new method based on sensor dimensions and focal length

gopro_8:7/unfish_gopro_8:7.glsl

@@ -1,14 +1,15 @@
 // RozK
 // Fisheye removal for GoPro 11+, 8:7 ratio, without hypersmooth
-// Adapted from https://github.com/duducosmos/defisheye
-// itself based on http://www.fmwconcepts.com/imagemagick/defisheye/index.php
 
 #extension GL_ARB_texture_rectangle: enable
 
 // TODO: investigate
 // precision highp float;
 
+// #define debug_borders
+
 // uniforms
+
 uniform sampler2DRect myTextureY;
 uniform sampler2DRect myTextureU;
 uniform sampler2DRect myTextureV;
@@ -16,25 +17,47 @@ uniform vec2 myResolution;
 uniform float pts;
 
 // parameters
-const float input_fov = 156.0;
-const float output_fov = 124.45;
-const vec2 pixel_scale = vec2(0.652485, 1.0);
+
+const vec2 sensor_dimensions = vec2(5.949440, 5.205760);
+const float fisheye_focal_length = 2.92;
+const float rectilinear_focal_length = 2.102263;
 const int subsampling = 4;
 
-// subsampling constants
-const float substep = 1.0 / float(subsampling);
-const float substart = substep * 0.5 - 0.5;
-const float subscale = 1.0 / float(subsampling * subsampling);
+// constants
+
+const float subsampling_step = 1.0 / float(subsampling);
+const float subsampling_start = subsampling_step * 0.5 - 0.5;
+const float subsampling_denominator = 1.0 / float(subsampling * subsampling);
 
 // variables
-vec2 center;
-float diameter;
-float input_len;
-float inv_output_len;
 
-vec4 unfish(const in vec2 coord) {
-    float len = max(0.001, length(coord));
-    vec2 y_coord = center + coord * ((input_len / len) * atan(len * inv_output_len));
+vec2 texture_center;
+vec2 texture_to_sensor;
+vec2 sensor_to_texture;
+
+void initialize() {
+    texture_center = myResolution * 0.5;
+    texture_to_sensor = (sensor_dimensions / myResolution);
+    sensor_to_texture = (myResolution / sensor_dimensions);
+}
+
+vec2 unfish_coord(const in vec2 coord) {
+    float rectilinear_distance = length(coord);
+    float rectilinear_angle = atan(rectilinear_distance / rectilinear_focal_length);
+    float fisheye_distance = rectilinear_angle * fisheye_focal_length;
+    return coord * (fisheye_distance / rectilinear_distance);
+}
+
+vec4 unfish_pixel(const in vec2 coord) {
+    vec2 unfished = unfish_coord((coord - texture_center) * texture_to_sensor);
+    vec2 y_coord = texture_center + unfished * sensor_to_texture;
+
+#ifdef debug_borders
+    if (y_coord.x < 0.0 || y_coord.y < 0.0 || y_coord.x > myResolution.x || y_coord.y > myResolution.y) {
+        return vec4(1.0, 1.0, 1.0, 1.0);
+    }
+#endif
+
     vec2 uv_coord = y_coord * 0.5;
     return vec4(
         texture2DRect(myTextureY, y_coord).r,
@@ -45,21 +68,18 @@ vec4 unfish(const in vec2 coord) {
 }
 
 void main() {
-    center = myResolution * 0.5;
-    diameter = length(myResolution);
-    input_len = diameter / radians(input_fov);
-    inv_output_len = (2.0 * tan(radians(output_fov * 0.5))) / diameter;
+    initialize();
 
-    vec2 coord = gl_TexCoord[0].xy - center;
+    vec2 coord = gl_TexCoord[0].xy;
     vec4 pixel = vec4(0.0, 0.0, 0.0, 0.0);
 
-    float x, y = substart;
-    for (int column = 0; column < subsampling; column++, y += substep) {
-        x = substart;
-        for (int row = 0; row < subsampling; row++, x += substep) {
-            pixel += unfish((coord + vec2(x, y)) * pixel_scale);
+    float x, y = subsampling_start;
+    for (int column = 0; column < subsampling; column++, y += subsampling_step) {
+        x = subsampling_start;
+        for (int row = 0; row < subsampling; row++, x += subsampling_step) {
+            pixel += unfish_pixel(coord + vec2(x, y));
         }
     }
 
-    gl_FragColor = pixel * subscale;
+    gl_FragColor = pixel * subsampling_denominator;
 }