Output: default to PNG with PPM fallback

Makefile

@@ -2,12 +2,15 @@ CC ?= cc
 CFLAGS ?= -std=c11 -O2 -Wall -Wextra -Wpedantic
 OPENMP_FLAGS ?= -fopenmp
 LDLIBS ?= -lm
-ENABLE_PNG ?= 0
+ENABLE_PNG ?= 1
 SPACETIME ?= minkowski
 
 ifeq ($(ENABLE_PNG),1)
 CPPFLAGS += -DENABLE_PNG
 LDLIBS += -lpng
+IMAGE_EXT := png
+else
+IMAGE_EXT := ppm
 endif
 
 COMMON_SOURCES := $(filter-out src/main.c src/spacetime_minkowski.c src/spacetime_schwarzschild.c,$(wildcard src/*.c))
@@ -40,7 +43,7 @@ build:
 
 run: $(TARGET)
 	mkdir -p output/imgs
-	./$(TARGET) --catalog assets/sky_grid_5deg.csv --output output/imgs/$(SPACETIME)_sky.ppm
+	./$(TARGET) --catalog assets/sky_grid_5deg.csv --output output/imgs/$(SPACETIME)_sky.$(IMAGE_EXT)
 
 minkowski:
 	$(MAKE) SPACETIME=minkowski all

README.md

@@ -21,8 +21,7 @@ which equalizes their CIE/linear-sRGB luminance under the renderer's blackbody
 integration. Longitude boundaries belong to the sector to their east and the
 equator to the northern hemisphere, so boundary stars have a deterministic
 color.
-The output is a binary PPM at `output/imgs/minkowski_sky.ppm`; it can be inspected by
+The default output is PNG at `output/imgs/minkowski_sky.png`.
-most image viewers or converted to PNG with ImageMagick.
 
 ## Movie PNG sequence (Phase A)
 
@@ -33,7 +32,7 @@ Minkowski benchmark—two coordinate seconds at 30 fps, accelerating from rest
 to about `0.95c`—then render its numbered PNG frames:
 
 ```sh
-make clean && make ENABLE_PNG=1
+make clean && make
 mkdir -p output/imgs
 ./build/minkowski_sky --write-minkowski-accel-track output/minkowski_accel_2s.csv \
   --duration 2 --fps 30 --proper-acceleration 1.52
@@ -55,15 +54,18 @@ synthetic test catalog uses global default exposure `1e-3`; the accelerated
 benchmark explicitly uses `1e-5` because its physical Doppler blue shift
 otherwise clips the later frames.
 
-PNG output is optional so the default build has no `libpng` dependency. Build
+PNG is the default output and the default build links `libpng`:
-with `make ENABLE_PNG=1`, then select it with a `.png` output path:
 
 ```sh
-make clean && make ENABLE_PNG=1
+make clean && make
 mkdir -p output/imgs
 ./build/minkowski_sky --output output/imgs/minkowski_sky.png
 ```
 
+If `libpng` is unavailable, rebuild with `make clean && make ENABLE_PNG=0`.
+That intentionally selects the binary-PPM fallback, whose default path is
+`output/imgs/minkowski_sky.ppm`; pass a `.ppm` path for explicit output.
+
 Run the flat-spacetime geodesic regression with:
 
 ```sh
@@ -79,7 +81,7 @@ Kerr--Schild coordinates (regular at the horizon), then render the test catalog
 to PNG:
 
 ```sh
-make clean && make SPACETIME=schwarzschild ENABLE_PNG=1
+make clean && make SPACETIME=schwarzschild
 mkdir -p output/imgs
 ./build/schwarzschild_sky --catalog assets/sky_grid_5deg.csv \
   --width 640 --height 360 --coarse-cell-pixels 8 --fov-deg 60 \
@@ -113,13 +115,13 @@ Useful options:
 
 ```sh
 ./build/minkowski_sky --width 1920 --height 1080 --fov-deg 30 \
-  --catalog assets/sky_grid_5deg.csv --output output/imgs/frame.ppm
+  --catalog assets/sky_grid_5deg.csv --output output/imgs/frame.png
 ./build/minkowski_sky --catalog assets/2mass/processed/2mass_psc_m31_0p5deg_stars.csv \
   --look-ra-deg 10.6847083 --look-dec-deg 41.26875 --fov-deg 1.8 \
-  --exposure 1e15 --output output/imgs/2mass_m31.ppm
+  --exposure 1e15 --output output/imgs/2mass_m31.png
 ./build/minkowski_sky --catalog assets/2mass/processed/2mass_psc_m44_1p0deg_stars.csv \
   --look-ra-deg 129.99165 --look-dec-deg 19.54139 --fov-deg 2.0 \
-  --exposure 1e15 --width 1920 --height 1920 --output output/imgs/2mass_m44.ppm
+  --exposure 1e15 --width 1920 --height 1920 --output output/imgs/2mass_m44.png
 ./build/minkowski_sky --write-catalog assets/sky_grid_5deg.csv
 ```
 

assets/2mass/processed/all_sky/README.md

@@ -38,7 +38,7 @@ tile tables after successful cleaning unless `--keep-raw` is passed.
 Use the directory mode rather than `--catalog` for this partitioned dataset:
 
