C# iText7文本坐标提取题

Question

I am working on a PDF text extractor with iText7 and am noticing strange text coordinates on a certain PDF. Most documents appear to yield x and y coordinates within the height and width of the page, but one seems to yield negatives. I was wondering if there was a standard approach to dealing with negative coordinates here. This basic approach is to use positive inch measurements from a PDF and to map them to iText7 extracted text and coordinates with a 1/72 scale value for inches per dot.

I am deriving from the LocationTextExtractionStrategy and code is as follows:

        private class LocationTextListStrategy : LocationTextExtractionStrategy
        {
            private readonly List<TextRect> _textRects = new List<TextRect>();

            public List<TextRect> TextRects() => _textRects;

            public override void EventOccurred(IEventData data, EventType type)
            {
                if (!type.Equals(EventType.RENDER_TEXT))
                    return;

                var renderInfo = (TextRenderInfo)data;
                var text = renderInfo.GetCharacterRenderInfos();

                foreach (var t in text)
                {
                    if (string.IsNullOrWhiteSpace(t.GetText()))
                        continue;

                    AddTextRect(t);
                }
            }

            private void AddTextRect(TextRenderInfo t)
            {
                var letterStart = t.GetBaseline().GetStartPoint();
                var letterEnd = t.GetAscentLine().GetEndPoint();

                var newTextRect = new TextRect(
                    text: t.GetText(),
                    l: letterStart.Get(0),
                    r: letterEnd.Get(0),
                    t: letterEnd.Get(1),
                    b: letterStart.Get(1));
                
                _textRects.Add(newTextRect);
            }
        }

Answer 1

Each PDF page can have its own, custom coordinate system. It is common to have the origin in the lower left corner of the page but it is not required.

Key	Type	Value
MediaBox	rectangle	(Required; inheritable) A rectangle (see 7.9.5, "Rectangles"), expressed in default user space units, that shall define the boundaries of the physical medium on which the page shall be displayed or printed (see 14.11.2, "Page boundaries").
CropBox	rectangle	(Optional; Inheritable) A rectangle, expressed in default user space units, that shall define the visible region of default user space. When the page is displayed or printed, its contents shall be clipped (cropped) to this rectangle (see 14.11.2, "Page boundaries"). Default value: the value of MediaBox .

(ISO 32000-2:2017, Table 31 — Entries in a page)

Thus, always interpret coordinates in respect to the crop box of the page they refer to.

The iText 7 class PdfPage has matching getters.