below for reference:

#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {
  Display *d;
  Window w;
  XEvent e;
  const char *msg = "Hello, World!";
  int s;

  d = XOpenDisplay(NULL);
  if (d == NULL) {
    fprintf(stderr, "Cannot open display\n");
    exit(1);
  }

  s = DefaultScreen(d);
  w = XCreateSimpleWindow(d, RootWindow(d, s), 10, 10, 500, 500, 1,
                          BlackPixel(d, s), WhitePixel(d, s));
  
  GC gc = XCreateGC(d, w, 0, NULL);

  XSelectInput(d, w, ExposureMask | KeyPressMask);
  XMapWindow(d, w);

  int width = 100;
  int height = 100;

  int data[width * height * 4];

  for (int i = 0; i < width * height; i++){

    data[i] = 255;

  }

  XImage *image  = XCreateImage(d, DefaultVisual(d, 0), 24, ZPixmap, 0, (char*) data, 100, 100, 32, 0); 

  Pixmap pm = XCreatePixmap(d, w, 100, 100, 24);

  XPutImage(d, pm, gc, image, 0,0,0,0, 100, 100);

  while (1) {
    XNextEvent(d, &e);
    if (e.type == Expose) {
        XCopyArea(d, pm, w, gc, 0, 0, width, height, 10, 10);
    }
    if (e.type == KeyPress)
        break;
  }

  XCloseDisplay(d);
  return 0;
}

Answer 1

Finally figured it out. If I remember where I found the original sources for some of this stuff, I'll link it in an edit. But basically below is a working for loop. I'm not entirely sure why I need it, but after every RGB triplet, we need a byte of 0x00 . This is on my computer where I have 24-bit true color depth (which is probably the same as every modern computer). So if you have 24 bits, then you have 3 bytes on every line which, for many reasons that I don't know, is illegal. If someone could explain this to me, I'd really love to know. The code is below:

unsigned char data[width * height * 4];

  for (int i = 0; i < width * height * 4; i+=4){

    data[i] = 255; // blue channel
    data[i + 1] = 0; // green channel
    data[i + 2] = 255; // red channel
    data[i + 3] = 0; // dead pixel that is always zero

  }