Unicode support in cv::addText via Qt (Bugfix #4128)

Passing a Unicode UTF-8 string into cv::addText (or cvAddText) results in extraneous characters being drawn (often â or Â) and other being dropped (if their Unicode is greater than 255). This is a symptom of ignoring the multi-byte UTF-8 escape encoding.

Fortunately the Qt library is fully Unicode compliant so can render all glyphs defined by the standard if provided by the fonts in use. (Which is a lot, tens of thousands, see: http://unicode-table.com/en/) All that is needed to fix this bug was to call Qt's QString::fromUtf8 function to indicate the array of 8-bit values should be interpreted as UTF-8 rather than as a vanilla C char string.

Note that this patch provides support for drawing Unicode with Qt, but does not help with the conceptually related problem of using Unicode in pathnames.

I thought I was supposed to provide the URL for my own Pull Request (https://github.com/cwreynolds/opencv/pull/1) in the box below, but it said that was invalid. Perhaps only PRs on https://github.com/Itseez/opencv/pull/... can be listed there? Please advise, this is my first contribution to OpenCV.

Example before this change:

Example after this change:

C++ program to draw example:

#include <opencv/highgui.h>
int main (int argc, char** argv)
{
    cv::namedWindow ("Test", 1);
    cv::moveWindow ("Test", 10, 50);
    cv::Mat image (cv::Size (1000, 200), CV_8UC3, cv::Scalar(0, 111, 222));
    CvFont fontB = cv::fontQt ("Times", 30, cv::Scalar (255, 255, 255), CV_FONT_BOLD);
    CvFont fontW = cv::fontQt ("Helvetica", 30, cv::Scalar (0, 0, 0), CV_FONT_NORMAL);
    cv::addText (image,
                 "Here is some vanilla 7-bit 'ascii' text.",
                 cv::Point (50, 60),
                 fontB);
    cv::addText (image,
                 "Here is some text with extended “Unicode” glyphs, like circle-R: ®, ",
                 cv::Point (50, 110),
                 fontW);
    cv::addText (image,
                 "superscript TM: ™ and the point-of-interest symbol (OSX cmd): ⌘",
                 cv::Point (50, 145),
                 fontW);
    cv::imshow ("Test", image);
    cv::waitKey(0);
    return 0;
}