OpenCV: How to use other font than HERSHEY with cvPutText (like Arial) - image

Question

I want to write to an image a formatted text. OpenCV offers only a limited set of default fonts. Is it possible to use others? For example to read them from the *.ttf file (in Ubuntu)?

Answer 1

If you cannot or don't want to use the Qt bindings, here is a way to do it with CAIRO:
#include <opencv2/opencv.hpp>
#include <cairo/cairo.h>
#include <string>
void putTextCairo(
cv::Mat &targetImage,
std::string const& text,
cv::Point2d centerPoint,
std::string const& fontFace,
double fontSize,
cv::Scalar textColor,
bool fontItalic,
bool fontBold)
{
// Create Cairo
cairo_surface_t* surface =
cairo_image_surface_create(
CAIRO_FORMAT_ARGB32,
targetImage.cols,
targetImage.rows);
cairo_t* cairo = cairo_create(surface);
// Wrap Cairo with a Mat
cv::Mat cairoTarget(
cairo_image_surface_get_height(surface),
cairo_image_surface_get_width(surface),
CV_8UC4,
cairo_image_surface_get_data(surface),
cairo_image_surface_get_stride(surface));
// Put image onto Cairo
cv::cvtColor(targetImage, cairoTarget, cv::COLOR_BGR2BGRA);
// Set font and write text
cairo_select_font_face(
cairo,
fontFace.c_str(),
fontItalic ? CAIRO_FONT_SLANT_ITALIC : CAIRO_FONT_SLANT_NORMAL,
fontBold ? CAIRO_FONT_WEIGHT_BOLD : CAIRO_FONT_WEIGHT_NORMAL);
cairo_set_font_size(cairo, fontSize);
cairo_set_source_rgb(cairo, textColor[2], textColor[1], textColor[0]);
cairo_text_extents_t extents;
cairo_text_extents(cairo, text.c_str(), &extents);
cairo_move_to(
cairo,
centerPoint.x - extents.width/2 - extents.x_bearing,
centerPoint.y - extents.height/2- extents.y_bearing);
cairo_show_text(cairo, text.c_str());
// Copy the data to the output image
cv::cvtColor(cairoTarget, targetImage, cv::COLOR_BGRA2BGR);
cairo_destroy(cairo);
cairo_surface_destroy(surface);
}
Example call:
putTextCairo(mat, "Hello World", cv::Point2d(50,50), "arial", 15, cv::Scalar(0,0,255), false, false);
It assumes that the target image is BGR.
It puts the text's center to the given point. If you want some different positioning, you have to modify the cairo_move_to call.

Answer 2

It's possible to use other fonts but you need to link the Qt library to OpenCV and use the cvAddText function with a cvFontQt
http://docs.opencv.org/modules/highgui/doc/qt_new_functions.html#addtext
http://docs.opencv.org/modules/highgui/doc/qt_new_functions.html#fontqt
There are other solutions you might try, with more or less the same performance as OpenCV. For instance, you can use CAIRO to write fonts into the image.