Calculate text size - c++

Question

Note: This question How to put text into a bounding box in OpenCV? is in some ways similar to this one but it is not the same question. The OP of the questions tried to spread a text to the whole size of his image & the code in the answer that gots the mark is just resizing the text using a mask.
I'm using openCV combined with C++ to do some image detection & manipulation.
So I want to align a text with a unknown length at a specific origin. The font-scale should be calculated because I'd like to specify a width factor for the maximum Text-width like you can see in the image below:
This is the code I got so far:
int fontFace = cv::FONT_HERSHEY_DUPLEX,
fontScale = myTextString.size() / 10;
cv::Size textSize = getTextSize(image, fontFace, fontScale, 0, 0);
putText(image, myTextString, cv::Point( (origin.x + textSize.width)/2, (origin.y + textSize.height)/2 ), fontFace, fontScale, Scalar(255, 0, 0));

Answer 1

Something like this should do it. You can change how the margins are calculated to change the horizontal/vertical alignment of the font.
If the height doesn't matter, you can just leave target.height a large number.
void drawtorect(cv::Mat & mat, cv::Rect target, int face, int thickness, cv::Scalar color, const std::string & str)
{
cv::Size rect = cv::getTextSize(str, face, 1.0, thickness, 0);
double scalex = (double)target.width / (double)rect.width;
double scaley = (double)target.height / (double)rect.height;
double scale = std::min(scalex, scaley);
int marginx = scale == scalex ? 0 : (int)((double)target.width * (scalex - scale) / scalex * 0.5);
int marginy = scale == scaley ? 0 : (int)((double)target.height * (scaley - scale) / scaley * 0.5);
cv::putText(mat, str, cv::Point(target.x + marginx, target.y + target.height - marginy), face, scale, color, thickness, 8, false);
}
* edit *
// Sample code
int L = 80; // width and height per square
int M = 60;
cv::Mat m( 5*M, 7*L,CV_8UC3,cv::Scalar(0,0,0) );
// create checkerboard
for ( int y=0,ymax=m.rows-M;y<=ymax; y+=M)
{
int c = (y/M)%2 == 0 ? 0 : 1;
for ( int x=0,xmax=m.cols-L;x<=xmax;x+=L)
{
if ( (c++)%2!=0 )
continue; // skip odd squares
// convenient way to do this
m( cv::Rect(x,y,L,M) ).setTo( cv::Scalar(64,64,64) );
}
}
// fill checkerboard ROIs by some text
int64 id=1;
for ( int y=0,ymax=m.rows-M;y<=ymax; y+=M)
{
for ( int x=0,xmax=m.cols-L;x<=xmax;x+=L)
{
std::stringstream ss;
ss<<(id<<=1); // some increasing text input
drawtorect( m, cv::Rect(x,y,L,M), cv::FONT_HERSHEY_PLAIN,1,cv::Scalar(255,255,255),ss.str() );
}
}