invisible text layer

[lbr991]

I'm trying to take a non-searchable pdf and convert it to searchable pdf by superimposing an invisible text layer.

        #Parse blocks received from textract response
        blocks = response['Blocks']
        
        for block in blocks:
            if block['BlockType'] == 'WORD':
                page_number = block['Page'] - 1  # pages in PyMuPDF start from 0
                pdf_page = doc.load_page(page_number)
        
                bbox = block['Geometry']['BoundingBox']
                bbox_mupdf = fitz.Rect(
                    bbox['Left'] * pdf_page.rect.width,
                    bbox['Top'] * pdf_page.rect.height,
                    (bbox['Left'] + bbox['Width']) * pdf_page.rect.width,
                    (bbox['Top'] + bbox['Height']) * pdf_page.rect.height
                )
                
                pdf_page.insert_textbox(
                    rect=bbox_mupdf,
                    buffer=block['Text'],
                    color=None,  # Invisible color
                    overlay=True  # Overlay the text on top of existing content
                )

I want the invisible bboxes to align exactly with the original bboxes (extracted by Textract), but with this code the resulting bboxes are off (smaller).
Is insert_textbox the right way to do this?
I don't want to specify a font and font_size because then the bboxes wouldn't align perfectly.

[JorjMcKie]

There are a few options to improve things:

• Textract coordinates are relative to a page rectangle (0, 0, 1, 1). So you have to transform every Textract bbox to the bbox of the page.
• This works best by computing a matrix that does this transformation: matrix = fitz.Rect(0, 0, 1, 1).torect(page.rect).
• For every bbox in Textract you need do this to compute the bbox on the page: `bbox_mupdf = fitz.Rect(
  bbox['Left'], bbox['Top'], bbox['Left'] + bbox['Width'], bbox['Top'] + bbox['Height']) * matrix
• Textract also does not deliver fonts, so you must decide about one. Suggestion: Helvetica, i.e. font = fitz.Font("helv").
• Before you can insert text, you must compute the font size, whereas you only have bbox.width. So do tl = font.text_length(text, fontsize=1) with the text of e.g. the WORD. Then compute font size = bbox.width / tl.
• the insertion point is the next issue: if text contains no descender characters below the baseline, choose bbox.bl as insertion point: page.insert_text(bbox.bl, text, fontsizefontsize), render_mode=3).
• If a character extending below baseline exists (y, g, ...), then the insertion point must be a little higher: bbox.bl + (0, font.descender*fontsize) (font.descender is negative).
• In any case use ender_mode=3, which creates invisible text - no need to fiddle around with text colors.
• In any case do not use insert_textbox(), but insert_text() as described above.

As an aside: this is a typcal "Discussions" item, so I will transfer it.