PngPongFont

A text writing transformer for PngPong.

What is it?

PngPong lets you draw shapes and copy sections of images. PngPongFont lets you write text. Much like PngPong, there are some major restrictions - primarily that it requires you to use a bespoke file format for font definitions, and that file can only contain one font size.

This is because PngPongFont basically does the same thing PngPong does when copying an image, directly copying pixels from source to destination. So it doesn't try to deal with scaling, anti-aliasing, etc. etc. - that is all contained in the source file.

The PngPongFont file format

PngPongFont uses a special file format, which uses the same basic structure as a PNG file - chunks, with headers and CRC checks. It has four blocks:

IHDR: a header with basic info like the font size, line height, width of the bitmap font image, and file format version.
GLPH: an index describing which glyphs match up to which numeric indexes in the next block.
OFFS: pixel offsets for each font glyph in the actual source image.
KERN: an optional block describing the kerning pairs between each glyph. NOTE: this is not currently used in rendering (but will be!)
ALPH: a long data block containing the alpha values for each glyph. Each glyph is written next to each other in one long horizontal line. A combination of the bitmap width, glyph index and glyph offset allow us to 'slice' out the appropriate glyph when drawing.

Much like our PngPong source PNGs, the font files are not compressed at all, so it's strongly recommended you GZIP them when serving to users.

Making a font file

The NPM package has a CLI tool to create font files. First run:

npm install -g png-pong-font

then you can run:

create-pngpong-font --input my-font-file.ttf --output font.pngpongfont --size 20

to generate a file. Run create-pngpng-font --help to get a list of all arguments.

Right now the tool only outputs a basic character set (as defined in character-sets.ts) - more options will be available eventually. The input file must be parsable by opentype.js, and your computer must have meet all of the dependency requirements for node-canvas.

Using the font file with PngPong.

There are two main components to the client side of PngPongFont - PngPongFontReader and PngPongTextWriter. Use them like so:

import { PngPongFontReader, PngPongTextWriter } from 'png-pong-font';

fetch("./font.pngpongfont")
.then((res) => res.arrayBuffer())
.then((ab) => {

    let font = new PngPongFontReader(ab);
    let writer = new PngPongTextWriter(pngPongInstance);

    // Draw "Hello!" in red, on a white background, at 10px from the top
    // and 20px from the left.

    writer.draw("Hello!", font, [255, 0, 0], [255, 255, 255], 20, 10);

    pngPongInstance.run();

})

You can use more than one font with a writer.

Next steps

Right now the library does not use kerning pairs when rendering text. opentype.js returns them as an absolute value, and I can't work out how to make that relative to font size (and didn't have time to work it out).

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