opencv_contrib/freetype Module

[hoyiliang]

Is your feature request related to a problem? Please describe.
The Hershey font used by putText is severely limited on what characters it can draw.

Describe the solution you'd like
https://github.com/opencv/opencv_contrib/tree/master/modules/freetype

If possible, I would like to learn how you would implement the bindings from the compiled binaries to Dart. Have been developing in Flutter for 2 years but have never tried to develop with native binaries.

Describe alternatives you've considered

1. Pillow (PIL), an image processing library in Python, is not available in Dart.

2. Image library (source: https://pub.dev/packages/image), has a severely limited support with font manipulation (especially in font sizes, you can only make a font file with a static size like 12, 24, 48).

Additional context
Even single quote ' is not able to be drawn by the putText method.

With this module, it also opens up the possibility to draw other languages' characters

[rainyl]

@hoyiliang Hi

Okay, it's considerable. I have just looked at freeType module and it's not complicate.

EDIT:

Do you find any alternatives? I am replying and find you have just closed it.

[hoyiliang]

@rainyl Hi

I found a hacky alternative by delegating the text drawing to Flutter first (using TextPainter and export the text as image), then apply the text image into the base image. This workaround is moderately slower on processing but works.

However, since it is considerable, let's reopen this issue and discuss the implementation soon.

[rainyl]

Well, I just found that the freetype wrappers provided by opencv is simple, while the freetype itself is kind of complicate to build and bundle with opencv.

Since Opencv doesn't add freetype as a 3rd_party library officially, to reduce the impact of platform specific differences, I'd like to use a self-built freetype and harffbuzz libs, it's kind of complicate and I have no much time to resolve it, if you or anyone can build freetype and harfbuzz for all five platforms (windows, linux, macos, ios, android) and at least x64+arm64 for each platform, we can try to support this feature.

In summary, I am afraid I can't implement this feature in the near future.

[hoyiliang]

I see, I'll research on building freetype and harfbuzz libs on the platforms and architectures you specified when I have free time. No ETA but I'll update if I have any progress.

[rainyl]

Since the freetype module of opencv is simple, I have just copied the source code to dartcv and linked to libfreetype and libharfbuzz, now it works as expected. refer to https://github.com/rainyl/opencv_dart/blob/2.x/test/contrib/freetype_test.dart for more details.

But currently we focus on the supporting of native-assets feature, so it only works on 2.x branch, the support of 1.x will be added in the future when there are enough new features to merge.

Will leave this issue open until it is merged to 1.x.