Interpolate palette colors in order to ensure colormaps always have the same number of colors

arpav_ppcv/config.py

@@ -113,6 +113,7 @@ class ArpavPpcvSettings(BaseSettings):  # noqa
     static_dir: Optional[Path] = Path(__file__).parent / "webapp/static"
     thredds_server: ThreddsServerSettings = ThreddsServerSettings()
     palettes_dir: Path = Path(__file__).parents[1] / "data/palettes"
+    palette_num_stops: int = 5
     prefect: PrefectSettings = PrefectSettings()
     martin_tile_server_base_url: str = "http://localhost:3000"
     nearest_station_radius_meters: int = 200

arpav_ppcv/palette.py

@@ -1,10 +1,38 @@
 import logging
 from pathlib import Path
-from typing import Optional
+from typing import (
+    Optional,
+    Sequence,
+)
+
+import matplotlib as mpl
 
 logger = logging.getLogger(__name__)
 
 
+def apply_palette(
+    colors: Sequence[str], minimum: float, maximum: float, num_stops: int
+) -> list[tuple[float, str]]:
+    # this converts from the AARRGGBB format which is what the .pal files have
+    # to RRGGBB
+    rgb_colors = [f"#{c[3:]}" for c in colors]
+    cmap = mpl.colors.ListedColormap(rgb_colors)
+    result = []
+    step = (maximum - minimum) / (num_stops - 1)
+    for current_stop in range(num_stops):
+        interval_stop = minimum + current_stop * step
+        normalized_stop = current_stop * step / abs(maximum - minimum)
+        color = mpl.colors.to_hex(cmap(normalized_stop), True)
+        thredds_representation = "#{alpha}{red}{green}{blue}".format(
+            red=color[1:3],
+            green=color[3:5],
+            blue=color[5:7],
+            alpha=color[7:9],
+        )
+        result.append((interval_stop, thredds_representation))
+    return result
+
+
 def parse_palette(palette: str, palettes_dir: Path) -> Optional[list[str]]:
     palette_name = palette.split("/")[-1].lower()
     if is_inverted := "-inv" in palette_name:
@@ -28,15 +56,3 @@ def parse_palette(palette: str, palettes_dir: Path) -> Optional[list[str]]:
     if is_inverted:
         colors.reverse()
     return colors if len(colors) > 0 else None
-
-
-def apply_palette(
-    colors: list[str], minimum: float, maximum: float
-) -> list[tuple[float, str]]:
-    minmax_range = maximum - minimum
-    step_increment = minmax_range / (len(colors) - 1)
-    result = []
-    for i, current_color in enumerate(colors):
-        current_value = minimum + i * step_increment
-        result.append((current_value, current_color))
-    return result

arpav_ppcv/webapp/api_v2/routers/coverages.py

@@ -188,14 +188,22 @@ def get_coverage_configuration(
     palette_colors = palette.parse_palette(
         db_coverage_configuration.palette, settings.palettes_dir
     )
+    applied_colors = []
     if palette_colors is not None:
-        applied_colors = palette.apply_palette(
-            palette_colors,
-            db_coverage_configuration.color_scale_min,
-            db_coverage_configuration.color_scale_max,
-        )
+        minimum = db_coverage_configuration.color_scale_min
+        maximum = db_coverage_configuration.color_scale_max
+        if abs(maximum - minimum) > 0.001:
+            applied_colors = palette.apply_palette(
+                palette_colors, minimum, maximum, num_stops=settings.palette_num_stops
+            )
+        else:
+            logger.warning(
+                f"Cannot calculate applied colors for coverage "
+                f"configuration {db_coverage_configuration.name!r} - check the "
+                f"colorscale min and max values"
+            )
     else:
-        applied_colors = []
+        logger.warning(f"Unable to parse palette {db_coverage_configuration.palette!r}")
     return coverage_schemas.CoverageConfigurationReadDetail.from_db_instance(
         db_coverage_configuration, allowed_coverage_identifiers, applied_colors, request
     )

pyproject.toml

@@ -42,6 +42,7 @@ cftime = "^1.6.4"
 babel = "^2.15.0"
 pyloess = "^0.1.0"
 prefect = {version = "^3.0.0rc14", allow-prereleases = true}
+matplotlib = "^3.9.2"
 
 
 [tool.poetry.group.dev]