CLAUDE: RT zero-DC conditioning + decoupled fat zero + borrowed-center DC appropriation

Three coupled improvements from tonight's design discussion with Andrey:

1. ZERO-DC in CuasConditioning (Config default ON): LWIR microbolometers are
   ADDITIVE (fixed tau=8ms exposure, F#~1 for NETD, ~100 counts/K) - day/night
   and seasons SHIFT the level, never scale it; the MCLT has no clean DC/AC
   separation, so a large DC (current ~-3500 counts under ~300 counts AC)
   spreads over many TD points and costs float32 mantissa bits. Subtract ONE
   common constant from all 16 sensors (pure shift, never rescale). AUTO level
   = MEDIAN OF THE TOP (SKY) HALF, robust to the snowy-ground-vs-summer-
   calibration mismatch and to targets.

2. RT-DECOUPLED FAT ZERO (curt.fz_inter=10000, curt.fz_intra=2000 reserved):
   the lean code erroneously used the INTRA getGpuFatZero (2000) instead of
   the INTER value - Andrey's FZ sweep (200/1000/3000/10000: corrRMS
   .54/.39/.37/.31, roll bias +.80/+.62/+.50/+.39) independently rediscovered
   the oracle's INTER operating point (10000). RT now owns its FZ pair, no
   AUX switch; legacy gpu_fatz*/FOPEN configs untouched. Sweep recorded as
   characterization: FZ tolerant over a broad range; revisit only together
   with the argmax method redesign (sharp peaks starve the centroid).

3. BORROWED-CENTER APPROPRIATION (curt.dc_center, 0=auto): the restored
   CenterClt reflects its own footage vintage - offset it ONCE at the borrow
   boundary (firewall: never re-compensate downstream) to match the zero-sky
   scenes. Auto level = sky-half median of the borrowed center average,
   ADOPTED into curt.dc_center (persisted with the config - borrowable later
   like the photometric calibration). Mechanics: DC pattern = CLT of a
   uniform image via the existing back-propagation convert (setImageCenter +
   no_kernels + use_center_image, the CorrectionFPN recipe); failure-safe
   (any missing piece leaves the reference unchanged, with a warning).

Roadmap recorded: per-pixel offset (recalibrated per-sensor + FPN) and
per-pixel scale maps to the GPU; the average stored in TD AFTER LoG
(aberration+LoG combined kernels) for fpixels subtraction - possibly for
poses too; center-average window length trades reference stability vs
extremely-slow-target detectability.

Verified: mvn -DskipTests clean package OK.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/java/com/elphel/imagej/cuas/rt/CuasConditioning.java

@@ -57,6 +57,16 @@ public class CuasConditioning {
 		public double  rowcol_weight_outlier= 0.01;   // small weight for out-of-range pixels
 		public boolean photometric          = true;   // apply scale*(raw-offset) [+ scales2 quadratic]
 		public boolean apply_fpn            = true;    // subtract per-pixel FPN (the offset map's per-pixel part)
+		// Zero-DC (Andrey 2026-07-05, restoring a lost pre-CUAS invariant): the MCLT has no
+		// clean DC/AC separation - a DC constant spreads over many TD points, costing float32
+		// mantissa bits under the small AC signal (LWIR: ~100 counts/K, typical scene AC
+		// ~300 counts = ~3K, current DC ~ -3500). Subtract ONE constant from ALL sensors
+		// (pure shift - never rescale, counts stay physically meaningful; a common constant
+		// preserves the photometric equalization). dc_level = the constant to subtract;
+		// NaN = AUTO (the global mean over all sensors of this scene). A calibration-
+		// maintained fixed level (no scene-to-scene DC flicker) is the production target.
+		public boolean zero_dc              = true;   // By Claude on 07/05/2026
+		public double  dc_level             = Double.NaN; // NaN = auto (per-scene global mean) // By Claude on 07/05/2026
 		public Config() {}
 	}
 
