revert gpr heatmap threshold, keep it for sar3d only

The display threshold from 'add treshhold' (e113ef6) was meant for the
sar3d projections; the GPR heatmap goes back to full-range levels and
the constant moves into sar3d_plot_mixin.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

python_app/gui/controllers/app_window_plot/gpr_plot_mixin.py

@@ -14,12 +14,6 @@ from python_app.orchestration.gpr_locator import (
     gpr_object_rows as extract_gpr_object_rows,
 )
 
-# Fraction of the frame maximum below which the heatmap is not highlighted
-# (same as cloud-threshold=0.75 in the VNA_AlGo 3D SAR animation): the color
-# scale is stretched over [frac*max, max], everything below falls into the
-# darkest LUT color.
-HEATMAP_DISPLAY_THRESHOLD_FRAC = 0.75
-
 
 class AppWindowGprPlotMixin:
     """Renders GPR accumulator heatmaps and detected object overlays."""
@@ -250,9 +244,7 @@ class AppWindowGprPlotMixin:
             self._gpr_image_item.setImage(image, autoLevels=False)
             self._gpr_image_item.setRect(rect)
             self._gpr_image_item.setLookupTable(self._gpr_lookup_table)
-            image_max = float(np.max(image))
-            self._gpr_image_item.setLevels(
-                (HEATMAP_DISPLAY_THRESHOLD_FRAC * image_max, image_max + 1e-6))
+            self._gpr_image_item.setLevels((float(np.min(image)), float(np.max(image) + 1e-6)))
             self._gpr_image_item.show()
 
             visible_x_min, visible_x_max, visible_z_min, visible_z_max = self._gpr_visible_bounds()

python_app/gui/controllers/app_window_plot/sar3d_plot_mixin.py

@@ -21,12 +21,15 @@ from PyQt6.QtCore import QRectF
 import numpy as np
 import pyqtgraph as pg
 
-from python_app.gui.controllers.app_window_plot.gpr_plot_mixin import (
-    HEATMAP_DISPLAY_THRESHOLD_FRAC,
-)
 from python_app.gui.runtime.sar3d_views import sar3d_bottom_view_deg_from_label
 from python_app.models.dataset_model import ResultCollection
 
+# Fraction of the projection maximum below which the heatmap is not highlighted
+# (same as cloud-threshold=0.75 in the VNA_AlGo 3D SAR animation): the color
+# scale is stretched over [frac, 1], everything below falls into the darkest
+# LUT color.
+HEATMAP_DISPLAY_THRESHOLD_FRAC = 0.75
+
 
 class AppWindowSar3dPlotMixin:
     """Renders 3D SAR volume projections published by the sar3d processor."""