Puttext docs (nihui#136)

Author: nihui

README.md

@@ -23,6 +23,8 @@
 
 :heavy_check_mark: All the binaries are compiled from source on github action, **no virus**, **no backdoor**, **no secret code**.
 
+:heavy_check_mark: ***NEW FEATURE*** [`cv::putText` supports full-width CJK characters](#cvputtext-supports-full-width-cjk-characters)
+
 |opencv 4.10.0 package size|The official opencv|opencv-mobile|
 |:-:|:-:|:-:|
 |source zip|95.2 MB|8.25 MB|
@@ -481,6 +483,40 @@ zip -r -9 opencv-mobile-4.10.0-mypackage.zip install
 
 * C++ RTTI and exceptions are disabled for minimal build on mobile platforms and webassembly build. Be careful when you write ```cv::Mat roi = image(roirect);```  :P
 
+## `cv::putText` supports full-width CJK characters
+
+1. Open https://nihui.github.io/opencv-mobile/patches/fontface.html or `opencv-mobile-X.Y.Z/fontface.html` in your browser.
+2. In the opened page, enter all the text to be drawn, select the TTF font file (optional), click the `Convert to Font Header` button to download the fontface header. This step is completely local operation, without connecting to a remote server, your data is private and safe.
+3. Include the generated fontface header, initialize a fontface instance, and pass it as the argument to `cv::putText`. The source file must be encoded in UTF-8.
+
+Since all characters have been converted to embedded bitmap, the drawing routine does not depend on freetype library or any font files at runtime.
+
+```cpp
+#include <opencv2/core/core.hpp>
+#include <opencv2/highgui/highgui.hpp>
+#include <opencv2/imgproc/imgproc.hpp>
+#include "myfontface.h"
+
+int main()
+{
+    cv::Mat bgr = cv::imread("atari.jpg", 1);
+
+    // use this font
+    MyFontFace myfont;
+
+    // draw full-width text with myfont
+    const char* zhtext = "称呼机器人为破铜烂铁，\n违反了禁止歧视机器人法！";
+    cv::putText(bgr, zhtext, cv::Point(30, 250), cv::Scalar(127, 0, 127), myfont, 20);
+
+    // get bounding rect
+    cv::Rect rr = cv::getTextSize(bgr.size(), zhtext, cv::Point(30, 250), myfont, 20);
+
+    cv::imwrite("out.jpg", bgr);
+
+    return 0;
+}
+```
+
 # opencv modules included
 
 |module|comment|