Is there a precise way to measure text size in a specific font in Python (3.7)?

Question

I've been through several questions and unsatisfying answers about this. Pillow can use ttf fonts but draw.textsize(text,font) and font.getsize(text) return the same wrong dimensions.

opencv.getTextSize is correct, but only offers a few fonts.

I got precise results by drawing text with pillow on an image dimensioned larger than the approximate size it tells to need, in step 2 converting that image to an opencv pixel array and in step 3 cropping it precisely.

That has two problems:

1. It's too slow. So is there a fast way to extract a central part of an array cutting off boundaries with all white pixels or everything under a threshold value?
2. Even knowing the exact dimensions, drawing a text at the origin computed with the exact textsize within a larger box, ie centering a digit in a square, it's still not drawn centered. I assume pillow counts empty pixels for kerning or such font related dimensions into the area textsize or getsize returns. So ultimately this might also be a thing about specific bad fonts, but I get the same wrong results for Arial, Arialbd, Times New Roman or Cambria, to name jus a few.

Is there any chance to get this precise? In the end I'd like to produce boxes quite narrow bounding an inner number (for math related learning/demo video). It's not just to be pedantic, but fiddling with manual corrections would make generating such videos a nightmare, the base tools I now build up should be as precise as possible.

Edit: In my case the texts are numbers, and they neither have ascenders nor descenders. I also observe the horizontal centering works okay, it's just the vertical position is too low. I'll see what I get when taking the offsets into account.

Answer 1

A font has several different metrics, as illustrated by this image from the Pillow appendix on Text Anchors:

The font.getsize() method gives you the size of the text to the ascender line, and not from the top line. If your actual text doesn't include ascenders, that can look like there is too much space.

Pillow 8.0.0 (released a few weeks ago, 2020-10-14) has added methods that address these issues, at least for TypeFont fonts, and you really want to use the new ImageDraw.textbbox() or FreeTypeFont.getbbox() methods, with the anchor set to 'lt' (left, top). The former transposes the result of the latter to the chosen (x, y) coordinates.

The documentation for FreeTypeFont.getsize() has been updated to add a note to this effect:

For historical reasons this function measures text height from the ascender line instead of the top, see Text anchors. If you wish to measure text height from the top, it is recommended to use the bottom value of getbbox() with anchor='lt' instead.

You may also be interested in one of the issues that were addressed by these new API additions, where the developer that added these methods demonstrates that ImageDraw.getbbox() can be used to get exact bounding boxes per character in a given text.