Line anti-aliasing issues due to missing sRGB mishandling

[Witold Baryluk]

Hi,

I just started using aggdraw, and it definitively draws nicer lines than PIL for my application (opacity and antialiassing does help), but I noticed still imperfection and possibly a bug:

Be sure to open it and view at 100% scale.

The issue is visible as uneven luminosity of the lines. I.e. if you look at the top center, and then count 3 or 4 lines to the right, it looks to be oscillating in brightness.

I am not talking about a Moiré pattern that appears in the center, that is somehow expected, but possibly is related partially. I am talking about straight slightly diagonal lines far from the center.

It feels like maybe the sRGB is not used correctly? Which is a bit strange, as I see there is support for gamma, linear and sRGB in the aggdraw.

# Python 3.8.1
from PIL import Image, ImageDraw  # Pillow 6.2.1
import aggdraw  # Version 1.3.11
w, h = 1024, 1024
img = Image.new("RGB", (w, h))
img1 = aggdraw.Draw(img)
img1.setantialias(True)
pen = aggdraw.Pen("white", 1.0, opacity=127)

img1.line(xyxyxy, pen)
# ...

img1.flush()
img.save("dump.png")
img.show()

With opacity=255 effect is visible even more:

When drawing a similar type of lines (width and angle) for example in Inkscape, I see much more uniform luminance and the oscillations are there, but essentially invisible.

[Witold Baryluk]

Update: Yes, it looks like sRGB issue, either with PIL, with aggdraw, or with the PNG writer in PIL.

I added this piece of code at the end:

from PIL.ImageCms import buildTransform, applyTransform
# SRGB_PROFILE = 'sRGB.icc'
SRGB_PROFILE = '/usr/share/color/icc/sRGB.icc'
LINEARIZED_PROFILE = 'linearized-sRGB.icc'
LINEARIZED_TO_SRGB = buildTransform(LINEARIZED_PROFILE, SRGB_PROFILE, 'RGB', 'RGB')
img = applyTransform(img, LINEARIZED_TO_SRGB)
img.save("dump2.png")

The color profile icc file I got from: python-pillow/Pillow#1604 (comment) based on this post: https://stackoverflow.com/questions/31300865/srgb-aware-image-resize-in-pillow/46613620#46613620

And the result looks much better:

[David Hoese]

Wow, nice examples. Thanks for reporting this.

Could you do a couple things for me?

1. Could you make a simple example script that includes all the data (x/y locations) needs to reproduce an image that shows this problem? That way I can test it locally.

2. Could you try this with installing from the current aggdraw master branch? You should be able to do pip install git+https://github.com/pytroll/aggdraw.git. Note that there are other issues we've discovered in the master branch that are being talked about in other issues/PRs so results may not be perfect.

I'm not sure there is much aggdraw, the python package, can do to resolve this unless it is specifically a bug of aggdraw improperly passing things between python and C. Aggdraw depends almost entirely on the antigrain (agg) C library. We keep a copy of it alongside aggdraw in this repository, but it is coming from another group. The reason I asked about you using the master branch is that we are working on switching to agg 2.4 (I think that's the version, can't remember offhand) which had a lot of changes done from the previous versions we used.

If the master branch gives you "better" results then this must be an agg C problem. If not, then it could still be an agg problem or a PIL problem as you've deduced already.

[Witold Baryluk]

A full script to reproduce the issue in the attachment:

agg-lines-issue.tar.gz

[David Hoese]

I'm pretty swamped this month, but might have time to look at this this weekend. @baryluk Did you have time to try the master branch and compare it?

Edit: And of course, thank you for putting together this example code and images.

[Witold Baryluk]

Tested the master branch at commit 6c1cf87 , and confirmed the code is using the locally compiled version.

Same results.

It looks the issue is that the PIL doesn't really understand color spaces, nor the image metadata carry this information. If the PIL supported 16-bit per channel, or multi channel f32 formats, that would be less of an issue.

I did modify aggdraw.cxx line 407 , in the method draw_adaptor::setantialias I did this:

    void setantialias(bool flag)
    {
        if (flag) {
            // rasterizer.gamma(agg::gamma_power(1.0/2.4));
            rasterizer.gamma(&agg::linear_to_sRGB);
        } else
            rasterizer.gamma(agg::gamma_threshold(0.5));
    };

Instead of using rasterizer.gamma(agg::gamma_linear());, and this works and fixes the issue.