pbio/image: Improve circle and disc shapes.

Previous code was too strict on the circle radius, resulting in pointy
circles.

Do not touch rounded rectangles, they looks better with small radii.

Refs: pybricks/support#2154

Author: schodet

lib/pbio/src/image/image.c

@@ -693,7 +693,7 @@ void pbio_image_draw_circle(pbio_image_t *image, int x, int y, int r,
 
     // Draw.
     int dx = 0, dy = r;
-    int r2 = r * r;
+    int err = 5 - r * 4;
     while (dx <= dy) {
         pbio_image_draw_pixel(image, x + dx, y + dy, value);
         pbio_image_draw_pixel(image, x - dx, y + dy, value);
@@ -703,10 +703,12 @@ void pbio_image_draw_circle(pbio_image_t *image, int x, int y, int r,
         pbio_image_draw_pixel(image, x - dy, y + dx, value);
         pbio_image_draw_pixel(image, x + dy, y - dx, value);
         pbio_image_draw_pixel(image, x - dy, y - dx, value);
-        dx++;
-        if (dx * dx + dy * dy > r2) {
+        if (err > 0) {
             dy--;
+            err -= dy * 8;
         }
+        dx++;
+        err += 4 + dx * 8;
     }
 }
 
@@ -729,7 +731,7 @@ void pbio_image_fill_circle(pbio_image_t *image, int x, int y, int r,
 
     // Draw.
     int dx = 0, dy = r;
-    int r2 = r * r;
+    int err = 5 - r * 4;
     while (dx <= dy) {
         // Optimization opportunity: when dy is not moving, the same pixels
         // are drawn again and again.
@@ -739,10 +741,12 @@ void pbio_image_fill_circle(pbio_image_t *image, int x, int y, int r,
             pbio_image_draw_hline(image, x - dy, y + dx, 1 + 2 * dy, value);
             pbio_image_draw_hline(image, x - dy, y - dx, 1 + 2 * dy, value);
         }
-        dx++;
-        if (dx * dx + dy * dy > r2) {
+        if (err > 0) {
             dy--;
+            err -= dy * 8;
         }
+        dx++;
+        err += 4 + dx * 8;
     }
 }
 

lib/pbio/test/src/test_image.c

@@ -747,23 +747,23 @@ static void test_image_draw_circle(void *env) {
     test_image_prepare_small_image(&small);
     pbio_image_draw_circle(&small, 3, 3, 3, '*');
     tt_want_small_image(
-        "...*................"
-        ".**.**.............."
+        "..***..............."
         ".*...*.............."
         "*.....*............."
+        "*.....*............."
+        "*.....*............."
         ".*...*.............."
-        ".**.**.............."
-        "...*................");
+        "..***...............");
 
     test_image_prepare_small_image(&small);
     pbio_image_draw_circle(&small, 6, 6, 6, '*');
     tt_want_small_image(
-        "......*............."
-        "...***.***.........."
+        "....*****..........."
+        "...*.....*.........."
         "..*.......*........."
         ".*.........*........"
-        ".*.........*........"
-        ".*.........*........"
+        "*...........*......."
+        "*...........*......."
         "*...........*.......");
 
     test_image_prepare_images(&outer, &inner, &full);
@@ -786,23 +786,23 @@ static void test_image_fill_circle(void *env) {
     test_image_prepare_small_image(&small);
     pbio_image_fill_circle(&small, 3, 3, 3, '*');
     tt_want_small_image(
-        "...*................"
-        ".*****.............."
+        "..***..............."
         ".*****.............."
         "*******............."
+        "*******............."
+        "*******............."
         ".*****.............."
-        ".*****.............."
-        "...*................");
+        "..***...............");
 
     test_image_prepare_small_image(&small);
     pbio_image_fill_circle(&small, 6, 6, 6, '*');
     tt_want_small_image(
-        "......*............."
+        "....*****..........."
         "...*******.........."
         "..*********........."
         ".***********........"
-        ".***********........"
-        ".***********........"
+        "*************......."
+        "*************......."
         "*************.......");
 
     test_image_prepare_images(&outer, &inner, &full);