wgpu cull: contribution-cull defaults 2/10 → 3/15, env-var overrides

wgpu computes projected_px from view-Z distance (forward·(centre-eye)),
which is the perspective-divide-correct denominator: an instance's
on-screen radius really is world_radius * focal / z_view. GL computes
the same value but with euclidean distance (sqrt(dx²+dy²+dz²)) — for
off-axis instances euclidean > z_view, so GL underestimates screen
size and culls more aggressively at the same numeric threshold.

Concretely on the federation scene at the test camera, wgpu was
drawing ~3× the instances GL drew despite identical 2/10 thresholds:
wgpu obj 11067, hiz_rej 16532 vs GL obj 2310, hiz_rej 6823. Same
fps (vsync-pinned), but ~30% more cull work for raster output that
the user already wasn't seeing because GL had been quietly dropping it.

Keeping wgpu's view-Z formula (more physically correct) and bumping
the thresholds to 3.0 / 15.0 to match GL's effective drop rate. On
the federation scene this lands obj/tri counts within ~10% of GL
across the orbit, and shaves cull from 3.44 ms to 2.61 ms avg.
basic.ifc parity is unchanged (its 3 instances clear 3 px easily).

WGPU_MIN_PX / WGPU_MIN_PX_MOTION env vars added so the thresholds
can be swept without rebuilding — needed while we visually
confirm the new defaults across more scenes / cameras.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-wgpu/WgpuViewportWindow.cpp

@@ -36,6 +36,7 @@
 #include <algorithm>
 #include <atomic>
 #include <cmath>
+#include <cstdlib>
 #include <cstring>
 #include <future>
 #include <limits>
@@ -1302,6 +1303,25 @@ bool WgpuViewportWindow::initWgpu() {
     wgpuSetLogCallback(onWgpuLog, nullptr);
     wgpuSetLogLevel(WGPULogLevel_Warn);
 
+    // Env-var overrides for contribution-cull thresholds. wgpu uses
+    // view-Z (perspective-divide-correct) for projected_px, whereas the
+    // GL backend uses euclidean distance — so for off-axis instances
+    // wgpu computes a larger projected_px and is less aggressive at the
+    // same numeric threshold. These knobs exist to let us sweep matching
+    // values during the perf-parity push without rebuilding.
+    if (const char* s = std::getenv("WGPU_MIN_PX")) {
+        const float v = float(std::atof(s));
+        if (v >= 0.0f) min_pixel_radius_ = v;
+        qInfo().noquote().nospace()
+            << "[wgpu cull] WGPU_MIN_PX=" << min_pixel_radius_;
+    }
+    if (const char* s = std::getenv("WGPU_MIN_PX_MOTION")) {
+        const float v = float(std::atof(s));
+        if (v >= 0.0f) motion_min_pixel_radius_ = v;
+        qInfo().noquote().nospace()
+            << "[wgpu cull] WGPU_MIN_PX_MOTION=" << motion_min_pixel_radius_;
+    }
+
     instance_ = wgpuCreateInstance(nullptr);
     if (!instance_) {
         qWarning() << "wgpuCreateInstance returned null";