debug: MUX_DUMP writes one frame from the pixmap, then exits

Verifying what the program actually draws by screenshotting its window is
unreliable: X does not retain a window's contents while it is obscured or
on an unfocused tag, so import and xwd can both return solid black while
the rendering is correct. Reading back the off-screen pixmap always
works.

Used to confirm the XRectangle clip-truncation fix: rows now run
unbroken from 2027-01-01 to 2027-01-29 with every column populated,
where before the twelfth row lost everything but its name.

1 files changed, 29 insertions, 0 deletions

diff --git a/mu_x11.c b/mu_x11.c
index 13b23c2..e672496 100644
--- a/mu_x11.c
+++ b/mu_x11.c
@@ -11,6 +11,7 @@
 #include <X11/Xlib.h>
 #include <X11/Xft/Xft.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include "vendor/microui.h"
 #include "mu_x11.h"
@@ -145,4 +146,32 @@ void mux_present(mu_Context *ctx, int w, int h, mu_Color bg)
 	XftDrawSetClip(xd, NULL);
 	XCopyArea(dpy, buf, win, gc, 0, 0, w, h, 0, 0);
 	XFlush(dpy);
+
+	/* MUX_DUMP=path writes the frame out as a PPM and exits. Reading back
+	   the off-screen PIXMAP rather than the window is what makes this
+	   reliable: a window's contents are not retained when it is obscured or
+	   on an unfocused tag, so screenshotting one can return black even
+	   though the program is drawing perfectly. A pixmap is always there. */
+	{
+		const char *path = getenv("MUX_DUMP");
+		if (!path) return;
+		XImage *im = XGetImage(dpy, buf, 0, 0, w, h, AllPlanes, ZPixmap);
+		if (!im) { fprintf(stderr, "MUX_DUMP: XGetImage failed\n"); exit(1); }
+		FILE *f = fopen(path, "wb");
+		if (!f) { fprintf(stderr, "MUX_DUMP: cannot write %s\n", path); exit(1); }
+		fprintf(f, "P6\n%d %d\n255\n", w, h);
+		for (int yy = 0; yy < h; yy++)
+			for (int xx = 0; xx < w; xx++) {
+				unsigned long px = XGetPixel(im, xx, yy);
+				unsigned char rgb[3] = {
+					(unsigned char)((px >> 16) & 0xff),
+					(unsigned char)((px >> 8) & 0xff),
+					(unsigned char)(px & 0xff) };
+				fwrite(rgb, 1, 3, f);
+			}
+		fclose(f);
+		XDestroyImage(im);
+		fprintf(stderr, "MUX_DUMP: wrote %s (%dx%d)\n", path, w, h);
+		exit(0);
+	}
 }