LAXMAYDAY/krea2-scene-linear-hdr-dataset

Krea 2 scene-linear HDR — LogC4 training dataset

320 scene-linear HDR training images for the Krea 2 HDR LoRA (see model: LAXMAYDAY/krea2-scene-linear-hdr-lora · tools: krea2-hdr-toolkit).

Built by re-projecting CC0 Poly Haven HDRIs into normal perspective scene images, exposure-normalizing (mid-gray → 0.18), and LogC4-encoding to [0,1] so a wide scene-linear range is carried through an ordinary 8-bit pipeline.

What each sample is

• images/<slug>__v<k>.png — an 8-bit RGB PNG holding the LogC4 code. It looks flat/ grey (normal for log footage); the HDR lives in the encoding. Recover scene-linear radiance with inverse-LogC4 (code 1.0 → ~470× mid-gray).
• images/<slug>__v<k>.txt — caption (trigger phrase scene-linear HDR, high dynamic range, physically based lighting).
• manifest.jsonl — one row per image: encoding (LogC4), anchor (0.18), exposure_scale, crop yaw/pitch/fov, and decoded HDR stats (hdr_peak, hdr_stops, frac_gt1).
• manifest_hdr_rich.jsonl — the recommended training subset (229 crops with source peak > 2× and ≥ 3 stops). Train on this.

Decode to scene-linear (EXR)

import numpy as np, imageio.v2 as iio
_A=(2**18-16)/117.45; _B=(1023-95)/1023; _C=95/1023
import math
_S=(7*math.log(2)*2**(7-14*_C/_B))/(_A*_B); _T=(2**(14*(-_C/_B)+6)-64)/_A
def logc4_to_linear(V):            # V in [0,1] -> scene-linear (>>1 for highlights)
    V=np.asarray(V,np.float64)
    return np.where(V>=_C,(2.0**(14*(V-_C)/_B+6)-64)/_A, V*_S+_T)
lin = logc4_to_linear(iio.imread("images/....png").astype(np.float64)/255.0)  # scene-linear HDR

(the toolkit's decode_to_exr.py writes this straight to 32-bit EXR.)

Verified

93% of images decode to real HDR (peak > 1.0). Same PNG read naively as SDR peaks at 1.0; read correctly via inverse-LogC4 it reaches ~470×. The HDR is in the file, log-encoded.

Notes & limits

• 8-bit is precision, not range: 256 log-spaced codes cover 0–470× (~17 codes/stop). The frozen VAE is ~8-bit effective anyway, so 8-bit LogC4 is adequate.
• Single-plane LogC4 caps peak radiance ~470×; the sun disk clips (true sun magnitude needs a multi-plane scheme). ~9% of crops are genuinely low-DR (overcast) — excluded by the hdr-rich manifest.
• Source HDRIs are Poly Haven (CC0); this dataset is released CC0.
• Caption caveat: some "visible sun" captions sit on crops where the sun is out of frame (a cropping artifact documented in the project report).