@@ -101,6 +111,34 @@ public class CuasConditioning {
 				}
 			}
 		}
+		// Zero-DC: one common constant subtracted from ALL sensors (see Config docs).
+		// AUTO level = MEDIAN OF THE TOP HALF of the frame (y < height/2), pooled over all
+		// sensors: for CUAS the sky background is what matters, and the median is robust to
+		// both the (snowy March Latvia vs summer-calibration) ground and to targets. The
+		// same level must be subtracted from the BORROWED averaged center image at borrow
+		// time (firewall: borrowed data zeroed once at the boundary, never re-compensated).
+		// By Claude on 07/05/2026, from Andrey's design.
+		if (cfg.zero_dc) {
+			double level = cfg.dc_level;
+			if (Double.isNaN(level)) { // auto: median of the top (sky) half, all sensors pooled
+				final int height = ((image_data[0] != null) && (image_data[0][0] != null)) ?
+						image_data[0][0].length / width : 0;
+				final int top_pix = (height / 2) * width;
+				double [] samples = new double [num_sens * top_pix];
+				int n = 0;
+				for (int s = 0; s < num_sens; s++) if (image_data[s] != null && image_data[s][0] != null) {
+					final double [] px = image_data[s][0];
+					for (int i = 0; i < top_pix; i++) if (!Double.isNaN(px[i])) samples[n++] = px[i];
+				}
+				if (n == 0) return;
+				java.util.Arrays.sort(samples, 0, n);
+				level = samples[n/2];
+			}
+			for (int s = 0; s < num_sens; s++) if (image_data[s] != null && image_data[s][0] != null) {
+				final double [] px = image_data[s][0];
+				for (int i = 0; i < px.length; i++) px[i] -= level;
+			}
+		}
 	}
 
 	/**

src/main/java/com/elphel/imagej/cuas/rt/CuasPoseRT.java

@@ -398,7 +398,7 @@ public class CuasPoseRT {
 				gpuQuad,
 				td_tiles,
 				0xFE,                           // corr_type (sum slot, as CuasMotion)
-				clt_parameters.getGpuFatZero(scene.isMonochrome()),
+				clt_parameters.curt.fz_inter, // RT-decoupled INTER fat zero (was erroneously the INTRA getGpuFatZero) // By Claude on 07/05/2026
 				debugLevel);
 		if (corr_pd == null) {
 			System.out.println("leanMeasure(): convertTDtoPD returned null for scene "+scene.getImageName());
@@ -442,6 +442,111 @@ public class CuasPoseRT {
 		return com.elphel.imagej.gpu.GPUTileProcessor.DTT_SIZE;
 	}
 