 ```sh
-make ENABLE_PNG=1 SPACETIME=minkowski all
+make SPACETIME=minkowski all
 ./build/minkowski_sky --all-sky-catalog assets/2mass/processed/all_sky \
   --fov-deg 2 --look-ra-deg 180 --look-dec-deg -75 \
   --exposure 1e15 \
@@ -87,8 +87,9 @@ with the rotating observer fixture:
 Missing tiles are skipped until the downloader has produced them. Fully
 contained tiles skip per-star source-triangle containment; partial tiles retain
 that exact check. For 2MASS apparent-solid-angle amplitudes, start with
-`--exposure 1e15`; use PNG (`make ENABLE_PNG=1`) for substantially smaller
+`--exposure 1e15`; PNG is the default output format. If libpng is unavailable,
-single-frame output than binary PPM.
+use the PPM fallback with `make clean && make ENABLE_PNG=0` and a `.ppm`
+output path.
 
 The two existing fields imply roughly 15--17 GiB of cleaned CSV for the full
 PSC. Their raw response rows imply 70--75 GiB if every source appeared once.

src/main.c

@@ -96,6 +96,11 @@ static int parse_moffat_beta(const char *text, double *value) {
 
 static int parse_args(int argc, char **argv, Settings *s,
                       const char **write_path) {
+#ifdef ENABLE_PNG
+  const char *default_output_path = "output/imgs/minkowski_sky.png";
+#else
+  const char *default_output_path = "output/imgs/minkowski_sky.ppm";
+#endif
   *s = (Settings){.width = 1280,
                   .height = 720,
                   .coarse_cell_pixels = 16,
@@ -106,7 +111,7 @@ static int parse_args(int argc, char **argv, Settings *s,
                   .observer_radius = 30.0,
                   .psf = {2.7, 4.5},
                   .catalog_path = "assets/sky_grid_5deg.csv",
-                  .output_path = "output/imgs/minkowski_sky.ppm",
+                  .output_path = default_output_path,
                   .frames_prefix = "frame",
                   .movie_duration = 2.0,
                   .movie_fps = 30.0,
@@ -246,13 +251,18 @@ static int render_frame(const Settings *s, StarCatalog *catalog,
 
 static int frame_output_path(char path[PATH_MAX], const Settings *s,
                              size_t frame_id) {
+#ifdef ENABLE_PNG
+  const char *extension = "png";
+#else
+  const char *extension = "ppm";
+#endif
   struct stat st;
   if (s->frames_dir == NULL || s->frames_prefix == NULL ||
       strchr(s->frames_prefix, '/') != NULL || stat(s->frames_dir, &st) ||
       !S_ISDIR(st.st_mode))
     return -1;
-  const int written = snprintf(path, PATH_MAX, "%s/%s_%06zu.png", s->frames_dir,
+  const int written = snprintf(path, PATH_MAX, "%s/%s_%06zu.%s", s->frames_dir,
-                               s->frames_prefix, frame_id);
+                               s->frames_prefix, frame_id, extension);
   return written < 0 || written >= PATH_MAX ? -1 : 0;
 }
 

src/optics.c

@@ -120,6 +120,7 @@ static unsigned char tonemap_channel(double hdr_value)
     return (unsigned char)lround(255.0 * clamp(display, 0.0, 1.0));
 }
 
+#ifndef ENABLE_PNG
 static int write_tonemapped_ppm(const char *path, const double *hdr, int width,
                                 int height)
 {
@@ -132,6 +133,7 @@ static int write_tonemapped_ppm(const char *path, const double *hdr, int width,
     }
     return fclose(file) == 0 ? 0 : -1;
 }
+#endif
 
