Output: default to PNG with PPM fallback

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
-most image viewers or converted to PNG with ImageMagick.
+The default output is PNG at `output/imgs/minkowski_sky.png`.
 
 ## 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
-with `make ENABLE_PNG=1`, then select it with a `.png` output path:
+PNG is the default output and the default build links `libpng`:
 
 ```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
 ```