Stirling-PDF/shared-operations/src/functions/common/detectEmptyPages.ts

import { PdfFile } from "../../wrappers/PdfFile";
import { PDFPageProxy } from "pdfjs-dist/types/src/display/api";
import { Image, ImageKind } from "image-js";

import { getImagesOnPage, PDFJSImage } from "./getImagesOnPage.js";

export async function detectEmptyPages(file: PdfFile, whiteThreashold: number): Promise<number[]> {
    const pdfDoc = await file.pdfJsDocument;

    const emptyPages: number[] = [];
    for (let i = 1; i <= pdfDoc.numPages; i++) {
        const page = await pdfDoc.getPage(i);
        console.log("Checking page " + i);

        if(!await hasText(page)) {
            console.log(`Found text on Page ${i}, page is not empty`);
            continue;
        }

        if(!await areImagesBlank(page, whiteThreashold)) {
            console.log(`Found non white image on Page ${i}, page is not empty`);
            continue;
        }

        console.log(`Page ${i} is empty.`);
        emptyPages.push(i - 1);
    }
    return emptyPages;
}

async function hasText(page: PDFPageProxy): Promise<boolean> {
    const textContent = await page.getTextContent();
    return textContent.items.length === 0;
}

async function areImagesBlank(page: PDFPageProxy, threshold: number): Promise<boolean> {
    const images = await getImagesOnPage(page);
    for (const image of images) {
        if(!await isImageBlank(image as any, threshold))
            return false;
    }
    return true;
}

// TODO: Fix this function
async function isImageBlank(image: PDFJSImage, threshold: number): Promise<boolean> {
    const img = new Image(image.width, image.height, image.data, { kind: "RGB" as ImageKind }); // TODO: Maybe respect image.kind and convert accordingly, needs to be tested with a pdf with alpha-image
    const grey = img.grey();
    const mean = grey.getMean();
    return mean[0] <= threshold;
}