ray: add rudimentary gamma correction

color banding has been quite visible, and somewhat expected with a
direct conversion from the linear float color space to the 8-bit
integral rgb color components.

A simple lookup table is used here to non-linearly map the values, table
generation is taken from Greg Ward's REAL PIXELS gem in Graphics Gems II.

5 files changed, 58 insertions, 2 deletions

diff --git a/src/libs/ray/Makefile.am b/src/libs/ray/Makefile.am
index dc13a0d..f055d94 100644
--- a/src/libs/ray/Makefile.am
+++ b/src/libs/ray/Makefile.am
@@ -1,4 +1,4 @@
 noinst_LIBRARIES = libray.a
-libray_a_SOURCES = ray_3f.h ray_camera.c ray_camera.h ray_color.h ray_euler.c ray_euler.h ray_light_emitter.h ray_object.h ray_object_light.h ray_object_plane.h ray_object_point.h ray_object_sphere.h ray_object_type.h ray_ray.h ray_render.c ray_render.h ray_render_object.h ray_render_object_plane.h ray_render_object_point.h ray_render_object_sphere.h ray_scene.h ray_surface.h
+libray_a_SOURCES = ray_3f.h ray_camera.c ray_camera.h ray_color.h ray_euler.c ray_euler.h ray_gamma.c ray_gamma.h ray_light_emitter.h ray_object.h ray_object_light.h ray_object_plane.h ray_object_point.h ray_object_sphere.h ray_object_type.h ray_ray.h ray_render.c ray_render.h ray_render_object.h ray_render_object_plane.h ray_render_object_point.h ray_render_object_sphere.h ray_scene.h ray_surface.h
 libray_a_CFLAGS = -ffast-math
 libray_a_CPPFLAGS = -I@top_srcdir@/src
diff --git a/src/libs/ray/ray_gamma.c b/src/libs/ray/ray_gamma.c
new file mode 100644
index 0000000..21a9afc
--- /dev/null
+++ b/src/libs/ray/ray_gamma.c
@@ -0,0 +1,15 @@
+#include <math.h>
+
+#include "ray_gamma.h"
+
+void ray_gamma_prepare(float gamma, ray_gamma_t *res_gamma)
+{
+	if (res_gamma->gamma == gamma)
+		return;
+
+	/* This is from graphics gems 2 "REAL PIXELS" */
+	for (unsigned i = 0; i < 1024; i++)
+		res_gamma->table[i] = 256.0f * powf((((float)i + .5f) / 1024.0f), 1.0f/gamma);
+
+	res_gamma->gamma = gamma;
+}
diff --git a/src/libs/ray/ray_gamma.h b/src/libs/ray/ray_gamma.h
new file mode 100644
index 0000000..c46d631
--- /dev/null
+++ b/src/libs/ray/ray_gamma.h
@@ -0,0 +1,37 @@
+#ifndef _RAY_GAMMA_H
+#define _RAY_GAMMA_H
+
+#include <stdint.h>
+
+#include "ray_color.h"
+
+typedef struct ray_gamma_t {
+	float	gamma;
+	uint8_t	table[1024];
+} ray_gamma_t;
+
+void ray_gamma_prepare(float gamma, ray_gamma_t *res_gamma);
+
+/* convert a color into a gamma-corrected, packed, 32-bit rgb pixel value */
+static inline uint32_t ray_gamma_color_to_uint32_rgb(ray_gamma_t *gamma, ray_color_t color) {
+	uint32_t	pixel;
+
+	if (color.x > 1.0f)
+		color.x = 1.0f;
+
+	if (color.y > 1.0f)
+		color.y = 1.0f;
+
+	if (color.z > 1.0f)
+		color.z = 1.0f;
+
+	pixel = (uint32_t)gamma->table[(unsigned)floorf(1023.0f * color.x)];
+	pixel <<= 8;
+	pixel |= (uint32_t)gamma->table[(unsigned)floorf(1023.0f * color.y)];
+	pixel <<= 8;
+	pixel |= (uint32_t)gamma->table[(unsigned)floorf(1023.0f * color.z)];
+
+	return pixel;
+}
+
+#endif
diff --git a/src/libs/ray/ray_render.c b/src/libs/ray/ray_render.c
index e551d8c..1c7c79c 100644
--- a/src/libs/ray/ray_render.c
+++ b/src/libs/ray/ray_render.c
@@ -5,6 +5,7 @@
 
 #include "ray_camera.h"
 #include "ray_color.h"
+#include "ray_gamma.h"
 #include "ray_render_object.h"
 #include "ray_ray.h"
 #include "ray_scene.h"
@@ -18,6 +19,7 @@ typedef struct ray_render_t {
 
 	ray_color_t		ambient_light;
 	ray_camera_frame_t	frame;
+	ray_gamma_t		gamma;
 
 	ray_render_object_t	objects[];
 } ray_render_t;
@@ -207,7 +209,7 @@ void ray_render_trace_fragment(ray_render_t *render, fb_fragment_t *fb_fragment)
 	ray_camera_fragment_begin(&render->frame, fb_fragment, &ray, &fragment);
 	do {
 		do {
-			*buf = ray_color_to_uint32_rgb(trace_ray(render, &ray));
+			*buf = ray_gamma_color_to_uint32_rgb(&render->gamma, trace_ray(render, &ray));
 			buf++;
 		} while (ray_camera_fragment_x_step(&fragment));
 
@@ -236,6 +238,7 @@ ray_render_t * ray_render_new(const ray_scene_t *scene, const ray_camera_t *came
 	render->camera = camera;
 
 	render->ambient_light = ray_3f_mult_scalar(&scene->ambient_color, scene->ambient_brightness);
+	ray_gamma_prepare(scene->gamma, &render->gamma);
 	ray_camera_frame_prepare(camera, &render->frame);
 
 	for (i = 0, object = scene->objects; object->type; object++)
diff --git a/src/libs/ray/ray_scene.h b/src/libs/ray/ray_scene.h
index ff9c440..2c7308f 100644
--- a/src/libs/ray/ray_scene.h
+++ b/src/libs/ray/ray_scene.h
@@ -11,6 +11,7 @@ typedef struct ray_scene_t {
 
 	ray_color_t	ambient_color;
 	float		ambient_brightness;
+	float		gamma;
 } ray_scene_t;
 
 #endif