The aspect ratio of A4 paper is different from standard specifications.

[asylee02]

Summary

When converting an A4 PDF, the rendered image has an aspect ratio of ~1.403
instead of the expected 1.4142, due to grid quantization in scale_to_fit.

Root Cause

Step 1 — input.py: load_pdf_images renders A4 correctly at 192 DPI:

• 595.28 × 841.89 pt → 1587 × 2245 px → ratio 1.4147 ✓

Step 2 — model/util.py: scale_to_fit snaps to multiples of grid_size=28:

w_blocks = round(1587 / 28) = round(56.68) = 57  →  new_width  = 1596
h_blocks = round(2245 / 28) = round(80.18) = 80  →  new_height = 2240
Width rounds up (+9 px), height rounds down (−5 px) — both errors push
the aspect ratio in the same direction:

2240 / 1596 = 1.4035 ✗

Impact

The distortion is ~0.8%, which is negligible for OCR text accuracy, but causes
systematic error when mapping bounding box coordinates back to the original PDF
coordinate space.

Environment

- chandra-ocr v0.2.0
- IMAGE_DPI = 192, MIN_PDF_IMAGE_DIM = 1024, grid_size = 28