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Get the [arti]facts: Browser image scaling

Posted by chet on October 2, 2006 at 8:03 PM PDT

I've been playing around with various web and desktop technologies lately, and
noticed a funny thing when passing images back and forth between the browser
and an applet: IE7 thinks my image is a different size than it actually is.

Here is the scenario: I have an image on my web page (a URL in an
tag) that I'd like to hand off to an applet.  I want the applet to be
sized appropriately, so I set the applet width/height according to the size of
the image.  Then I display the image in the applet ... and the image is
smaller than the applet.

Let's try that again.  I have an image that is 100x100, I create an applet
of size 100x100, I use a simple drawImage(img, 0, 0, null) call in the applet,
and I have a significant border to the right and bottom of the image; the image
is not taking up the entire size of the applet, even though I specifically
created the applet to be the same size as the image.  Also, the image
displayed in the applet is clearly smaller than the image displayed through the
tag in HTML.

Then I run it in Firefox and it works the way I thought it should to begin with.

I pounded my head against this for a while, and finally debugged the problem to
my "DPI setting" on my Windows XP box.

Apparently, IE7 (and possibly IE6, although I no longer have it
available on my system) auto-scales images in the browser according to
your current DPI setting.  It considers 96 DPI the default, where one
physical pixel on the screen equals one pixel in the image (so an image won't
be scaled).  But any other DPI setting will cause auto-scaling.  On
my system, a laptop with a relatively dense 1400x1050 screen, the setting is
120 DPI, or 125% of the default.  The net effect is that images will be
scaled to 125% of their physical size when displayed in IE7 on my laptop.

Here is a screenshot that show the problem.  Ideally I would use
PNG, a nice lossless image format, but IE's support for PNGs is somewhat
broken; hopefully this JPEG will do the trick (if none of the images look right, save the image locally and open it up in a different image viewer):

IE7Results.jpg width="879"> 

The image I used is a simple black-line grid on a white background, where the
black lines and white pixels are spaced very regularly away from each other
(always one pixel of separation).  With this simple image, it is easy to
see the scaling artifacts that are bugging me.

What I wanted was that the images on the first line would all look good. If
I simply load an image with no width/height specified, it should create a space
of exactly the right size.  Similarly, if I specify a width/height that
matches the size of the image (I tried this with both "100" and "100px" to see
if it mattered), then I should get an image area of the right size. 

What I got was an image area that was clearly not the right size; the distortion
you see in these top three images shows an irregular grouping of black/white
pixels, which shows that the image is not being mapped one-to-one to the
onscreen pixels.

To test the theory further, I loaded the images in the second row with
width/height specifications of "80" and "80px".  Magically, these images
look perfect on my system.  I pre-scaled the images to account for the
125% scaling that I now knew IE7 was going to do.

Now, if we look at what happens on Firefox, we see the reverse (again, if you're not seeing what I'm describing, save the image and view it in a different application):

FirefoxResults.jpg width="903"> 

Firefox shows the first three images all looking good, and the bottom two images
distorted. Firefox is clearly not auto-scaling the images, and when I
pre-scale the images, it ends up causing distortion for Firefox where it fixed
the problem for IE7.

Why Should We Care?

For the most part, images are being viewed in the browser and these kinds of
scaling differences are probably irrelevant in that context; you want your
photos to look nice, the browser does a decent job of scaling things, and the
photos look fine. 

So what's the problem?

The problem is when we need to actually access the pixel information of these
images, we're going to need finer control.  And if the browser is hiding
that information from us, the browser makes a poor UI engine.

For example, if I need to edit a specific area of pixel data on an image, I'm
going to need to find out exactly where those pixels are.  Or if I want to
work on individual pixels, I don't want to be working on some pre-scaled
version of the image, but rather the raw image data.

But here's a better example, and one that you might have already hit: Google

I've been using Google Maps off and on since it was introduced; it's so nice to
have a dynamic mapping interface instead of the old batch retrieval system of
the original mapping sites.  But most of the time, the "Directions"
results of Google were just ridiculous.  That is, the directions themselves were
correct, but the path drawn on the map was way off.  I don't mean
that it had me taking the wrong highway to work, but that it had me driving
through the middle of the bay (where there ain't no bridge). 

