Question

Note: I am cross-posting this from App Engine group because I got no answers there.

As part of my site about Japan, I have a feature where the user can
get a large PNG for use as desktop background that shows the user’s
name in Japanese. After switching my site hosting entirely to App
Engine, I removed this particular feature because I could not find any
way to render text to a PNG using the image API.

In other words, how would you go about outputting an unicode string on
top of an image of known dimensions (1024×768 for example), so that
the text will be as large as possible horizontally, and centered
vertically? Is there a way to do this is App Engine, or is there some
external service besides App Engine that could make this easier for
me, that you could recommend (besides running ImageMagick on your own
server)?

Answer 1

Solution #1. Pure Python image library.

You can try to bundle PyPNG with your application. PyPNG is a pure Python library to create PNG images. It depends on zlib module, which is allowed on AppEngine, so PyPNG should work on AppEngine. Just use StringIO objects instead of files and write PNG data to them.

Shamelessly adapting PyPNG example how to make a bitmap PNG image:

import png
from StringIO import StringIO

# bitmap data
s = ['110010010011',
     '101011010100',
     '110010110101',
     '100010010011']
s = map(lambda x: map(int, x), s)

f = StringIO()
w = png.Writer(len(s[0]), len(s), greyscale=True, bitdepth=1)
w.write(f, s)

# binary PNG data
print f.getvalue()

I suspect suboptimal performance, but as far as I know there is no other way to generate images on GAE.

And you still need to figure out how to rasterize text to produce bitmap data. The easiest way, probably, is just to keep bitmaps of all the symbols around (essentially, using a bitmap font).

To render ASCII text with PyPNG take a look at texttopng script.

So, limitations are:

• Probably slow (needs to be checked)
• Glyph rasterization is to be addressed

Solution #2. Off-site text-to-image rendering.

Google AppEngine does not provide tools to render text as raster images, but Google Charts does. With a proper choice of parameters, the outline text chart just renders simple text to PNG images.

For example, http://chart.apis.google.com/chart?chst=d_text_outline&chld=000000|32|h|FFFFFF|_|Render text to image|with Google Charts.|Some Unicode too:|Здра́вствуйте|こんにちは|नमस्ते|你好|שלו produces this:

Limitations:

• You cannot generate images bigger than 300000 pixels
• Style and font customizations are limited
• Some Unicode scripts are not available
• White background only