[ginp-users] Question about jpeg scaling

[Brian B. <Br...@Pi...>]

I suspect this is a well-understood issue, but it isn't mentioned in the mailing list archives. If someone knows about it, please take the time to point me in the right direction. I don't want to waste a lot of time reinventing 
the wheel.

I can see that the javascript from ginpservlet retrieves the Window.innerHeigth and Window.innerWidth values from the browser. I can see the image displayed in the browser window is a jpeg, scaled to fit. For 
example, I'm looking at one which is 700 x 525 pixels. The original jpeg file accesed by ginp is 2848 x 2136 pixels.

The resultant image is counter-intuitively poor quality. I have in mind that analogue photographic images get more grainy when you enlarge them, and more precise when you reduce them. I realise something more 
subtle must happen when you scale-down a digital image... after all, you can't put 4 pixels of image into a single pixel of a display panel. An algorithm must be deciding how best to summarise the image into the 
target pixel, and that decision must be influenced by the neighbouring partial-image properties. As you can tell, this is not a subject I've thought seriously about, but I'm sure lots of clever people have!

Now, before we start on a discussion of the limitations and merits of different algorithms, let me say that this isn't what I want to discuss. My issues are much simpler.

I run ginp under java 1.5 with ubuntu 5.10. I also run a much older photo album webapp, written in perl and running on an old redhat 7 system. They BOTH scale the SAME photos to very similar sizes and quality, so I 
deduce that I'm not dealing with a subtle algorithmic issue.

I've looked at image quality with windoze, OS2 and linux. I've used firefox, mozilla, internet exploder and even opera. I've used 5 different hardware platforms, each with different video cards and displays. The 
results are near enough to identical that I deduce I'm not dealing with a specific browser/OS/hardware problem.

Here's the wierd thing. When I view the original jpeg file under ANY of the os's native image viewers, it ALWAYS has crisper details than when displayed via a browser/webapp on the same system. I could be 
convinced that a modern native viewer could beat remote scaling and browser rendering, if that really is the case, but I don't believe it is....

e.g. a 10-year old OS2 image viewer shows a crisper rendering of an image of a particular size than firefox/ginp on very new hardware... and mozilla under OS2 doesn't do any better or worse than the linux/firefox 
combination. Similarly, eye-of-gnome-2.12.1 doesn't do much better than my OS2 viewer, but (of course) it does a lot better than the browser/webapp image.

So, to summarise, it seems to be an architectural issue - scaling on the server and rendering on the browser seems to give a (subjective) 15 percent drop in image quality when compared to scaling-and-rendering 
on a single platform. This effect appears to be independent of hardware, operating system, browser, viewer, display type, scaling algorithm, etc.

What is the missing factor? Wouldn't it be nice if we could "adjust" the server-side logic to improve the browser-side image quality by 15 percent?

p.s. you can try this at home. Just look at the same photo on the same machine with a) a browser talking to ginp, and b) the native OS image viewer. Size the two images so they take up roughly the same real-estate 
and reload if necessary to get them re-scaled. Look for some detail in the distance, such as lamp posts, yacht masts, people, dogs, railings... and compare the resolution of these details between the 2 images on 
your screen.

I hope someone can help me understand this puzzling issue.



Regards,

Brian