libs/puddle: add a classic 2D raindrop sim lib

These were commonish in the 90s demo days, done as a library to encourage
use by different modules.

You can simply render this directly onto a frame buffer like the old days,
or sample it as a height map or density field for more complex compositions.

3 files changed, 169 insertions, 0 deletions

diff --git a/src/libs/puddle/Makefile.am b/src/libs/puddle/Makefile.am
new file mode 100644
index 0000000..faacb49
--- /dev/null
+++ b/src/libs/puddle/Makefile.am
@@ -0,0 +1,3 @@
+noinst_LIBRARIES = libpuddle.a
+libpuddle_a_SOURCES = puddle.c puddle.h
+libpuddle_a_CPPFLAGS = -I@top_srcdir@/src
diff --git a/src/libs/puddle/puddle.c b/src/libs/puddle/puddle.c
new file mode 100644
index 0000000..e5c0dd4
--- /dev/null
+++ b/src/libs/puddle/puddle.c
@@ -0,0 +1,136 @@
+/*
+ *  Copyright (C) 2020 - Vito Caputo - <vcaputo@pengaru.com>
+ *
+ *  This program is free software: you can redistribute it and/or modify it
+ *  under the terms of the GNU General Public License version 3 as published
+ *  by the Free Software Foundation.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <assert.h>
+#include <math.h>
+#include <stdlib.h>
+
+#include "puddle.h"
+
+typedef struct puddle_t {
+	int	w, h;
+	float	*a, *b;
+	float	floats[];
+} puddle_t;
+
+typedef struct v2f_t {
+	float	x, y;
+} v2f_t;
+
+
+puddle_t * puddle_new(int w, int h)
+{
+	puddle_t	*puddle;
+
+	puddle = calloc(1, sizeof(puddle_t) + sizeof(float) * w * (h + 2) * 2);
+	if (!puddle)
+		return NULL;
+
+	puddle->w = w;
+	puddle->h = h;
+
+	puddle->a = &puddle->floats[w];
+	puddle->b = &puddle->floats[w * 2 + w * h + w];
+
+	return puddle;
+}
+
+
+void puddle_free(puddle_t *puddle)
+{
+	free(puddle);
+}
+
+
+/* Run the puddle simulation for a tick, using the supplied viscosity value.
+ * A good viscosity value is ~.01, YMMV.
+ */
+void puddle_tick(puddle_t *puddle, float viscosity)
+{
+	float	*a, *b;
+
+	assert(puddle);
+
+	a = puddle->a;
+	b = puddle->b;
+
+	for (int y = 0, i = 0; y < puddle->h; y++) {
+		for (int x = 0; x < puddle->w; x++, i++) {
+			float	tmp =	a[i - puddle->w] +
+					a[i - 1] +
+					a[i + 1] +
+					a[i + puddle->w];
+
+			tmp -= b[i] * 2.f;
+			tmp *= .5f;
+			tmp -= tmp * viscosity;
+
+			b[i] = tmp;
+		}
+	}
+
+	puddle->b = a;
+	puddle->a = b;
+}
+
+
+/* Set a specific cell in the puddle to the supplied value */
+void puddle_set(puddle_t *puddle, int x, int y, float value)
+{
+	assert(puddle);
+	assert(x >= 0 && x < puddle->w);
+	assert(y >= 0 && y < puddle->h);
+
+	puddle->a[y * puddle->w + x] = value;
+}
+
+
+static inline float lerp(float a, float b, float t)
+{
+	return (1.0f - t) * a + t * b;
+}
+
+
+/* Sample the supplied puddle field at the specified coordinate.
+ *
+ * The puddle field is treated as a unit square mapped to the specified
+ * dimensions @ create time.  the sampled value is linearly interpolated from
+ * the data.
+ */
+float puddle_sample(const puddle_t *puddle, const v2f_t *coordinate)
+{
+	int	x0, y0, x1, y1;
+	float	x, y, tx, ty;
+
+	assert(puddle);
+	assert(coordinate);
+
+	x = .5f + (coordinate->x * .5f + .5f) * (puddle->w - 2);
+	y = .5f + (coordinate->y * .5f + .5f) * (puddle->h - 2);
+
+	x0 = floorf(x);
+	y0 = floorf(y);
+
+	x1 = x0 + 1;
+	y1 = y0 + 1;
+
+	tx = x - (float)x0;
+	ty = y - (float)y0;
+
+	return lerp(lerp(puddle->a[y0 * puddle->w + x0], puddle->a[y0 * puddle->w + x1], tx),
+		    lerp(puddle->a[y1 * puddle->w + x0], puddle->a[y1 * puddle->w + x1], tx),
+		    ty);
+}
diff --git a/src/libs/puddle/puddle.h b/src/libs/puddle/puddle.h
new file mode 100644
index 0000000..cc8988e
--- /dev/null
+++ b/src/libs/puddle/puddle.h
@@ -0,0 +1,30 @@
+/*
+ *  Copyright (C) 2020 - Vito Caputo - <vcaputo@pengaru.com>
+ *
+ *  This program is free software: you can redistribute it and/or modify it
+ *  under the terms of the GNU General Public License version 3 as published
+ *  by the Free Software Foundation.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef _PUDDLE_H
+#define _PUDDLE_H
+
+typedef struct puddle_t puddle_t;
+typedef struct v2f_t v2f_t;
+
+puddle_t * puddle_new(int w, int h);
+void puddle_free(puddle_t *puddle);
+void puddle_tick(puddle_t *puddle, float viscosity);
+void puddle_set(puddle_t *puddle, int x, int y, float v);
+float puddle_sample(const puddle_t *puddle, const v2f_t *coordinate);
+
+
+#endif