Implement EFFECT_VWAVE_SCROLL_CROSSFADE

This is the same as EFFECT_VWAVE_CROSSFADE, except with a different
maximum amplitude and the screen scrolls vertically.

(Used in Rance 9 when transitioning to a flashback scene.)

include/effect.h

@@ -72,6 +72,7 @@ enum effect {
 	EFFECT_BLUR_FADEOUT           = 50,
 	EFFECT_BLUR_CROSSFADE         = 51,
 	EFFECT_2ROT_ZOOM_BLEND_BLUR   = 52,
+	EFFECT_VWAVE_SCROLL_CROSSFADE = 55,
 	EFFECT_VWAVE_CROSSFADE        = 60,
 	NR_EFFECTS
 };

shaders/effects/vwave_scroll_crossfade.f.glsl

@@ -0,0 +1,58 @@
+/* Copyright (C) 2019 Nunuhara Cabbage <nunuhara@haniwa.technology>
+ *
+ * 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.
+ *
+ * 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://gnu.org/licenses/>.
+ */
+
+uniform sampler2D tex;   // the new scene
+uniform sampler2D old;   // the old scene
+uniform vec2 resolution; // the screen resolution
+uniform float progress;  // effect progress (0..1)
+
+in vec2 tex_coord;
+out vec4 frag_color;
+
+#define M_PI 3.1415926535897932384626433832795
+#define OLD_PERIOD ((2.0 * M_PI) / 360.0)
+#define NEW_PERIOD ((2.0 * M_PI) / 720.0)
+#define OLD_SHIFT (-M_PI)
+#define NEW_SHIFT (M_PI / 2.0)
+
+vec3 get_pixel(vec2 old_xy, vec2 new_xy) {
+	// influence of old color is described by: C_old = (1-t)^2
+	// influence of new color increases linearly: C_new = t
+	// the resulting blend is not normalized
+	vec3 old = texture(old, old_xy).rgb;
+	vec3 new = texture(tex, new_xy).rgb;
+	float old_weight = (1.0 - progress) * (1.0 - progress);
+	float new_weight = progress;
+	return (old * old_weight + new * new_weight);
+}
+
+float triangle(float f) {
+        return 1.0 - abs(f * 2.0 - 1.0);
+}
+
+void main() {
+	// old: x = amp * sin((2pi/360)*y + phase + -pi)
+	// new: x = amp * sin((2pi/720)*y + phase + pi/2)
+        vec2 p = tex_coord * resolution;
+        float amp = triangle(progress) * 160.0; // (0..0.5..1) -> (0..160..0);
+	float phase = progress * M_PI * 8.0;
+	float old_off_x = amp * sin(OLD_PERIOD * p.y + OLD_SHIFT + phase);
+	float new_off_x = amp * sin(NEW_PERIOD * p.y + NEW_SHIFT + phase);
+	float off_y = resolution.y * progress;
+	vec2 old_xy = fract(vec2(p.x + old_off_x, p.y + off_y) / resolution);
+	vec2 new_xy = fract(vec2(p.x - new_off_x, p.y + off_y) / resolution);
+        frag_color = vec4(get_pixel(old_xy, new_xy), 1.0);
+}

src/effect.c

@@ -75,6 +75,7 @@ const char *effect_names[NR_EFFECTS] = {
 	[EFFECT_BLUR_FADEOUT]           = "EFFECT_BLUR_FADEOUT",
 	[EFFECT_BLUR_CROSSFADE]         = "EFFECT_BLUR_CROSSFADE",
 	[EFFECT_2ROT_ZOOM_BLEND_BLUR]   = "EFFECT_2ROT_ZOOM_BLEND_BLUR",
+	[EFFECT_VWAVE_SCROLL_CROSSFADE] = "EFFECT_VWAVE_SCROLL_CROSSFADE",
 	[EFFECT_VWAVE_CROSSFADE]        = "EFFECT_VWAVE_CROSSFADE",
 };
 
@@ -156,6 +157,7 @@ static struct effect_shader turn_page_shader = EFFECT_SHADER("shaders/effects/tu
 static struct effect_shader sepia_noise_crossfade_shader = EFFECT_SHADER("shaders/effects/sepia_noise_crossfade.f.glsl");
 static struct effect_shader blur_fadeout_shader = EFFECT_SHADER("shaders/effects/blur_fadeout.f.glsl");
 static struct effect_shader blur_crossfade_shader = EFFECT_SHADER("shaders/effects/blur_crossfade.f.glsl");
+static struct effect_shader vwave_scroll_crossfade_shader = EFFECT_SHADER("shaders/effects/vwave_scroll_crossfade.f.glsl");
 static struct effect_shader vwave_crossfade_shader = EFFECT_SHADER("shaders/effects/vwave_crossfade.f.glsl");
 
 static struct effect_shader *effect_shaders[NR_EFFECTS] = {
@@ -173,6 +175,7 @@ static struct effect_shader *effect_shaders[NR_EFFECTS] = {
 	[EFFECT_SEPIA_NOISE_CROSSFADE] = &sepia_noise_crossfade_shader,
 	[EFFECT_BLUR_FADEOUT] = &blur_fadeout_shader,
 	[EFFECT_BLUR_CROSSFADE] = &blur_crossfade_shader,
+	[EFFECT_VWAVE_SCROLL_CROSSFADE] = &vwave_scroll_crossfade_shader,
 	[EFFECT_VWAVE_CROSSFADE] = &vwave_crossfade_shader,
 };