 #ifdef ENABLE_PNG
 static int write_tonemapped_png(const char *path, const double *hdr, int width,
@@ -171,13 +173,16 @@ done:
 int write_tonemapped_image(const char *path, const double *hdr, int width, int height)
 {
     const size_t path_length = strlen(path);
-    if (path_length >= 4 && strcmp(path + path_length - 4, ".png") == 0) {
 #ifdef ENABLE_PNG
+    if (path_length >= 4 && strcmp(path + path_length - 4, ".png") == 0)
         return write_tonemapped_png(path, hdr, width, height);
+    fputs("PNG output is enabled; use a .png output path.\n", stderr);
+    return -1;
 #else
-        fputs("PNG output is disabled; rebuild with make ENABLE_PNG=1.\n", stderr);
+    if (path_length >= 4 && strcmp(path + path_length - 4, ".ppm") == 0)
-        return -1;
+        return write_tonemapped_ppm(path, hdr, width, height);
+    fputs("PNG output is unavailable; use a .ppm output path or rebuild with libpng.\n",
+          stderr);
+    return -1;
 #endif
-    }
-    return write_tonemapped_ppm(path, hdr, width, height);
 }

src/optics.h

@@ -13,7 +13,7 @@ LinearRgb blackbody_to_linear_rgb(double temperature_K);
 void splat_moffat(double *hdr, int width, int height, double x, double y,
                   LinearRgb color, double flux,
                   const PointSpreadFunction *psf);
-/* Writes PPM by default; a .png path requires a build with ENABLE_PNG=1. */
+/* Writes PNG when built with libpng; non-libpng builds use PPM fallback. */
 int write_tonemapped_image(const char *path, const double *hdr, int width,
                            int height);
 

video_rendering_plan.md

@@ -41,8 +41,8 @@ SampleRequest -> RayPool (SoA, inactive / active / terminated)
 1. 新增 `observer_track.h/.c`，将目前的 `ObserverState` 提升为可插值的 `ObserverTrack`：每个 sample 保存 `t_coordinate`、`tau`、`x[3]` 和完整 tetrad。提供加载、严格单调时间校验、区间查找、插值和销毁函数。原有固定 observer 构造函数保留，作为只含一个 sample 的兼容快捷方式。
 2. 第一版轨迹输入采用人可审阅的 21 列 CSV，字段固定为 `t,tau,x,y,z,e0t,e0x,...,e3z`；不在 renderer 内从“速度曲线”临时猜 tetrad。loader 做完整性、有限数与严格单调时间检查；tetrad 的 metric 正交归一由轨迹生成器和 backend-aware regression 验证。另提供生成器工具/模式为测试写出 CSV。这样 NR 生产轨迹、解析测试轨迹和以后离线 trajectory generator 使用完全相同的 renderer 输入。
 3. 加入 `movie.h/.c`，定义 `MovieSettings`、`MovieFrame`（`frame_id`、`t_camera`、插值得到的 observer state、mesh、HDR image）与 `Movie`。用 `start-time`、`duration`、`fps` 生成 frame 时间表；先拒绝轨迹覆盖范围外的时间。
-4. 将 `render_frame()` 中与单帧无关的工作拆成“初始化 mesh / 安装 endpoint / splat HDR / 写出”四个明确步骤。保持现有单帧命令和 PPM/PNG 行为可用；movie 模式只是调用同一套 frame rendering 与 optics。
+4. 将 `render_frame()` 中与单帧无关的工作拆成“初始化 mesh / 安装 endpoint / splat HDR / 写出”四个明确步骤。默认构建写 PNG；未启用 libpng 时单帧和 movie 序列统一退回 PPM。movie 模式只是调用同一套 frame rendering 与 optics。
-5. 新增 `output.h/.c`，从 `optics` 移出文件命名和写图逻辑。PNG 仍复用当前 libpng 路径，movie PNG 命名固定为 `<output-dir>/<prefix>_%06u.png`，例如 `output/imgs/minkowski_accel_000042.png`。在创建前验证目录、prefix 与帧号范围，写入失败应指出 frame id 和路径。
+5. 新增 `output.h/.c`，从 `optics` 移出文件命名和写图逻辑。默认 movie PNG 仍复用当前 libpng 路径，命名固定为 `<output-dir>/<prefix>_%06u.png`，例如 `output/imgs/minkowski_accel_000042.png`；无 libpng 的 PPM fallback 只替换扩展名。创建前验证目录、prefix 与帧号范围，写入失败应指出 frame id 和路径。
 
 验收：原有 `make test` 和两种单帧 backend 的输出保持不变；新测试验证 CSV 拒绝缺列、非有限数与非单调时间，frame 时间表精确包含首帧且不越界，PNG 序号无冲突。此阶段不要求 FFmpeg，也不改变 ray tracing 的调度方式。