Drawing large PDF pages is slow, inefficient, and fails

[吴吴]

I think an API is needed to solve the problem of rendering large PDF pages. That is, you can specify the rendering area within the PDF page and draw it to a DC or hbitmap of a specified size. This can solve the problem of slow drawing of large pages by Pdfium, and you can draw PDF pages of any magnification in the program without any lag.

[geisserml]

Rendering only a specific sub-area of a page is already possible with the existing APIs.
The idea is basically to shift start_x/start_y into negatives and use a smaller bitmap.[^1]

size_x/size_y stay the non-cropped, scaled size (if you change these, I believe the rendering would be squashed or stretched).

This way, you should be able to avoid the need to create an extremely large bitmap that you mentioned in https://groups.google.com/g/pdfium/c/Bq488Mht6l8

By the way, please don't create multiple mailing list threads for the same topic, as this causes confusion for future readers.

[^1]: For an example, see e.g. how pypdfium2's higher-level rendering helper implements the `crop` parameter: https://github.com/pypdfium2-team/pypdfium2/blob/a40e02019517eb03b30bf7774fa21bec4aee9e17/src/pypdfium2/_helpers/page.py#L447-L482

[吴吴]

This is not what I want. What I want is to crop the area of this page and display it in the hbitmap. For example, the left: 200, top: 400, right: 400, bottom: 200 in the cropped page is displayed in the area left: 0 right: 600, bottom: 0, top: 600.

[geisserml]

I'm not sure I understand correctly, but it sounds to me as though you don't just want to render a sub-section of the page, but also draw it only onto a specific sub-rectangle of the bitmap?

Indeed, I suppose that's not possible with FPDF_RenderPage() or FPDF_RenderPageBitmap() directly.

A workaround might be to temporarily change the page cropbox using FPDFPage_SetCropBox() from fpdf_transformpage.h, which is taken into account on rendering.

Then you can use a larger bitmap and shift start_x/start_y inward for left/top margins.

There's also FPDF_RenderPageBitmapWithMatrix() which takes a `clipping` rectangle, but it seems that you want to use Windows GDI, whereas this API is for FPDF_BITMAP.

(Though, I seem to remember reading that pdfium's GDI renderer might be deprecated eventually?)