Manual of Lua-OOCairo: userfont

lua-oocairo - Lua API for the Cairo vector graphics library

Introduction

User fonts are created by calling the user_font_face_create function in the main module hash, and passing in a table of callbacks which Cairo will call when necessary. The render_glyph callback function must be supplied, but all the others are optional.

A user font can be created and set to be used like this:

local function render_glyph (...) end

local font = Cairo.user_font_face_create({
    init = function () print("initializing") end,
    render_glyph = render_glyph,
})
cr:set_font_face(font)
cr:set_font_size(12)

Callbacks

The following callbacks can be supplied (the names here being the keys you should use in the table when creating the font). The font value is always the scaled font object representing the user font at the size it's being used at.

init (font, cr, extents)

This is called once, and can be used to set up the font and provide Cairo with metrics information, by filling in the font extents table supplied. The extents information is in the same format as that returned by the font_extents method on context objects (see lua-oocairo-context(3)), except that all measurements are in font units, which will be scaled according to the font size.

The font value isn't fully created when this function is called, and using it is likely to break things.

Return values are ignored.

render_glyph (font, glyph, cr, extents)

This callback is required, and will be called for each glyph which needs to be displayed. It should draw the glyph using cr, which is a special Cairo context object which will be scaled to font coordinates. The callback must not change the source on cr (so multi-coloured fonts are out of the question, and rendering bitmap images from another surface needs to be done with the mask method).

The value of the glyph parameter will be the glyph index number of the glyph which Cairo wants drawn. This will be whatever is determined to be appropriate by the other callbacks, or the Unicode codepoint number as a default.

Glyph images are cached, so you can't have a font where each occurrence of the letter ‘e’ is slightly different, for example.

The callback should set the x_advance value in the extents table to an appropriate value (measured in font coordinates). Return values are ignored.

This example draws all glyphs as a simple triangle shape, except for the space character (assuming that doesn't end up with some other glyph index):

local function render_glyph (font, glyph, cr, extents)
    extents.x_advance = 1   -- distance to move to next glyph

    if glyph == 32 then return end  -- space, draw nothing

    cr:move_to(0, 0)
    cr:line_to(1, 0)
    cr:line_to(0, 1)
    cr:fill()
end

text_to_glyphs (font, text, want_clusters)

Called to translate text into glyphs. The text parameter is UTF-8 encoded text. The callback should return a table of glyphs which would be appropriate for rendering it, including appropriate x and y coordinates for each one, in the same format as accepted by the show_glyphs and show_text_glyphs methods on context objects. The coordinates should be in font coordinates.

If the want_clusters parameter is true then the font is being used on a surface which can make use of text cluster information (mapping bytes in text to glyphs returned). In this case the callback can optionally return a second value, an array of pairs in the same format as accepted by the show_text_glyphs method on context objects. If want_clusters is false then any text clusters information returned will just be ignored.

unicode_to_glyph (font, unicode)

Called when there is no text_to_glyphs callback, or it doesn't provide any glyphs. The unicode parameter is a Unicode codepoint for a character being drawn, and the callback should return the glyph index appropriate for rendering it. With this simpler callback each Unicode character has to map to exactly one glyph.