diff --git a/include/reign.h b/include/reign.h
index 801090c..90c30ad 100644
--- a/include/reign.h
+++ b/include/reign.h
@@ -105,6 +105,7 @@ struct RE_instance {
 	struct model *model;
 	struct motion *motion;
 	struct motion *next_motion;
+	struct height_detector *height_detector;
 
 	enum RE_instance_type type;
 	vec3 pos;
@@ -150,6 +151,7 @@ bool RE_instance_free_next_motion(struct RE_instance *instance);
 bool RE_instance_set_vertex_pos(struct RE_instance *instance, int index, float x, float y, float z);
 int RE_instance_get_bone_index(struct RE_instance *instance, const char *name);
 bool RE_instance_trans_local_pos_to_world_pos_by_bone(struct RE_instance *instance, int bone, vec3 offset, vec3 out);
+float RE_instance_calc_height(struct RE_instance *instance, float x, float z);
 
 int RE_motion_get_state(struct motion *motion);
 bool RE_motion_set_state(struct motion *motion, int state);
diff --git a/shaders/reign_shadow.f.glsl b/shaders/reign_shadow.f.glsl
index 5a1cfa4..4290140 100644
--- a/shaders/reign_shadow.f.glsl
+++ b/shaders/reign_shadow.f.glsl
@@ -14,5 +14,8 @@
  * along with this program; if not, see <http://gnu.org/licenses/>.
  */
 
+out uvec4 frag_color;
+
 void main() {
+	frag_color.r = uint(gl_FragCoord.z * 65535.0);
 }
diff --git a/src/3d/3d_internal.h b/src/3d/3d_internal.h
index 4172887..384afca 100644
--- a/src/3d/3d_internal.h
+++ b/src/3d/3d_internal.h
@@ -163,6 +163,9 @@ void motion_free(struct motion *motion);
 struct RE_renderer *RE_renderer_new(struct texture *texture);
 void RE_renderer_free(struct RE_renderer *r);
 bool RE_renderer_load_billboard_texture(struct RE_renderer *r, int cg_no);
+struct height_detector *RE_renderer_create_height_detector(struct RE_renderer *r, struct model *model);
+void RE_renderer_free_height_detector(struct height_detector *hd);
+float RE_renderer_detect_height(struct height_detector *hd, float x, float z);
 
 // parser.c
 
diff --git a/src/3d/reign.c b/src/3d/reign.c
index 92fe549..db9d236 100644
--- a/src/3d/reign.c
+++ b/src/3d/reign.c
@@ -66,6 +66,10 @@ static void unload_instance(struct RE_instance *instance)
 		free(instance->bone_transforms);
 		instance->bone_transforms = NULL;
 	}
+	if (instance->height_detector) {
+		RE_renderer_free_height_detector(instance->height_detector);
+		instance->height_detector = NULL;
+	}
 }
 
 static void free_instance(struct RE_instance *instance)
@@ -374,6 +378,17 @@ bool RE_instance_trans_local_pos_to_world_pos_by_bone(struct RE_instance *instan
 	return true;
 }
 
