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Feat: Add analytic moving Alcubierre spacetime backend

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Add a third analytic spacetime provider for the moving Alcubierre warp
bubble, x_s(t) = v_s*t with x_s(0) = 0.  The lab slices stay flat, so
alpha = 1, gamma_ij = delta_ij, beta^x = -v_s f(r_s), and K_ij follows
from the flat spatial metric; the time dependence enters through the
moving shape argument.  The exotic matter is treated as transparent, so
there is no capture: rays are only ACTIVE or ESCAPED, with a bubble-
centered escape radius R + 20/sigma.

Expose --alcubierre-vs, --alcubierre-radius, and --alcubierre-sigma
(|v_s| < 1).  Scale the per-ray step budget with the escape radius and
1/(1-|v_s|) so near-luminal grazing rays still escape, and reject
parameter combinations whose worst-case budget exceeds the cap.  Use a
cancellation-free shape formula for small sigma*R and reject derived
escape radii that overflow.

The regression test covers metric reconstruction, d_beta/K finite
differences, the translation isometry, small-sigma stability, the flat
limit, reflection symmetry, step convergence, and a near-luminal slow
ray.  build.md, usage.md, and README.md document the backend.
This commit is contained in:
wyj committed 2026-10-03 03:09:26 -04:00
1 parent 4e34780fa9
commit 3cbf5ba386
8 files changed
+688 -16
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Makefile
+15 -5
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@@ -29,14 +29,14 @@ else
IMAGE_EXT := ppm
endif
COMMON_SOURCES := $(filter-out src/main.c src/dummy_psf.c src/fast_psf_fftw.c src/spacetime_minkowski.c src/spacetime_schwarzschild.c,$(wildcard src/*.c))
COMMON_SOURCES := $(filter-out src/main.c src/dummy_psf.c src/fast_psf_fftw.c src/spacetime_minkowski.c src/spacetime_schwarzschild.c src/spacetime_alcubierre.c,$(wildcard src/*.c))
PROVIDER_SOURCE := src/spacetime_$(SPACETIME).c
BUILD_DIR := build/$(BUILD_TYPE)
TARGET_BASENAME := $(SPACETIME)_sky
OBJECT_DIR := $(BUILD_DIR)/obj/$(SPACETIME)
CORE_MINKOWSKI_SOURCES := $(COMMON_SOURCES) src/spacetime_minkowski.c
.PHONY: all backend clean run test tone-map-test sensor-bloom-test sensor-bloom-bench hip-psf-test hip-psf-bench fast-psf-fftw-bench minkowski schwarzschild FORCE
.PHONY: all backend clean run test tone-map-test sensor-bloom-test sensor-bloom-bench hip-psf-test hip-psf-bench fast-psf-fftw-bench minkowski schwarzschild alcubierre FORCE
ifneq ($(filter 0 1,$(PSF_EVENT_SINK)),$(PSF_EVENT_SINK))
$(error Unknown PSF_EVENT_SINK '$(PSF_EVENT_SINK)'; choose 0 or 1)
@@ -85,8 +85,10 @@ ifeq ($(SPACETIME),minkowski)
BACKEND_CPPFLAGS := -DSPACETIME_MINKOWSKI
else ifeq ($(SPACETIME),schwarzschild)
BACKEND_CPPFLAGS := -DSPACETIME_SCHWARZSCHILD
else ifeq ($(SPACETIME),alcubierre)
BACKEND_CPPFLAGS := -DSPACETIME_ALCUBIERRE
else
