Merge pull request #67 from dcbriccetti/gravity_pulls_pixel

Create “Gravity Pulls Pixel” program

.travis.yml

@@ -27,6 +27,6 @@ install:
 
 script:
   - pylint adafruit_circuitplayground/*.py
-  - ([[ ! -d "examples" ]] || pylint --disable=missing-docstring,invalid-name examples/*.py)
+  - ([[ ! -d "examples" ]] || pylint --disable=missing-docstring,invalid-name `find ./examples -name "*.py"`)
   - circuitpython-build-bundles --filename_prefix adafruit-circuitpython-circuitplayground --library_location .
   - cd docs && sphinx-build -E -W -b html . _build/html

examples/advanced_examples/circuitplayground_gravity_pulls_pixel.py

@@ -0,0 +1,85 @@
+"""Gravity Pulls Pixel
+
+This program uses the Circuit Playground Express’s accelerometer to position
+a white pixel as if gravity were pulling it.
+
+Flip the switch left (toward the notes) to turn on debugging messages and
+slow down the action. See a code walkthrough here: https://youtu.be/sZ4tNOUKRpw
+"""
+import time
+import math
+from adafruit_circuitplayground.express import cpx
+
+PIXEL_SPACING_ANGLE = 30
+STANDARD_GRAVITY = 9.81
+
+BACKGROUND_COLOR = 0, 0, 64
+MIN_BRIGHTNESS = 15  # Minimum brightness for anti-aliasing
+LIGHTING_ARC_LENGTH = 45
+
+
+def compute_pixel_angles():
+    """Return a list of rotation angles of the ten NeoPixels.
+
+    On the CPX there are ten pixels arranged like the 12 hours of a clock, except that positions
+    6 and 12 are empty. The numbers in the list are the angles from the (x, y) accelerometer
+    values for each pixel when the CPX is rotated with that pixel at the bottom. For example,
+    with pixel 0 at the bottom (and pixel 5 at the top), the accelerometer’s (x, y) values
+    give an angle of 300°. Rotated clockwise 1/12 turn (30°), so that pixel 1 is at the bottom,
+    the angle is 330°.
+    """
+    return [(300 + PIXEL_SPACING_ANGLE * n) % 360 for n in range(12) if n not in (5, 11)]
+
+
+def degrees_between(a1, a2):
+    smaller = min(a1, a2)
+    larger = max(a1, a2)
+    return min(larger - smaller, 360 + smaller - larger)
+
+
+def pixel_brightness(distance_from_down, accel_magnitude):
+    """Return the a brightness for a pixel, or None if the pixel is not in the lighting arc"""
+    half_lighting_arc_length = LIGHTING_ARC_LENGTH / 2
+
+    if accel_magnitude < 0.1 or distance_from_down > half_lighting_arc_length:
+        return None
+
+    normalized_nearness = 1 - distance_from_down / half_lighting_arc_length
+    scale_factor = (255 - MIN_BRIGHTNESS) * accel_magnitude
+    color_part = MIN_BRIGHTNESS + round(normalized_nearness * scale_factor)
+    return color_part
+
+
+def angle_in_degrees(x, y):
+    """Return the angle of the point (x, y), in degrees from -180 to 180"""
+    return math.atan2(y, x) / math.pi * 180
+
+
+def positive_degrees(angle):
+    """Convert -180 through 180 to 0 through 360"""
+    return (angle + 360) % 360
+
+
+cpx.pixels.brightness = 0.1  # Adjust overall brightness as desired, between 0 and 1
+pixel_positions = compute_pixel_angles()
+
+while True:
+    debug = cpx.switch  # True is toward the left
+    accel_x, accel_y = cpx.acceleration[:2]  # Ignore z
+    down_angle = positive_degrees(angle_in_degrees(accel_x, accel_y))
+    magnitude_limit = STANDARD_GRAVITY
+    normalized_magnitude = min(math.sqrt(accel_x * accel_x + accel_y * accel_y),
+                               magnitude_limit) / magnitude_limit
+
+    pixels_lit = []
+    for i, pixel_position in enumerate(pixel_positions):
+        pe = pixel_brightness(degrees_between(pixel_position, down_angle), normalized_magnitude)
+        cpx.pixels[i] = (pe, pe, pe) if pe else BACKGROUND_COLOR
+        if pe:
+            pixels_lit.append((i, pe))
+
+    if debug:
+        lit_formatted = ', '.join(('{}: {:>3d}'.format(p, i) for p, i in pixels_lit))
+        print('x: {:>6.2f}, y: {:>6.2f}, angle: {:>6.2f}, mag: {:>3.2f}, pixels: [{}]'.format(
+            accel_x, accel_y, down_angle, normalized_magnitude, lit_formatted))
+        time.sleep(0.5)