+	/**
+	 * Appropriate the borrowed center reference: subtract the DC level from the
+	 * reference CLT already uploaded to the GPU, so it matches the zero-sky conditioned
+	 * scenes (the MCLT has no clean DC/AC separation - a DC constant spreads over many
+	 * TD points and costs float32 mantissa bits; LWIR is ADDITIVE: day/night and seasons
+	 * shift the level, they do not scale it - the borrowed center reflects its own
+	 * footage vintage).
+	 * Level: curt.dc_center; 0 = AUTO - median of the top (sky) half of the borrowed
+	 * center average image, then ADOPTED into curt.dc_center (persisted on the next
+	 * configuration save - borrowable later, like the photometric calibration).
+	 * Mechanics: the CLT of a UNIFORM image (the DC pattern - identical for every
+	 * converted tile) is obtained through the existing back-propagation convert
+	 * (setImageCenter + no_kernels + use_center_image, the CorrectionFPN recipe), then
+	 * fclt_zeroed = fclt - level*pattern is re-uploaded to all reference slots. Only the
+	 * task tiles get the pattern (erase=NaN elsewhere) - sufficient: the correlation
+	 * reads task tiles only. Failure-safe: any missing piece leaves the reference as-is.
+	 * By Claude on 07/05/2026, from Andrey's design.
+	 */
+	public static void appropriateCenterDC(
+			final CLTParameters clt_parameters,
+			final QuadCLT       center_CLT,
+			final TpTask []     tp_tasks_center,
+			final float []      fclt0,
+			final int           debugLevel) {
+		final GpuQuad gpuQuad = center_CLT.getGPUQuad();
+		double level = clt_parameters.curt.dc_center;
+		if (level == 0.0) { // auto: median of the top (sky) half of the borrowed center average
+			final ImagePlus imp_avg = center_CLT.getCenterAverage();
+			if (imp_avg == null) {
+				System.out.println("appropriateCenterDC(): no center average image - reference NOT offset");
+				return;
+			}
+			final float [] apx = (float []) imp_avg.getProcessor().getPixels();
+			final int width = imp_avg.getWidth();
+			final int top_pix = (imp_avg.getHeight() / 2) * width;
+			final float [] samples = new float [top_pix];
+			int n = 0;
+			for (int i = 0; i < top_pix; i++) if (!Float.isNaN(apx[i])) samples[n++] = apx[i];
+			if (n == 0) {
+				System.out.println("appropriateCenterDC(): center average all-NaN in the sky half - reference NOT offset");
+				return;
+			}
+			Arrays.sort(samples, 0, n);
+			level = samples[n/2];
+			clt_parameters.curt.dc_center = level; // adopt: persisted on the next config save
+			System.out.println("appropriateCenterDC(): AUTO center DC level = "+level+
+					" (median of the sky half of the borrowed center average), adopted into curt.dc_center");
+		}
+		// the DC pattern: CLT of a uniform image through the back-propagation convert
+		final int [] wh = center_CLT.getGeometryCorrection().getSensorWH();
+		final double [][] saved_center = center_CLT.getImageCenter();
+		float [] pattern = null;
+		try {
+			final double [][] uniform = new double [][] {new double [wh[0]*wh[1]]};
+			Arrays.fill(uniform[0], 1.0);
+			center_CLT.setImageCenter(uniform);
+			final ImageDtt image_dtt = new ImageDtt(
+					center_CLT.getNumSensors(),
+					clt_parameters.transform_size,
+					clt_parameters.img_dtt,
+					center_CLT.isAux(),
+					center_CLT.isMonochrome(),
+					center_CLT.isLwir(),
+					clt_parameters.getScaleStrength(center_CLT.isAux()),
+					center_CLT.getGPU());
+			image_dtt.preSetReferenceTD( // tasks + offsets, no convert
+					clt_parameters.img_dtt,
+					tp_tasks_center,
+					true,                       // keep_tiles_offsets
+					clt_parameters.gpu_sigma_r,
+					clt_parameters.gpu_sigma_b,
+					clt_parameters.gpu_sigma_g,
+					clt_parameters.gpu_sigma_m,
+					debugLevel);
+			image_dtt.execConvertDirect(
+					false,                      // use_reference_buffer: SCENE buffer (scratch)
+					null,                       // wh
+					1,                          // erase_clt: NaN outside tasks
+					true,                       // no_kernels
+					true);                      // use_center_image (uniform)
+			pattern = gpuQuad.getCltData(false)[0];
+		} catch (Exception e) {
+			System.out.println("appropriateCenterDC(): DC-pattern conversion FAILED ("+e.getMessage()+
+					") - reference NOT offset");
+			pattern = null;
+		} finally {
+			center_CLT.setImageCenter(saved_center); // never leave back-propagate mode on
+		}
+		if (pattern == null) return;
+		final float [] fclt_zeroed = fclt0.clone();
+		int n_off = 0;
+		for (int i = 0; i < fclt_zeroed.length; i++) {
+			final float p = pattern[i];
+			if (!Float.isNaN(p) && !Float.isNaN(fclt_zeroed[i])) {
+				fclt_zeroed[i] -= (float) (level * p);
+				n_off++;
+			}
+		}
+		for (int ncam = 0; ncam < center_CLT.getNumSensors(); ncam++) {
+			gpuQuad.setCltData(ncam, fclt_zeroed, true); // reference buffer, all slots (merge_channels convention)
+		}
+		System.out.println("appropriateCenterDC(): reference CLT offset by "+level+
+				" over "+n_off+" TD points (task tiles)");
+	}
+
 	/**
 	 * Phase B lean per-scene fit: the measure<->solve cycle with leanMeasure() as the
 	 * measurement engine and the same IntersceneLma solver/exit rules as the oracle
@@ -667,6 +772,13 @@ public class CuasPoseRT {
 				mb_max_gain,      // double             mb_max_gain,
 				null,             // double [][]        mb_vectors (center does not move)
 				debugLevel);      // int                debug_level
+		// APPROPRIATE the borrowed center: offset the reference CLT so it matches the
+		// zero-sky conditioned scenes (curt.dc_center; 0 = auto from the borrowed center
+		// average, adopted for persistence). Firewall: the borrowed data is zeroed ONCE
+		// here at the boundary - nothing downstream compensates again. Failure-safe: any
+		// missing piece leaves the reference unchanged (a warning, not a break).
+		// By Claude on 07/05/2026, from Andrey's design.
+		appropriateCenterDC(clt_parameters, center_CLT, tp_tasks_center[0], fclt[0], debugLevel);
 
