Question

I’m trying to create thumbnails for uploaded images in a JRuby/Rails app using the Image Voodoo plugin – the problem is the resized thumbnails look like… ass.

It seems that the code to generate the thumbnails is absolutely doing everything correctly to set the interpolation rendering hint to “bicubic”, but it isn’t honoring them on our dev environment (OS X), or on the production web server (Linux).

I’ve extracted out the code to generate the thumbnails, rewritten it as a straight Java app (ie kicked off from a main() method) with the interpolation rendering hint explicitly set to “bicubic”, and have reproduced the (lack of) bicubic and bilinear resizing.

As expected on both OS X and Linux the thumbanils are ugly and pixelated, but on Windows, it resizes the images nicely with bicubic interpolation used.

Is there any JVM environment setting and/or additional libraries that I’m missing to make it work? I’m doing a lot of banging of head against wall for this one.

Answer 1

I realize this question was asked a while ago, but incase anyone else is still running into this.

The reason the thumbnails look like ass are caused by two things (primarily the first one):

• Non-incremental image scaling in Java is very rough, throws a lot of pixel data out and averages the result once regardless of the rendering hint.
• Processing a poorly supported BufferedImage type in Java2D (typically GIFs) can result in very poor looking/dithered results.

As it turns out the old AreaAveragingScaleFilter does a decent job of making good looking thumbnails, but it is slow and deprecated by the Java2D team — unfortunately they didn’t replace it with any nice out-of-the-box alternative and left us sort of on our own.

Chris Campbell (from the Java2D team) addressed this a few years ago with the concept of incremental scaling — instead of going from your starting resolution to the target resolution in one operation, you do it in steps, and the result looks much better.

Given that the code for this is decently large, I wrote all the best-practices up into a library called imgscalr and released it under the Apache 2 license.

The most basic usage looks like this:

BufferedImage img = ImageIO.read(...); // load image
BufferedImage scaledImg = Scalr.resize(img, 640);

In this use-case the library uses what is called it’s “automatic” scaling mode and will fit the resulting image (honoring it’s proportions) within a bounding box of 640×640. So if the image is not a square and is a standard 4:3 image, it will resize it to 640×480 — the argument is just it’s largest dimension.

There are a slew of other methods on the Scalr class (all static and easy to use) that allow you to control everything.

For the best looking thumbnails possible, the command would look like this:

BufferedImage img = ImageIO.read(...); // load image
BufferedImage scaledImg = Scalr.resize(img, Method.QUALITY, 
                                       150, 100, Scalr.OP_ANTIALIAS);

The Scalr.OP_ANTIALIAS is optional, but a lot of users feel that when you scale down to a small enough thumbnail in Java, some of the transitions between pixel values are a little too discrete and make the image look “sharp”, so a lot of users asked for a way to soften the thumbnail a bit.

That is done through a ConvolveOp and if you have never used them before, trying to figure out the right “kernel” to use is… a pain in the ass. That OP_ANTIALIAS constant defined on the class it the best looking anti-aliasing op I found after a week of testing with another user who had deployed imgscalr into their social network in Brazil (used to scale the profile photos). I included it to make everyone’s life a bit easier.

Also, ontop of all these examples, you might have noticed when you scale GIFs and some other types of images (BMPs) that sometimes the scaled result looks TERRIBLE compared to the original… that is because of the image being in a poorly supported BufferedImage type and Java2D falling back to using it’s software rendering pipeline instead of the hardware accelerated one for better supported image types.

imgscalr will take care of all of that for you and keep the image in the best supported image type possible to avoid that.

Anyway, that is a REALLY long way of saying “You can use imgscalr to do all that for you and not have to worry about anything”.