On Dec 8, 2010, at 12:13 PM, Peter Kasting wrote:

> If you never write layout tests, you can stop reading.
> 
> Right now few ports run pixel tests (a shame).  One reason is because we 
> frequently need different reference images for each port, and creating 
> hundreds or thousands of these is a hassle.  Maintenance burden also 
> increases.
> 
> We could greatly decrease the number of these baselines by following a simple 
> rule: don't display text unless you need to.  For example, let's say I have a 
> test that is supposed to display a green square.  If the test output is:
> 
> A green square means this test passes.  If there is any red below, fail.
> [green square here]
> 
> ...then for each platform that has different font rendering, whether that be 
> subpixel AA differences, font size differences, or anything else, we'll need 
> new baselines.  By contrast, if that explanation was in an HTML comment 
> inside the test code, and the output was a simple green square, one baseline 
> would serve for all ports.
> 
> I think this doesn't really hinder tracking down test failures, for a couple 
> of reasons:
> * Most tests are "green = pass, red = failure" by convention, so frequently a 
> pixel result that differs from the baseline is easily classifiable at a quick 
> glance.
> * When this isn't the case, one of the first steps in understanding the test 
> output is to read the test, at which point the HTML comment will explain 
> things.
> 
> Obviously, some tests need to display text, but this seems to me to be a good 
> rule of thumb.

Tests that involve text can still have cross-platform results if they use the 
Ahem font. See for example 
<http://trac.webkit.org/browser/trunk/LayoutTests/fast/css/first-letter-set-text.html?rev=55196>.

> 
> Am I missing anything?  If not, is anyone interested in helping to make this 
> change on existing tests where possible, to trim the number of baselines in 
> the tree and make it easier for new ports to bring up pixel tests?
> 
> PK
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