 		// ---- Stored poses (comparison truth + seed) ----
 		final ErsCorrection ers_center = center_CLT.getErsCorrection();

src/main/java/com/elphel/imagej/tileprocessor/CuasRtParameters.java

@@ -29,6 +29,9 @@ public class CuasRtParameters {
 	public boolean  pose_img_save =   false; // DEBUG: save the composite scenes actually correlated - imclt render of the consolidated 16-sensor TD average at the converged pose (-POSE-RT-COMPOSITE: z=scenes; NaN outside task tiles) + the virtual-center reference render (-POSE-RT-CENTER-REF). Lean engine only. // By Claude on 07/04/2026
 	public boolean  pose_stored =     false; // DEBUG: decouple rendering/measurement from the LMA - use the STORED (oracle vintage) pose for every scene, no adjustment; one leanMeasure per scene feeds -POSE-RT-HYPER/-CORR2D/-COMPOSITE. The composite must then match the oracle CUAS-MERGED-CUAS-DBG renders tile-for-tile. Lean engine only. // By Claude on 07/04/2026
 	public boolean  rend_test =       false; // MINIMAL RT ingest+render test (CuasRender.testRenderSequence, takes precedence over pose test): per scene raw /jp4/ -> conditionSceneToGpu -> virtual-grid render at BORROWED stored pose+ERS rates -> -CUAS-RT-RENDER hyperstack [s00..s15, merged][scenes], comparable to -CUAS-INDIVIDUAL/MERGED-CUAS-DBG. Certify THIS before the pose test. // By Claude on 07/05/2026
+	public double   fz_inter =        10000.0; // RT fat zero, INTER (pose/motion: scene x virtual-center correlation). DECOUPLED from the legacy gpu_fatz*/AUX switches (which stay untouched for non-RT/FOPEN). Value is for a single physical-pair amplitude; FZ ~ amplitude^2 in the GPU normalize - scale down x4..x16 for consolidated-average applications as needed. 10000 = the proven oracle INTER operating point (the lean code erroneously used the INTRA value before). // By Claude on 07/05/2026
+	public double   fz_intra =        2000.0; // RT fat zero, INTRA (ranging/disparity: sensor-pair correlations within one scene). Reserved for the RT ranging path; same decoupling/scaling notes as fz_inter. // By Claude on 07/05/2026
+	public double   dc_center =       0.0; // DC level of the BORROWED center (CLT-RESTORED/average): subtracted from the reference CLT at appropriation so it matches the zero-sky conditioned scenes (MCLT has no clean DC/AC split - a DC constant costs float32 accuracy). 0 = AUTO: compute as the median of the top (sky) half of the borrowed center average, adopt the value here (persisted on config save - borrowable later without our data, like the photometric calibration). Slow targets do not influence this shift. // By Claude on 07/05/2026
 	public double   psf_radius =      1.0;   // sensor PSF radius for LoG pre-filter
 	public double   n_sigma =         4.0;   // cutoff LoG kernel array, number of sigmas
 	public int      pyramid =         7;     // temporal pyramid levels
@@ -112,6 +115,12 @@ public class CuasRtParameters {
 				"DEBUG: measure/render every scene at its STORED pose, skip the LMA entirely - decouples the task/render/correlation chain from the solver. Lean engine only.");
 		gd.addCheckbox ("CUAS RT render test (ingest+render)",       this.rend_test, // By Claude on 07/05/2026
 				"MINIMAL standalone test (precedence over pose test): raw jp4 -> conditioning -> virtual-grid render at borrowed stored poses/ERS; saves -CUAS-RT-RENDER [s00..s15+merged][scenes] to compare against the -CUAS-*-DBG oracle products.");
+		gd.addNumericField("RT fat zero INTER (pose/motion)",        this.fz_inter, 1,9,"", // By Claude on 07/05/2026
+				"RT-decoupled fat zero for INTERscene (pose/motion) correlations - the legacy gpu_fatz*/AUX parameters are not used by RT. For a single physical-pair amplitude; FZ ~ amplitude^2.");
+		gd.addNumericField("RT fat zero INTRA (ranging)",            this.fz_intra, 1,9,"", // By Claude on 07/05/2026
+				"RT-decoupled fat zero for INTRAscene (ranging/disparity) correlations (reserved).");
+		gd.addNumericField("Center DC level (0 - auto)",              this.dc_center, 3,9,"counts", // By Claude on 07/05/2026
+				"DC level subtracted from the borrowed center reference at appropriation (match the zero-sky scenes). 0 = compute from the center average (median of the sky half) and adopt; persisted with the configuration.");
 
