ubb-vga2: added support for showing a PPM image

- Makefile (OBJS): added ppm.o and ppmimg.o
- ppm.h (load_ppm), ppm.c: PPM file loader, adapted from
  eda-tools/schhist/ppmdiff/ppmdiff.c
- ubb-vga.h (img_name, ppmimg), ppmimg.c: PPM image to frame buffer
  converter
- ubb-vga2.c (usage, main): the threshold is now set with the option -l
- ubb-vga2.c (usage, main): if a second argument is given, treat it as
  a PPM file
- ubb-vga2.c (usage): also documented option -t

ubb-vga/ppmimg.c

@@ -0,0 +1,63 @@
+/*
+ * ppmimg.c - Convert a PPM image
+ *
+ * Written 2011 by Werner Almesberger
+ * Copyright 2011 Werner Almesberger
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "ppm.h"
+#include "ubb-vga.h"
+
+
+char *img_name;
+
+
+static uint8_t pattern(int r, int g, int b)
+{
+	return ((r ? R_VAL : 0) | (g ? G_VAL : 0) | (b ? B_VAL : 0))*0x11;
+}
+
+
+static void convert(uint8_t *f, int xres, int yres, uint8_t *ppm)
+{
+	int x, y;
+	uint8_t v, last = 0;
+
+	for (y = 0; y != yres; y++)
+		for (x = 0; x != xres; x++) {
+			v = pattern(ppm[0] >= thres, ppm[1] >= thres,
+			    ppm[2] >= thres);
+			if (x & 1) {
+				*f++ = last | v;
+			} else {
+				last = v << 4;
+			}
+			ppm += 3;
+		}
+}
+
+
+void ppmimg(void *f, int xres, int yres)
+{
+	uint8_t *ppm;
+	int xr = 0, yr = 0;
+
+	ppm = load_ppm(img_name, &xr, &yr);
+	if (xr != xres || yr != yres) {
+		fprintf(stderr, "image is %dx%d, display is %dx%d\n",
+		    xr, yr, xres, yres);
+		exit(1);
+	}
+	convert(f, xres, yres, ppm);
+	free(ppm);
+}