Stirling-PDF/public/functions/removeBlankPages.js

export async function removeBlankPages(snapshot, whiteThreashold, PDFJS, OpenCV, PDFLib) {
    
    const emptyPages = await findEmptyPages(snapshot);

    console.log("Empty Pages: ", emptyPages);

    const pdfDoc = await PDFLib.PDFDocument.load(snapshot);

    // Reverse the array before looping in order to keep the indecies at the right pages. E.g. if you delete page 5 page 7 becomes page 6, if you delete page 7 page 5 remains page 5
    emptyPages.reverse().forEach(pageIndex => {
        pdfDoc.removePage(pageIndex);
    })

    return pdfDoc.save();

    async function findEmptyPages(snapshot) {
        const pdfDoc = await PDFJS.getDocument(snapshot).promise;

        const emptyPages = [];
        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 areImagesBlank(page, whiteThreashold) {
        const ops = await page.getOperatorList();
    
        for (var j=0; j < ops.fnArray.length; j++) {
            if (ops.fnArray[j] == PDFJS.OPS.paintJpegXObject || ops.fnArray[j] == PDFJS.OPS.paintImageXObject) {
                const image = page.objs.get(ops.argsArray[j][0]);
                if(image.data) {
                    return isImageBlank(image, whiteThreashold);
                }
            }
        }
        return true;
    }
    
    async function hasText(page) {
        const textContent = await page.getTextContent();
        return textContent.items.length === 0;
    }
    
    async function isImageBlank(image, threshold) {
        const src = new OpenCV.cv.Mat(image.width, image.height, OpenCV.cv.CV_8UC4);
        src.data.set(image.data);
        // Convert the image to grayscale
        const gray = new OpenCV.cv.Mat();
        OpenCV.cv.cvtColor(src, gray, OpenCV.cv.COLOR_RGBA2GRAY);
    
        // Calculate the mean value of the grayscale image
        const meanValue = OpenCV.cv.mean(gray);
    
        // Free memory
        src.delete();
        gray.delete();
    
        // Check if the mean value is below the threshold
        if (meanValue[0] <= threshold) {
            return true;
        } else {
            return false;
        }
    }
};