 		gd.addMessage("=== LoG prefilter ===");
 		gd.addNumericField("Optical PSF radius",                     this.psf_radius, 6,8,"pix",
@@ -248,6 +257,9 @@ public class CuasRtParameters {
 		this.pose_img_save =       gd.getNextBoolean();      // By Claude on 07/04/2026
 		this.pose_stored =         gd.getNextBoolean();      // By Claude on 07/04/2026
 		this.rend_test =           gd.getNextBoolean();      // By Claude on 07/05/2026
+		this.fz_inter =            gd.getNextNumber();       // By Claude on 07/05/2026
+		this.fz_intra =            gd.getNextNumber();       // By Claude on 07/05/2026
+		this.dc_center =           gd.getNextNumber();       // By Claude on 07/05/2026
 
 		this.psf_radius =          gd.getNextNumber();
 		this.n_sigma =             gd.getNextNumber();
@@ -328,6 +340,9 @@ public class CuasRtParameters {
 		properties.setProperty(prefix+"pose_img_save",   this.pose_img_save+"");  // boolean // By Claude on 07/04/2026
 		properties.setProperty(prefix+"pose_stored",     this.pose_stored+"");    // boolean // By Claude on 07/04/2026
 		properties.setProperty(prefix+"rend_test",       this.rend_test+"");      // boolean // By Claude on 07/05/2026
+		properties.setProperty(prefix+"fz_inter",        this.fz_inter+"");       // double  // By Claude on 07/05/2026
+		properties.setProperty(prefix+"fz_intra",        this.fz_intra+"");       // double  // By Claude on 07/05/2026
+		properties.setProperty(prefix+"dc_center",       this.dc_center+"");      // double  // By Claude on 07/05/2026
 
 		properties.setProperty(prefix+"psf_radius",      this.psf_radius+"");     // double
 		properties.setProperty(prefix+"n_sigma",         this.n_sigma+"");        // double
@@ -408,6 +423,9 @@ public class CuasRtParameters {
 		if (properties.getProperty(prefix+"pose_img_save")!=null)   this.pose_img_save=Boolean.parseBoolean(properties.getProperty(prefix+"pose_img_save"));   // By Claude on 07/04/2026
 		if (properties.getProperty(prefix+"pose_stored")!=null)     this.pose_stored=Boolean.parseBoolean(properties.getProperty(prefix+"pose_stored"));       // By Claude on 07/04/2026
 		if (properties.getProperty(prefix+"rend_test")!=null)       this.rend_test=Boolean.parseBoolean(properties.getProperty(prefix+"rend_test"));           // By Claude on 07/05/2026
+		if (properties.getProperty(prefix+"fz_inter")!=null)        this.fz_inter=Double.parseDouble(properties.getProperty(prefix+"fz_inter"));               // By Claude on 07/05/2026
+		if (properties.getProperty(prefix+"fz_intra")!=null)        this.fz_intra=Double.parseDouble(properties.getProperty(prefix+"fz_intra"));               // By Claude on 07/05/2026
+		if (properties.getProperty(prefix+"dc_center")!=null)       this.dc_center=Double.parseDouble(properties.getProperty(prefix+"dc_center"));             // By Claude on 07/05/2026
 
 		if (properties.getProperty(prefix+"psf_radius")!=null)      this.psf_radius=Double.parseDouble(properties.getProperty(prefix+"psf_radius"));
 		if (properties.getProperty(prefix+"n_sigma")!=null)         this.n_sigma=Double.parseDouble(properties.getProperty(prefix+"n_sigma"));
@@ -491,6 +509,9 @@ public class CuasRtParameters {
 		cp.pose_img_save = this.pose_img_save; // By Claude on 07/04/2026
 		cp.pose_stored = this.pose_stored; // By Claude on 07/04/2026
 		cp.rend_test = this.rend_test; // By Claude on 07/05/2026
+		cp.fz_inter = this.fz_inter; // By Claude on 07/05/2026
+		cp.fz_intra = this.fz_intra; // By Claude on 07/05/2026
+		cp.dc_center = this.dc_center; // By Claude on 07/05/2026
 		cp.psf_radius = this.psf_radius;
 		cp.n_sigma = this.n_sigma;
 		cp.pyramid = this.pyramid;