PIL how to scale text size in relation to the size

Question

I'm trying to dynamically scale text to be placed on images of varying but known dimensions. The text will be applied as a watermark. Is there any way to scale the text in relation to the image dimensions? I don't require that the text take up the whole surface area, just to be visible enough so its easily identifiable and difficult to remove. I'm using Python Imaging Library version 1.1.7. on Linux.

I would like to be able to set the ratio of the text size to the image dimensions, say like 1/10 the size or something.

I have been looking at the font size attribute to change the size but I have had no luck in creating an algorithm to scale it. I'm wondering if there is a better way.

Any ideas on how I could achieve this?

Thanks

Answer 1

In general when you change the font sizing its not going to be a linear change in size of the font.

Now this often depends on the software, fonts, etc... This example was taken from Typophile and uses LaTex + Computer Modern font. As you can see its not exactly a linear scaling. So if you are having trouble with non-linear font scaling then I'm not sure how to resolve it, but one suggestion maybe is to.

1. Render the font as closely to the size that you want, then scale that up/down via regular image scaling algorithm...
2. Just accept that it won't exactly be linear scaling and try to create some sort of table/algorithm that will select the closest point size for the font to match up with the image size.

Answer 2

You could just increment the font size until you find a fit. font.getsize() is the function that tells you how large the rendered text is.

import ImageFont, ImageDraw, Image

image = Image.open('hsvwheel.png')
draw = ImageDraw.Draw(image)
txt = "Hello World"
fontsize = 1  # starting font size

# portion of image width you want text width to be
img_fraction = 0.50

font = ImageFont.truetype("arial.ttf", fontsize)
while font.getsize(txt)[0] < img_fraction*image.size[0]:
    # iterate until the text size is just larger than the criteria
    fontsize += 1
    font = ImageFont.truetype("arial.ttf", fontsize)

# optionally de-increment to be sure it is less than criteria
fontsize -= 1
font = ImageFont.truetype("arial.ttf", fontsize)

print 'final font size',fontsize
draw.text((10, 25), txt, font=font) # put the text on the image
image.save('hsvwheel_txt.png') # save it

If this is not efficient enough for you, you can implement a root-finding scheme, but I'm guessing that the font.getsize() function is small potatoes compared to the rest of your image editing processes.