[webkit-dev] pixel tests and --tolerance (was Re: Pixel test experiment)

Dirk Pranke dpranke at chromium.org
Fri Oct 8 12:43:06 PDT 2010

Jeremy is correct; the Chromium port has seen real regressions that
virtually no concept of a fuzzy match that I can imagine would've
new-run-webkit-tests doesn't currently support the tolerance concept
at al, and I am inclined to argue that it shouldn't.

However, I frequently am wrong about things, so it's quite possible
that there are good arguments for supporting it that I'm not aware of.
I'm not particularly interested in working on a tool that doesn't do
what the group wants it to do, and I would like all of the other
WebKit ports to be running pixel tests by default (and
new-run-webkit-tests ;) ) since I think it catches bugs.

As far as I know, the general sentiment on the list has been that we
should be running pixel tests by default, and the reason that we
aren't is largely due to the work involved in getting them back up to
date and keeping them up to date. I'm sure that fuzzy matching reduces
the work load, especially for the sort of mismatches caused by
differences in the text antialiasing.

In addition, I have heard concerns that we'd like to keep fuzzy
matching because people might potentially get different results on
machines with different hardware configurations, but I don't know that
we have any confirmed cases of that (except for arguably the case of
different code paths for gpu-accelerated rendering vs. unaccelerated

If we made it easier to maintain the baselines (improved tooling like
the chromium's rebaselining tool, add reftest support, etc.) are there
still compelling reasons for supporting --tolerance -based testing as
opposed to exact matching?

-- Dirk

On Fri, Oct 8, 2010 at 11:14 AM, Jeremy Orlow <jorlow at chromium.org> wrote:
> I'm not an expert on Pixel tests, but my understanding is that in Chromium
> (where we've always run with tolerance 0) we've seen real regressions that
> would have slipped by with something like tolerance 0.1.  When you have
> 0 tolerance, it is more maintenance work, but if we can avoid regressions,
> it seems worth it.
> J
> On Fri, Oct 8, 2010 at 10:58 AM, Nikolas Zimmermann
> <zimmermann at physik.rwth-aachen.de> wrote:
>> Am 08.10.2010 um 19:53 schrieb Maciej Stachowiak:
>>> On Oct 8, 2010, at 12:46 AM, Nikolas Zimmermann wrote:
>>>> Am 08.10.2010 um 00:44 schrieb Maciej Stachowiak:
>>>>> On Oct 7, 2010, at 6:34 AM, Nikolas Zimmermann wrote:
>>>>>> Good evening webkit folks,
>>>>>> I've finished landing svg/ pixel test baselines, which pass with
>>>>>> --tolerance 0 on my 10.5 & 10.6 machines.
>>>>>> As the pixel testing is very important for the SVG tests, I'd like to
>>>>>> run them on the bots, experimentally, so we can catch regressions easily.
>>>>>> Maybe someone with direct access to the leopard & snow leopard bots,
>>>>>> could just run "run-webkit-tests --tolerance 0 -p svg" and mail me the
>>>>>> results?
>>>>>> If it passes, we could maybe run the pixel tests for the svg/
>>>>>> subdirectory on these bots?
>>>>> Running pixel tests would be great, but can we really expect the
>>>>> results to be stable cross-platform with tolerance 0? Perhaps we should
>>>>> start with a higher tolerance level.
>>>> Sure, we could do that. But I'd really like to get a feeling, for what's
>>>> problematic first. If we see 95% of the SVG tests pass with --tolerance 0,
>>>> and only a few need higher tolerances
>>>> (64bit vs. 32bit aa differences, etc.), I could come up with a per-file
>>>> pixel test tolerance extension to DRT, if it's needed.
>>>> How about starting with just one build slave (say. Mac Leopard) that
>>>> runs the pixel tests for SVG, with --tolerance 0 for a while. I'd be happy
>>>> to identify the problems, and see
>>>> if we can make it work, somehow :-)
>>> The problem I worry about is that on future Mac OS X releases, rendering
>>> of shapes may change in some tiny way that is not visible but enough to
>>> cause failures at tolerance 0. In the past, such false positives arose from
>>> time to time, which is one reason we added pixel test tolerance in the first
>>> place. I don't think running pixel tests on just one build slave will help
>>> us understand that risk.
>> I think we'd just update the baseline to the newer OS X release, then,
>> like it has been done for the tiger -> leopard, leopard -> snow leopard
>> switch?
>> platform/mac/ should always contain the newest release baseline, when
>> therere are differences on leopard, the results go into
>> platform/mac-leopard/
>>> Why not start with some low but non-zero tolerance (0.1?) and see if we
>>> can at least make that work consistently, before we try the bolder step of
>>> tolerance 0?
>>> Also, and as a side note, we probably need to add more build slaves to
>>> run pixel tests at all, since just running the test suite without pixel
>>> tests is already slow enough that the testers are often significantly behind
>>> the builders.
>> Well, I thought about just running the pixel tests for the svg/
>> subdirectory as a seperate step, hence my request for tolerance 0, as the
>> baseline passes without problems at least on my & Dirks machine already.
>> I wouldnt' want to argue running 20.000+ pixel tests with tolerance 0 as
>> first step :-) But the 1000 SVG tests, might be fine, with tolerance 0?
>> Even tolerance 0.1 as default for SVG would be fine with me, as long as we
>> can get the bots to run the SVG pixel tests :-)
>> Cheers,
>> Niko
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