Maybe Google Maps just had it in for me, but I don't think so.  When I
realized what was going on with my images, something clicked in the cobwebs of
my memory and I thought I'd see if Google was failing for me for similar

First, I needed a nice test.  The roads around here (the SF bay area) are
way too non-linear; it's hard to do easy comparisons.  So I did a search
around Northfield, Minnesota.  If there's one place in the world that
knows its cardinal directions, it's the American midwest.  (Aside: My
Iowan grandfather once gave directions like so: "If you're coming from the
North, then it'll be to the East".  And if we came from the South?).

I ran directions test from highway 280 to Cedar Avenue in Northfield, and ended
up with the following map:

googleMapsArtifact.jpg width="753">

You can see my difficulty (I hope).  The blue line should be drawn from the
start (the top black blob) to the end (the bottom black blob).  (Ignore
the blob graphics; this appears to be IE's continued hate-hate relationship
with PNG images).  Instead, the path starts to the left of where it should
and ends up far short of where it should, having taken an odd detour through
some corn fields. If you look at the actual path it should have drawn, you'll
see that it's basically correct, but that the path is drawn at a different
scale than the map image.  And if you pop up a screen magnifier with pixel
coordinates on it, you can calculate that the path on the map image is
exactly 125% of the size of the drawn path. Voila! It's the
Google Maps bug that's been dogging me all along.

Just to test this into the ground and then some, I reset my DPI to 75% (instead
of 125%), or 72 DPI (instead of 120 DPI).  Again, the path was drawn
incorrectly, but this time it overshoots the mapping area and the map path is
75% of the drawn path.

It's interesting (to me, at least), that in all of my tests the path appeared to
be correctly placed in the top-left-most position.  So, for example, in
the image above the first turn taken to the South (as my grandfather would say,
the first South turn if you're coming from the East) is actually in the correct
place.  I don't know enough (well, okay: anything) about the Google Maps
rendering algorithms, so I don't know why this would be.  But after that
one correct spot, it all falls apart and you find yourself off in the
weeds.  Or the cornfields. Or the bay.

But what if we don't have the problem on our system?

Granted, not everyone has this problem.  You may have never noticed it
before I pointed it out, even.  You would need a combination of a
high-density display and a non-default DPI setting (which was the case on my
laptop out of the box).  But with the increased availability of
high-resolution displays, both for desktop and laptops, the problem will only
get worse.  It is expected that resolutions of 200 DPI and up will be
commonplace in the next few years.  DPI settings would reflect this change
and more people will suffer.

What do we do about it?

Frankly, I'm not sure what we do about it; if the browser is telling us it's
doing one thing (sizing things according to pixel dimensions) and then doing
another (sizing them according to a scale factor applied to the pixel
dimensions), where do we go from there?

In a desktop application, this would not be a hard problem to solve; there are
APIs to do everything from querying the DPI settings to checking the size of
the image on the physical screen.  But in a browser application, there is
no such API (that I'm aware of).  We are reduced to try to divine the
information from side effects in the environment.  Like trying to predict
the weather by how your knee joint feels. Or trying to see whether a plant is poisonous by eating it.

But it's not clear to me now how to do this reverse-engineering; everything I've
seen from IE7 makes it seem like my images are the size I think they are; it is
only in measuring the screen pixels that I see the error of their ways.

For example, Dmitri pointed me to this
interesting blog
on the topic.  One of the comments claims that
you can determine the DPI settings by creating a div of a particular size and
then asking JavaScript how big it is.  So I tried this, creating a div:

        <DIV id="dpiDetector" style="WIDTH: 1in; HEIGHT: 1in;"/>

and calculating the width/height of the div in JavaScript like this:

    function detectDPI() {
        var IE = (navigator.appName.indexOf("Explorer") != -1);
        if (IE) {
            dpiDetectorElement = document.all.dpiDetector;
        } else {
            dpiDetectorElement = document.getElementById('dpiDetector');
        offsetW = dpiDetectorElement.offsetWidth;
        offsetH = dpiDetectorElement.offsetHeight;

The result?  IE7 reported the offsetW/offsetH as 96x96, and Firefox
reported them as 120x120.

Strike out.

Got any other thoughts on finding out this info?  Post it here; I'll try it
out.  I'm thinking that there are no obvious, good solutions here yet, or
else we wouldn't be seeing the problems in such a widely used application as
Google Maps; it's just that people have not yet hit the problem widely. 
But they will...

One solution, I feel compelled to point out, would be to use a toolkit more
suited to the purpose.  For example, an applet, even if sized incorrectly
by the browser, could adjust its size according to the physical size of the
image.  In fact, you could even use an applet just to do the DPI detection
step (have it detect the size the image it is, the size the browser thinks it
is, and then have it adjust the image width/height appropriately).

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