Opencv-Python, draw text with fontsize in pixels

Question

In the opencv python, we use fontscale or thickness to show the font size, but if I want my font exactly to be 20px, how can I do that? How can I change the scale/thickness in a way to compute my desired point size?

cv2.putText(img_inpainted,boxes['text'][i],(x,y),cv2.FONT_HERSHEY_PLAIN,1,(120,145,152),1,2)

I want the size of text exactly to be L px. How could I go doing that?

I searched and thought a lot, couldn't find any way

Answer 1

There's a roundabout way. First you need to know the size of text for a given arbitrary fontScale . Then, assuming the Hershey vector fonts scale linearly, you can calculate desired pixel height into a corresponding fontScale .

There exists the function cv.getTextSize() to get you the size of a not-quite -bounding box for some given text. It's giving a box including ascenders, but excluding descenders... and the baseline goes lower than any descenders among the regular alphabet.

Here's a rough outline because I won't explore the effect of the thickness parameter for now, except to assume it's 1.

# calculating a factor here
fontScale = 100
fontFace = cv.FONT_HERSHEY_PLAIN

((fw,fh), baseline) = cv.getTextSize(
    "", fontFace=fontFace, fontScale=fontScale, thickness=1) # empty string is good enough
factor = (fh-1) / fontScale

# that bounding box seems to ignore descenders...

# using the factor now
text = "Hello, World!"
thickness = 1
height_in_pixels = 30
fontScale = (height_in_pixels - thickness) / factor
(w,h) = (500, 100)
canvas = np.zeros((h, w), 'uint8')
# I'm gonna center it
((fw, fh), baseline) = cv.getTextSize(
    text=text, fontFace=fontFace, fontScale=fontScale, thickness=thickness)
org = ((w-fw)//2, (h+fh)//2)
cv.putText(
    img=canvas, text=text, org=org,
    fontFace=fontFace, fontScale=fontScale, color=255, thickness=thickness)

cv.rectangle(canvas, org, (org[0] + fw, org[1] - fh), 128)
cv.line(canvas, (org[0], org[1]+baseline), (org[0] + fw, org[1]+baseline), 128)