Building Fillable Forms: spec format and workflow

The same JSON spec drives both pdf_form_layout.py (design lint) and pdf_make_form.py (AcroForm build). Coordinates are PDF points, origin at the bottom-left of the page (1 pt = 1/72 inch; A4 is 595.27 x 841.89, letter is 612 x 792).

Spec shape

{
  "title": "Example Intake Form",
  "author": "example-author",
  "page_size": "A4",
  "page_count": 1,
  "fields": [
    {"name": "surname", "type": "text", "page": 1,
     "label": "Surname", "label_box": [72, 700, 150, 714],
     "entry_box": [160, 696, 400, 716],
     "value": "", "tooltip": "Family name"},

    {"name": "agree", "type": "checkbox", "page": 1,
     "label": "I agree", "label_box": [72, 660, 150, 674],
     "entry_box": [160, 658, 176, 674], "checked": false},

    {"name": "color", "type": "radio", "page": 1,
     "label": "Color", "label_box": [72, 620, 150, 634],
     "entry_box": [160, 616, 400, 636],
     "options": ["red", "blue"], "value": "blue"},

    {"name": "size", "type": "dropdown", "page": 1,
     "label": "Size", "label_box": [72, 580, 150, 594],
     "entry_box": [160, 576, 300, 596],
     "options": ["small", "large"], "value": "small"}
  ]
}

page_size: "A4", "letter", or [width, height] in points.
page_count: optional; extended automatically to the highest field page.
Boxes are [x0, y0, x1, y1] with x0 < x1, y0 < y1.
label is drawn as static text near label_box; omit it (and label_box) for unlabeled fields.
radio: the buttons are laid out left-to-right inside entry_box, one slot per option, each with a small static caption. value pre-selects an option by its export name.
dropdown maps to an AcroForm choice (combo) field.

Field types → what pdf_read.py --fields reports

Spec type	/FT	value format after fill
text	/Tx (`text`)	the string
checkbox	/Btn (`button`)	`/Yes` or `/Off`
radio	/Btn (`button`)	`/<export>`, e.g. `/red`
dropdown	/Ch (`choice`)	the option string

When filling with pdf_fill_form.py, checkboxes accept true/false; radio values need the leading slash ("/red"); dropdown values are the plain option string.

Layout lint rules (pdf_form_layout.py)

Per field, on its declared page:

boxes must be well-formed and inside the page bounds;
entry boxes must be at least 8x8 pt (12 pt tall for text/dropdown);
no two entry boxes on the same page may overlap (the second and later fields of an overlapping cluster are flagged);
a label must sit within 150 pt of its entry box and must not overlap it.

Exit code 0 = clean, 1 = at least one problem; the JSON report lists per-field problems. Lint the spec BEFORE building — fixing numbers in JSON is cheaper than debugging a rendered PDF.

Visual review loop

python3 scripts/pdf_form_layout.py spec.json --render-overlay overlay.png [--pdf built.pdf]

Red rectangles = entry boxes (with field names), blue = label boxes. Without --pdf the overlay is drawn on a blank page (PIL-only, always works); with --pdf the real page is rasterized underneath (needs pypdfium2 or pdftoppm — otherwise the report says "rendered": false with install hints). Feed the PNG to vision_analyze and ask specifically about collisions, alignment, and stray labels.

Radio-group quirks (reportlab + pypdf)

reportlab requires at least two radio() calls per group; a single-option radio group produces a broken field.
Pre-selecting is done at build time via "value"; changing selection later via pdf_fill_form.py needs the slashed export name ("/red").
Some viewers render reportlab radio appearances inconsistently after a pypdf fill; verify with --fields (data truth) plus a rendered page image (visual truth) rather than either alone.
Flattening radio groups is the least reliable flatten case — check the output image before shipping.

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