Freetype2: Assuming that glyph height is font height is wrong.

[Glandos]

System information (version)

• OpenCV => 3.2
• Operating System / Platform => Linux
• Compiler => ❔

Detailed description

Currently on this line, the code assumes that the font height will be the maximum height of a glyph. This is wrong, especially with diacritics.

The code cannot really cover all Unicode insanities (see http://stackoverflow.com/questions/10414864/whats-up-with-these-unicode-combining-characters-and-how-can-we-filter-them), but even some basic characters can have some higher metrics, either above or below the base height. I don't know if FreeType2 can extract this information from a complete font file, or only glyph by glyph.

As an example cairo provides a height attributes that is a good hint. It seems that this value is set from FT_Face.size->metrics

This is obviously an open discussion.

[Kumataro]

Thank you! I can reproduce this problem with following code..
I think harfbuzz library have to be used to determine position of the glyph which is combined.
I'm very sorry that It will request some time to solve this problem.

#include <iostream>
#include <opencv2/opencv.hpp>
#include <opencv2/freetype.hpp>

int main( int argc, char* argv[] )
{
    cv::Mat image = cv::imread( "lena.jpg" );
    if( image.empty() ){
        return -1;
    }

    cv::Ptr<cv::freetype::FreeType2> ft2 = cv::freetype::createFreeType2();
    ft2->loadFontData( "./NotoSansThai-Regular.ttf" , 0 );
    ft2->putText( image, "ก้กิก้ก็ก็", cv::Point( 50, 50 ), 30, cv::Scalar( 255, 255,   0 ), -1, cv::LINE_AA, true );
    ft2->putText( image, "ก้กิก้ก็ก็", cv::Point( 50, 50 ), 30, cv::Scalar( 255,   0, 255 ), -1, cv::LINE_AA, false );

    ft2->putText( image, "ก้กิก้ก็ก็", cv::Point(200, 50 ), 30, cv::Scalar( 255, 255,   0 ),  1, cv::LINE_AA, true );
    ft2->putText( image, "ก้กิก้ก็ก็", cv::Point(200, 50 ), 30, cv::Scalar( 255,   0, 255 ),  1, cv::LINE_AA, false );

    cv::imwrite("out.jpg", image);
    return 0;
}

[Glandos]

Don't be sorry. I've found that drawing text is an extraordinary complex task.

The question is whether it's better to use a fixed value (for performance), or a computed one from glyphs in the string (more accurate but CPU intensive).

[Kumataro]

Thank you very much for waiting.
Because I thought of the problem complicatedly, the answer was delayed.
(As first I thought there is a problem which there is overlapped area between top-left origin and bottom-left origin strings).

---

Q1. When bottomLeftOrigin is true, is usinig 'maximum glyph height' better than using 'font height' ?

A1. Using font height is better. bottomLeftOrigine option should be independence with calling count of putText().

bottomLeftOrigin option is to move string origin.

If the same y-position is used, the result of 1(splitted) is should be same as 2(non-splitted).

1. putText("A") and putText("g");
2. putText("Ag");

Using glyph height for bottomLeftOrigin option is effected by calling count of putText().It is not good for users.

---

Q2. When glyph height is larger than font glyph, does it be drawn well ?
A2. It is OK. Currently module supports it.

mOrg is "string origin".
gPos is "glyph position" in left-top.

Step 1. Initialize glyph origin with string origin.

    Point gPos = mOrg;

Step 2. Move glyph origin to top/left of glyph.

   gPos.y -= ( mFace->glyph->metrics.horiBearingY >> 6) ;
   gPos.x += ( mFace->glyph->metrics.horiBearingX >> 6) ;

horiBearingY means that ...

https://www.freetype.org/freetype2/docs/tutorial/step2.html
horiBearingY
For horizontal text layouts, this is the vertical distance from the current cursor position (on the baseline) to the topmost border of the glyph image's bounding box.

Step 3. Drawing pixels from left-top to right bottom.
bmp->rows is may or may not bigger than fontHeight.

        for (int row = 0; row < (int)bmp->rows; row ++) {
            if( gPos.y + row < 0 ) {
                continue;
            }
            if( gPos.y + row >= dst.rows ) {
                break;
            }

Step 4. Update String Origin for next glyph.

   mOrg.x += ( mFace->glyph->advance.x ) >> 6;
   mOrg.y += ( mFace->glyph->advance.y ) >> 6;