+float RE_instance_calc_height(struct RE_instance *instance, float x, float z)
+{
+	if (!instance || !instance->model)
+		return 0.0;
+	if (!instance->height_detector) {
+		instance->height_detector = RE_renderer_create_height_detector(instance->plugin->renderer, instance->model);
+	}
+	float y = RE_renderer_detect_height(instance->height_detector, x, z);
+	return y;
+}
+
 void RE_instance_update_local_transform(struct RE_instance *inst)
 {
 	vec3 euler = {
diff --git a/src/3d/renderer.c b/src/3d/renderer.c
index 0c6511f..e36eda9 100644
--- a/src/3d/renderer.c
+++ b/src/3d/renderer.c
@@ -620,3 +620,97 @@ void RE_render(struct sact_sprite *sp)
 	glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, 0);
 	gfx_reset_framebuffer(GL_DRAW_FRAMEBUFFER, fbo);
 }
+
+struct height_detector {
+	GLushort *buf;
+	vec3 aabb[2];
+};
+
+struct height_detector *RE_renderer_create_height_detector(struct RE_renderer *r, struct model *model)
+{
+	struct height_detector *hd = xcalloc(1, sizeof(struct height_detector));
+	vec3 aabb[2];
+	memcpy(aabb, model->aabb, sizeof(aabb));
+	// Add some Y padding.
+	aabb[0][1] -= 1.0;
+	aabb[1][1] += 1.0;
+	memcpy(hd->aabb, aabb, sizeof(aabb));
+
+	// Set up an orthographic projection matrix looking down the instance from
+	// right above.
+	mat4 view_matrix, proj_matrix;
+	glm_mat4_identity(view_matrix);
+	glm_rotate_x(view_matrix, glm_rad(90.0), view_matrix);
+	glm_aabb_transform(aabb, view_matrix, aabb);
+	glm_ortho_aabb(aabb, proj_matrix);
+
+	GLint orig_fbo, orig_viewport[4];
+	glGetIntegerv(GL_FRAMEBUFFER_BINDING, &orig_fbo);
+	glGetIntegerv(GL_VIEWPORT, orig_viewport);
+
+	// Since OpenGL ES does not allow reading from depth textures, create a
+	// color texture and render depth values into it.
+	GLuint color_texture;
+	glGenTextures(1, &color_texture);
+	glBindTexture(GL_TEXTURE_2D, color_texture);
+	glTexImage2D(GL_TEXTURE_2D, 0, GL_R16UI, SHADOW_WIDTH, SHADOW_HEIGHT, 0, GL_RED_INTEGER, GL_UNSIGNED_SHORT, NULL);
+
+	glBindFramebuffer(GL_FRAMEBUFFER, r->shadow.fbo);
+	GLenum draw_buffers[1] = { GL_COLOR_ATTACHMENT0 };
+	glDrawBuffers(1, draw_buffers);
+	glReadBuffer(GL_COLOR_ATTACHMENT0);
+	glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, color_texture, 0);
+
+	glViewport(0, 0, SHADOW_WIDTH, SHADOW_HEIGHT);
+	glClear(GL_DEPTH_BUFFER_BIT);
+	glUseProgram(r->shadow.program);
+	glUniformMatrix4fv(r->shadow.world_transform, 1, GL_FALSE, view_matrix[0]);
+	glUniformMatrix4fv(r->shadow.view_transform, 1, GL_FALSE, proj_matrix[0]);
+	glUniform1i(r->shadow.has_bones, GL_FALSE);
+
+	glEnable(GL_DEPTH_TEST);
+
+	// Render the model.
+	for (int i = 0; i < model->nr_meshes; i++) {
+		struct mesh *mesh = &model->meshes[i];
+		glBindVertexArray(mesh->vao);
+		glDrawArrays(GL_TRIANGLES, 0, mesh->nr_vertices);
+	}
+	glBindVertexArray(0);
+	glFinish();
+
+	hd->buf = xmalloc(SHADOW_WIDTH * SHADOW_HEIGHT * sizeof(GLushort));
+	glReadPixels(0, 0, SHADOW_WIDTH, SHADOW_HEIGHT, GL_RED_INTEGER, GL_UNSIGNED_SHORT, hd->buf);
+
+	glDisable(GL_DEPTH_TEST);
+	glDrawBuffers(0, NULL);
+	glReadBuffer(GL_NONE);
+	glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, 0, 0);
+	glBindFramebuffer(GL_FRAMEBUFFER, orig_fbo);
+	glViewport(orig_viewport[0], orig_viewport[1], orig_viewport[2], orig_viewport[3]);
+
+	glDeleteTextures(1, &color_texture);
+	return hd;
+}
+
+void RE_renderer_free_height_detector(struct height_detector *hd)
+{
+	free(hd->buf);
+	free(hd);
+}
+
+float RE_renderer_detect_height(struct height_detector *hd, float x, float z)
+{
+	float minx = hd->aabb[0][0];
+	float maxx = hd->aabb[1][0];
+	float miny = hd->aabb[0][1];
+	float maxy = hd->aabb[1][1];
+	float minz = hd->aabb[0][2];
+	float maxz = hd->aabb[1][2];
+	if (x < minx || x > maxx || z < minz || z > maxz)
+		return 0.0;
+	int px = glm_percent(minx, maxx, x) * SHADOW_WIDTH;
+	int pz = glm_percent(maxz, minz, z) * SHADOW_HEIGHT;
+	float py = hd->buf[pz * SHADOW_WIDTH + px] / 65535.0;
+	return glm_lerp(maxy, miny, py);
+}
diff --git a/src/hll/ReignEngine.c b/src/hll/ReignEngine.c
index 1ad1a89..d89ca8f 100644
--- a/src/hll/ReignEngine.c
+++ b/src/hll/ReignEngine.c
@@ -523,7 +523,12 @@ static bool ReignEngine_TransInstanceLocalPosToWorldPosByBone(int plugin, int in
 //int ReignEngine_GetInstanceTextureMemorySize(int plugin, int instance);
 //void ReignEngine_GetInstanceInfoText(int plugin, int instance, struct string **pIText);
 //void ReignEngine_GetInstanceMaterialInfoText(int plugin, int instance, struct string **pIText);
-HLL_WARN_UNIMPLEMENTED(0.0, float, ReignEngine, CalcInstanceHeightDetection, int plugin, int instance, float x, float z);
+
+static float ReignEngine_CalcInstanceHeightDetection(int plugin, int instance, float x, float z)
+{
+	return RE_instance_calc_height(get_instance(plugin, instance), x, -z);
+}
+
 //float ReignEngine_GetInstanceSoftFogEdgeLength(int plugin, int instance);
 HLL_WARN_UNIMPLEMENTED(false, bool, ReignEngine, SetInstanceSoftFogEdgeLength, int plugin, int instance, float length);