$(error Unknown SPACETIME '$(SPACETIME)'; choose minkowski or schwarzschild)
$(error Unknown SPACETIME '$(SPACETIME)'; choose minkowski, schwarzschild, or alcubierre)
endif
ifeq ($(ENABLE_HDR),1)
@@ -106,6 +108,7 @@ TEST_OUT_DIR := $(OBJECT_DIR)/$(HDR_BUILD_TAG)
TEST_TARGET := $(TEST_OUT_DIR)/test_geodesic
FRAME_TEST_TARGET := $(TEST_OUT_DIR)/test_frame
SCHWARZSCHILD_TEST_TARGET := $(TEST_OUT_DIR)/test_schwarzschild
ALCUBIERRE_TEST_TARGET := $(TEST_OUT_DIR)/test_alcubierre
OBSERVER_TRACK_TEST_TARGET := $(TEST_OUT_DIR)/test_observer_track
CATALOG_PREFETCH_TEST_TARGET := $(TEST_OUT_DIR)/test_catalog_prefetch
HIP_PSF_TEST_TARGET := $(TEST_OUT_DIR)/test_hip_psf
@@ -142,7 +145,7 @@ RENDER_DEPS := $(RENDER_OBJECTS:.o=.d)
ifneq ($(filter command\ line environment environment\ override,$(origin SPACETIME)),)
all: backend
else
all: minkowski schwarzschild
all: minkowski schwarzschild alcubierre
endif
minkowski:
@@ -151,6 +154,9 @@ minkowski:
schwarzschild:
$(MAKE) SPACETIME=schwarzschild ENABLE_HDR=$(ENABLE_HDR) backend
alcubierre:
$(MAKE) SPACETIME=alcubierre ENABLE_HDR=$(ENABLE_HDR) backend
# Build exactly the selected backend/configuration, e.g.
# make SPACETIME=schwarzschild ENABLE_HDR=1 backend
backend: $(TARGET)
@@ -203,6 +209,9 @@ $(FRAME_TEST_TARGET): tests/test_frame.c $(CORE_MINKOWSKI_SOURCES) $(CPU_FFTW_SO
$(SCHWARZSCHILD_TEST_TARGET): tests/test_schwarzschild.c $(COMMON_SOURCES) src/spacetime_schwarzschild.c $(CPU_FFTW_SOURCES) | $(TEST_OUT_DIR)
$(CC) $(CPPFLAGS) $(BUILD_CPPFLAGS) $(CFLAGS) $(BUILD_CFLAGS) $(OPENMP_FLAGS) -Isrc $^ $(LDLIBS) -o $@
$(ALCUBIERRE_TEST_TARGET): tests/test_alcubierre.c $(COMMON_SOURCES) src/spacetime_alcubierre.c $(CPU_FFTW_SOURCES) | $(TEST_OUT_DIR)
$(CC) $(CPPFLAGS) $(BUILD_CPPFLAGS) $(CFLAGS) $(BUILD_CFLAGS) $(OPENMP_FLAGS) -Isrc $^ $(LDLIBS) -o $@
$(OBSERVER_TRACK_TEST_TARGET): tests/test_observer_track.c $(COMMON_SOURCES) $(CPU_FFTW_SOURCES) | $(TEST_OUT_DIR)
$(CC) $(CPPFLAGS) $(BUILD_CPPFLAGS) $(CFLAGS) $(BUILD_CFLAGS) $(OPENMP_FLAGS) -Isrc $^ $(LDLIBS) -o $@
@@ -244,12 +253,13 @@ FAST_PSF_FFTW_TEST_DEP :=
FAST_PSF_FFTW_TEST_RUN :=
endif
test: $(CAMERA_TEST_TARGETS) $(TEST_TARGET) $(FRAME_TEST_TARGET) $(SCHWARZSCHILD_TEST_TARGET) $(OBSERVER_TRACK_TEST_TARGET) $(CATALOG_PREFETCH_TEST_TARGET) $(FAST_PSF_FFTW_TEST_DEP) $(TONE_MAP_TEST_TARGET) $(SENSOR_BLOOM_TEST_TARGET)
test: $(CAMERA_TEST_TARGETS) $(TEST_TARGET) $(FRAME_TEST_TARGET) $(SCHWARZSCHILD_TEST_TARGET) $(ALCUBIERRE_TEST_TARGET) $(OBSERVER_TRACK_TEST_TARGET) $(CATALOG_PREFETCH_TEST_TARGET) $(FAST_PSF_FFTW_TEST_DEP) $(TONE_MAP_TEST_TARGET) $(SENSOR_BLOOM_TEST_TARGET)
$(TEST_OUT_DIR)/test_observer_minkowski
$(TEST_OUT_DIR)/test_observer_schwarzschild
$(TEST_TARGET)
$(FRAME_TEST_TARGET)
$(SCHWARZSCHILD_TEST_TARGET)
$(ALCUBIERRE_TEST_TARGET)
$(OBSERVER_TRACK_TEST_TARGET)
$(CATALOG_PREFETCH_TEST_TARGET)
$(FAST_PSF_FFTW_TEST_RUN)