pdfcraft/crates/engine/src/scan_colours.rs

//! Edit text on a scanned page: an OCR word is invisible text over the scan, so rewriting it must
//! also hide the scanned word. The paper colour around the word becomes the cover drawn over it
//! ([`pdfcraft_edit::BlockStyle::cover`]) and the colour of its ink becomes the new text's.

use std::sync::Arc;

use pdfcraft_edit::TextBlock;
use pdfcraft_render::{PageInfo, PageRenderer, RenderConfig, RenderRequest, Tile};

/// Device pixels per point the word's surroundings are read at.
const SCALE: f32 = 3.0;
/// The most pixels read for one paragraph; a larger area is read at a lower resolution.
const MAX_PIXELS: f32 = 4_000_000.0;
/// How far around the word (points) the paper is read.
const RING: f32 = 4.0;

/// The paper and ink colours (RGB 0–1) of invisible paragraph `b` on `page`, or `None` when the
/// paragraph is visible text or the page can't be read.
pub(crate) fn sample(cos: &pdfcraft_cos::Document, info: &PageInfo, page: usize, b: &TextBlock) -> Option<([f64; 3], [f64; 3])> {
    if !b.invisible {
        return None;
    }
    // The word's box in view space (points, y down).
    let corners = [[b.rect[0], b.rect[1]], [b.rect[2], b.rect[1]], [b.rect[0], b.rect[3]], [b.rect[2], b.rect[3]]]
        .map(|[x, y]| info.user_to_view(x as f32, y as f32));
    let (x0, x1) = corners.iter().fold((f32::INFINITY, f32::NEG_INFINITY), |(a, z), c| (a.min(c[0]), z.max(c[0])));
    let (y0, y1) = corners.iter().fold((f32::INFINITY, f32::NEG_INFINITY), |(a, z), c| (a.min(c[1]), z.max(c[1])));
    let word = [x0, y0, x1, y1];
    let area = [x0 - RING, y0 - RING, x1 + RING, y1 + RING];
    if word.iter().any(|v| !v.is_finite()) || x1 <= x0 || y1 <= y0 {
        return None;
    }
    let pixels = (area[2] - area[0]) * (area[3] - area[1]) * SCALE * SCALE;
    let scale = if pixels > MAX_PIXELS { SCALE * (MAX_PIXELS / pixels).sqrt() } else { SCALE };
    let (page_w, page_h) = (info.width * scale, info.height * scale);
    let clamp = |v: f32, max: f32| v.clamp(0.0, max.max(0.0)).floor() as u32;
    let (tx0, ty0) = (clamp(area[0] * scale, page_w), clamp(area[1] * scale, page_h));
    let (tx1, ty1) = (clamp(area[2] * scale, page_w), clamp(area[3] * scale, page_h));
    if tx1 <= tx0 || ty1 <= ty0 {
        return None;
    }
    let tile = Tile { x: tx0, y: ty0, w: tx1 - tx0, h: ty1 - ty0 };
    // The working file as it is: written out only if the document holds unsaved changes, which
    // it doesn't between edits (each one reopens the file it wrote).
    let bytes = if cos.is_modified() {
        Arc::new(pdfcraft_cos::write_incremental(cos, &pdfcraft_cos::SaveOptions::default()).ok()?)
    } else {
        cos.bytes().clone()
    };
    // The document's true colours, whatever the reading colours are set to.
    let shot = PageRenderer::new(bytes, RenderConfig::default()).render(RenderRequest { page, scale, tile: Some(tile), ..Default::default() });
    if shot.error.is_some() {
        return None;
    }
    let pad = ((y1 - y0) * 0.1).clamp(0.5, 2.0);
    let (mut paper, mut ink) = (Vec::new(), Vec::new());
    for (i, px) in shot.rgba.as_chunks::<4>().0.iter().enumerate() {
        let (col, row) = (i % shot.width.max(1) as usize, i / shot.width.max(1) as usize);
        let x = (tx0 as f32 + col as f32 + 0.5) / scale;
        let y = (ty0 as f32 + row as f32 + 0.5) / scale;
        let rgb = [px[0], px[1], px[2]].map(|c| f64::from(c) / 255.0);
        if x >= x0 && x <= x1 && y >= y0 && y <= y1 {
            ink.push(rgb);
        } else if x < x0 - pad || x > x1 + pad || y < y0 - pad || y > y1 + pad {
            paper.push(rgb);
        }
    }
    let luma = |c: &[f64; 3]| 0.299 * c[0] + 0.587 * c[1] + 0.114 * c[2];
    let by_luma = |a: &[f64; 3], b: &[f64; 3]| luma(a).total_cmp(&luma(b));
    paper.sort_by(by_luma);
    ink.sort_by(by_luma);
    // The paper is the ring's median, so a table rule or a neighbouring letter in it doesn't count.
    let paper = paper.get(paper.len() / 2).copied().unwrap_or([1.0; 3]);
    // The ink is the average of the word's darkest pixels; a box without any is printed in black.
    let darkest = ink.get(..ink.len().div_ceil(8)).unwrap_or_default();
    let ink = if darkest.is_empty() {
        [0.0; 3]
    } else {
        let sum = darkest.iter().fold([0.0; 3], |s, c| [s[0] + c[0], s[1] + c[1], s[2] + c[2]]);
        sum.map(|v| v / darkest.len() as f64)
    };
    let ink = if luma(&paper) - luma(&ink) < 0.25 { [0.0; 3] } else { ink };
    Some((paper, ink))
}