SealEngine: Implement hemisphere lighting

This implements the sky/middle/ground hemisphere ambient driven by
light_params[8..19].

SealEngine has no separate specular-light instance; its specular instead
shares the hemisphere light vector.

shaders/reign.f.glsl

@@ -50,7 +50,7 @@ in vec3 light_dir[NR_DIR_LIGHTS];
 
 uniform vec3 global_ambient;
 
-vec3 dir_lights_diffuse(vec3 norm)
+vec3 compute_diffuse(vec3 norm)
 {
 	vec3 diffuse = vec3(0.0);
 	for (int i = 0; i < NR_DIR_LIGHTS; i++) {
@@ -60,11 +60,29 @@ vec3 dir_lights_diffuse(vec3 norm)
 	return diffuse;
 }
 
-#else // ENGINE == REIGN_ENGINE
+#elif ENGINE == SEAL_ENGINE
 
 const vec3 global_ambient = vec3(0.0);
 
-vec3 dir_lights_diffuse(vec3 norm)
+// Hemisphere lighting (sky/middle/ground).
+uniform vec3 hemi_light_dir;
+uniform vec3 hemi_sky_color;
+uniform vec3 hemi_mid_color;
+uniform vec3 hemi_ground_color;
+
+vec3 compute_diffuse(vec3 norm)
+{
+	float t = 0.5 * dot(norm, hemi_light_dir) + 0.5;
+	vec3 low_mid = mix(hemi_ground_color, hemi_mid_color, t);
+	vec3 mid_high = mix(hemi_mid_color, hemi_sky_color, t);
+	return mix(low_mid, mid_high, t);
+}
+
+#else // TAPIR_ENGINE
+
+const vec3 global_ambient = vec3(0.0);
+
+vec3 compute_diffuse(vec3 norm)
 {
 	return vec3(1.0);
 }
@@ -138,7 +156,7 @@ void main() {
 			texel = mix(texel, texture(blend_tex, blend_tex_coord), blend_weight);
 		}
 		if (diffuse_type != DIFFUSE_EMISSIVE) {
-			diffuse = dir_lights_diffuse(norm);
+			diffuse = compute_diffuse(norm);
 			if (diffuse_type == DIFFUSE_LIGHT_MAP) {
 				diffuse *= texture(light_texture, light_tex_coord).rgb;
 			}

src/3d/3d_internal.h

@@ -188,6 +188,10 @@ struct RE_renderer {
 	GLint ls_light_dir;
 	GLint ls_light_color;
 	GLint ls_sun_color;
+	GLint hemi_light_dir;
+	GLint hemi_sky_color;
+	GLint hemi_mid_color;
+	GLint hemi_ground_color;
 	GLint alpha_mode;
 	GLint alpha_texture;
 	GLint uv_scroll;

src/3d/renderer.c

@@ -259,6 +259,10 @@ struct RE_renderer *RE_renderer_new(void)
 	r->ls_light_dir = glGetUniformLocation(r->program, "ls_light_dir");
 	r->ls_light_color = glGetUniformLocation(r->program, "ls_light_color");
 	r->ls_sun_color = glGetUniformLocation(r->program, "ls_sun_color");
+	r->hemi_light_dir = glGetUniformLocation(r->program, "hemi_light_dir");
+	r->hemi_sky_color = glGetUniformLocation(r->program, "hemi_sky_color");
+	r->hemi_mid_color = glGetUniformLocation(r->program, "hemi_mid_color");
+	r->hemi_ground_color = glGetUniformLocation(r->program, "hemi_ground_color");
 	r->alpha_mode = glGetUniformLocation(r->program, "alpha_mode");
 	r->alpha_texture = glGetUniformLocation(r->program, "alpha_texture");
 	r->uv_scroll = glGetUniformLocation(r->program, "uv_scroll");
@@ -1133,6 +1137,26 @@ static void setup_light_scattering(struct RE_plugin *plugin)
 	}
 }
 
+static void setup_hemisphere_light(struct RE_plugin *plugin)
+{
+	struct RE_renderer *r = plugin->renderer;
+	const float *lp = plugin->light_params;
+	vec3 hemi_dir = {
+		-lp[SEAL_LP_HEMI_VEC], -lp[SEAL_LP_HEMI_VEC + 1], lp[SEAL_LP_HEMI_VEC + 2]
+	};
+	glm_vec3_normalize(hemi_dir);
+	glUniform3fv(r->hemi_light_dir, 1, hemi_dir);
+	glUniform3fv(r->hemi_sky_color, 1, &lp[SEAL_LP_HEMI_SKY_COLOR]);
+	glUniform3fv(r->hemi_mid_color, 1, &lp[SEAL_LP_HEMI_MID_COLOR]);
+	glUniform3fv(r->hemi_ground_color, 1, &lp[SEAL_LP_HEMI_GROUND_COLOR]);
+	// SealEngine's specular uses the hemisphere light vector. hemi_dir points
+	// toward the light, but the shader's specular formula expects the light
+	// propagation direction.
+	vec3 spec_dir;
+	glm_vec3_negate_to(hemi_dir, spec_dir);
+	glUniform3fv(r->specular_light_dir, 1, spec_dir);
+}
+
 static void setup_lights(struct RE_plugin *plugin)
 {
 	struct RE_renderer *r = plugin->renderer;
@@ -1168,6 +1192,9 @@ static void setup_lights(struct RE_plugin *plugin)
 		glUniform3fv(r->dir_lights[light_index].diffuse, 1, zero);
 		glUniform3fv(r->dir_lights[light_index].globe_diffuse, 1, zero);
 	}
+
+	if (re_plugin_version >= RE_SEAL_PLUGIN)
+		setup_hemisphere_light(plugin);
 }
 
 static int cmp_instances_by_z(const void *lhs, const void *rhs)