Re: [ft-devel] glyph metrics don't match bitmap size (2.8.1)

[Jan Alexander Steffens]

On Tue, Sep 19, 2017 at 4:13 AM Nicolas Jinchereau <address@hidden> wrote:

When using FT_LOAD_TARGET_LCD, the size of the rendered bitmap doesn't seem to match the size in the metrics. It's usually off by 1~2 units - is this normal?

Example:

#include <ft2build.h>

#include FT_FREETYPE_H

#include FT_LCD_FILTER_H

int main()

{

    FT_Library _library;

    FT_Init_FreeType(&_library);

    FT_Library_SetLcdFilter(_library, FT_LCD_FILTER_DEFAULT);

    FT_Face face;

    FT_New_Face(_library, "arial.ttf", 0, &face);

    FT_Set_Pixel_Sizes(face, 0, 14);

    

    FT_Int32 flags = 0;

    flags |= FT_LOAD_TARGET_LCD;

    flags |= FT_LOAD_RENDER;

    string sample = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

    for(char c : sample)

    {

        FT_Load_Char(face, (int)c, flags);

        cout << (face->glyph->bitmap.width / 3) << " ==? ";

        cout << (face->glyph->metrics.width >> 6) << endl;

    }

    FT_Done_Face(face);

    return 0;

}

Any help with this would be appreciated

I assume you used to build 2.8 and earlier without subpixel rendering. In that case, FT_LOAD_TARGET_LCD doesn't really do LCD rendering and just gives you bitmaps three times wider without increasing the resolution.

With "subpixel rendering" enabled, glyphs are rendered at higher resolution with LCD filtering which blurs the image horizontally. This requires up to one pixel (three bitmap columns) of padding at each side.

With "subpixel rendering" disabled, 2.8.1 now includes a patent-free LCD rendering technique which does not use filters but effectively multisamples the outline. It has similar padding requirements.