Building PDF Stamp Placement Without a Framework — Click to Place, Drag to Move

#rust #tauri #programming #pdf

All tests run on an 8-year-old MacBook Air.

Sign & Fill lets users click anywhere on a PDF page to place a stamp or signature image — then drag it to the exact position before committing.

No PDF form framework. No annotation API. Just a coordinate system and a content stream injection.

The coordinate problem

PDF coordinates start at the bottom-left. Canvas/screen coordinates start at the top-left.

Every click position needs conversion:

function screenToPdfCoords(
  clickX: number,
  clickY: number,
  canvasHeight: number,
  pageHeight: number,
  scale: number
): { x: number; y: number } {
  return {
    x: clickX / scale,
    // Flip Y axis: PDF origin is bottom-left
    y: pageHeight - (clickY / scale),
  };
}

Get this wrong and stamps appear at the mirror position of where you clicked.

Drag-to-position UI

The stamp renders as an absolutely-positioned overlay on the canvas. Dragging updates state, not the PDF:

const [stampPos, setStampPos] = useState({ x: 0, y: 0 });
const [isDragging, setIsDragging] = useState(false);

const handleMouseMove = (e: React.MouseEvent) => {
  if (!isDragging) return;
  setStampPos({
    x: e.clientX - dragOffset.x,
    y: e.clientY - dragOffset.y,
  });
};

Only when the user clicks "Commit" does the position get converted to PDF coordinates and written to the content stream.

Writing the stamp to the PDF

pub fn stamp_page(
    doc: &mut Document,
    page_id: ObjectId,
    image_id: ObjectId,
    x: f64,
    y: f64,
    width: f64,
    height: f64,
) -> Result<(), lopdf::Error> {
    let content = format!(
        "q {} 0 0 {} {} {} cm /HiyokoImg Do Q\n",
        width, height, x, y
    );

    // Register image in page resources
    ensure_image_resource(doc, page_id, image_id)?;

    // Append to page content stream
    append_to_page_content(doc, page_id, content.as_bytes())?;

    Ok(())
}

The cm operator sets the transformation matrix. The Do operator renders the image resource. No annotation layer — the stamp is burned directly into the content stream.