Erroneous handling of out of range values on the 'opsz'-axis of variable fonts

[fkett]

For testing purposes, I tried to run this little script using my newly finished variable font. Unsatisfyingly, it did not work as expected (let's be honest: it didn't work at all).

Info: the font has three axes:

• wght {'name': 'Weight', 'minValue': 100, 'maxValue': 900, 'defaultValue': 400}
• wdth {'name': 'Width', 'minValue': 75, 'maxValue': 125, 'defaultValue': 100}
• opsz {'name': 'Optical Size', 'minValue': 16, 'maxValue': 36, 'defaultValue': 16}

After some further research (and simplifying the test setup), I found that DrawBot was only able to use those axis coordinates that were also defined as instances. I checked the font file (very thoroughly) to make sure that everything was according to specification and there were no errors causing this unexpected behaviour. I did find some problems, but even after fixing all of them, I wouldn't get any different results.

Eventually, my second finding was that when I set the optical size value to its minimum (16) or below using the fontVariations() method, it did not work. Some further try and error led me to the conclusion, that DrawBot's variable font axes handling caused the problems:

Apparently, DrawBot automatically sets the value for the optical size (opsz) axis according to the specified font size. I had used text with a font size of 60px (which is bigger than the axis' defined maximum value of 36). It seems like DrawBot did not handle this discrepancy correctly: According to the OpenType specification, “if an application specifies a value greater than the maxValue defined in the font, then maxValue must be used”. Instead, the whole variable fonts functionality responded in an unexpectable way.

Conclusion:
example: opsz (16–36, default 16)

Input	Expected Behaviour	DrawBots behaviour
fontVariations(opsz=8)	opsz <- 16	💥
fontVariations(opsz=16)	opsz <- 16	💥
fontVariations(opsz=24)	opsz <- 24	✔️
fontVariations(opsz=36)	opsz <- 36	✔️
fontVariations(opsz=48)	opsz <- 36	✔️
fontSize(8)	opsz <- 16	💥
fontSize(16)	opsz <- 16	✔️
fontSize(24)	opsz <- 24	✔️
fontSize(36)	opsz <- 36	✔️
fontSize(48)	opsz <- 36	💥

There are a few problems, obviously:

• “error” handling of fontVariations does not work properly
• “error” handling of fontSize does not work properly
• fontVariations and fontSize work differently in similar situations
• “automatic opsz adjustment” functionality is not mentioned in the documentation

[justvanrossum]

This issue may be related to #305 and could easily be an OS bug. On which OS are you?

[justvanrossum]

See also this Twitter thread: https://twitter.com/jenskutilek/status/1197128145683566592

[typemytype]

I cannot reproduce this anymore on 10.15

can you provide your OS? as this seems to be the same bug as #305

and maybe provide an example script with an example font generating this issue

thanks!

[fkett]

Sorry for the late response.

The OS is macOS 10.13.6.

I was able to reproduce the bug with a test font that I created and exported using only Glyphs (build 1271). This is the script that I used:

font_name = "TestFontVF.ttf"

variations = listFontVariations(font_name)
if variations: 
    print(font_name)
    for axis_name, dimensions in variations.items():
        print (axis_name, dimensions)
    print ()

####################################################

# font only contains 'n' & 'o'
txt = 'nononono'
txt_x = 100
txt_y = 100
txt_x_space = 800
txt_y_space = 20

# scale to this size
target_font_size = 128

# define testing `wght` values
wght_val = [
    100, # minimum
    400, # default
    630, # arbitrary
    900, # maximum
]

# define testing font size / `opsz` values
size_val = [
    8,  # below minimum
    16, # minimum & default
    16.1, # slightly above minimum
    24, # in between
    35.9, # slightly below maximum
    36, # maximum
    64, # above maximum
]

curr_wdth = 100

####################################################

# iterate over font sizes (set per fontSize())
for k, curr_size in enumerate(size_val):
    scaling = target_font_size / curr_size

    newPage(6400, 700)
    font(font_name)
    
    # iterate over weights, do not touch `opsz` axis manually
    for i, curr_wght in enumerate(reversed(wght_val)):
        with savedState():
            fontVariations(wght=curr_wght)
            
            scale(scaling)
            
            x = txt_x
            y = txt_y + (i * (txt_y_space + target_font_size))
            
            fontSize(curr_size)
            text(txt, (x / scaling, y / scaling))
    
    fontSize(20)
    fontVariations(wght= 400 )
    
    text('size: %04.1f, opsz: auto (=size), wdth: %04.1f' % (curr_size, curr_wdth), (txt_x, 20))
    
    #############################################
    
    # iterate over weights and `opsz` axis (manually)
    for j, curr_opsz in enumerate(size_val):
        for i, curr_wght in enumerate(reversed(wght_val)):
            with savedState():
                fontSize(curr_size)
                
                scale(scaling)
            
                x = txt_x + ((j + 1) * txt_x_space)
                y = txt_y + (i * (txt_y_space + target_font_size))
                
                fontVariations(opsz=curr_opsz)
                fontVariations(wght=curr_wght)
                
                text(txt, (x / scaling, y / scaling))
    
        fontSize(20)
        fontVariations(wght= 400 )

        text('size: %04.1f, opsz: %04.1f, wdth: %04.1f' % (curr_size, curr_opsz, curr_wdth), (x, 20))
    
    ##############################################
        
    saveImage("vf-issue340-test-%i.png" % k)

And the output images. The iterations marked in red are erroneous.

You can find the font file I used on WeTransfer: https://we.tl/t-zp1TEH0v2o