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From "Steve Miller (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLEX-17337) FlexPrintJob making extra blank pages
Date Fri, 22 May 2015 22:28:17 GMT

    [ https://issues.apache.org/jira/browse/FLEX-17337?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14556929#comment-14556929

Steve Miller commented on FLEX-17337:

This problem is still there in 4.14.1, and is because of a rounding error in the addObject()
function. If the pageWidth is 576, then when the ratio is calculated and used the result can
be something like 576.0000000000001
Then when
			var hPages:int = Math.ceil(objWidth / _pageWidth);
is run, hPages becomes 2 when it should be one. 
I fixed this locally by rounding objWidth and objHeight before calling Math.ceil(), ie: 

			objWidth = Math.round(objWidth);
			objHeight = Math.round(objHeight);
			// Find the number of pages required in vertical and horizontal.
			var hPages:int = Math.ceil(objWidth / _pageWidth);
			var vPages:int = Math.ceil(objHeight / _pageHeight);

> FlexPrintJob making extra blank pages
> -------------------------------------
>                 Key: FLEX-17337
>                 URL: https://issues.apache.org/jira/browse/FLEX-17337
>             Project: Apache Flex
>          Issue Type: Bug
>          Components: Print Management
>    Affects Versions: Adobe Flex SDK 3.2 (Release)
>         Environment: Affected OS(s): All OS Platforms
> Affected OS(s): All OS Platforms
> Language Found: English
>            Reporter: Adobe JIRA
> Let me start by saying that this problem seems to occur randomly between machines, but
is consistent within each machine.  What I mean is that if breaks on machine A using Flash
10 and print driver X, it will always break using that combination.  However, if you go to
another machine, it may not break with those settings.  Though, you will find on another machine
it may break when you use Flash 9 and print driver Z.
> Luckily, I was able to find a repeatable condition on my development machine and was
able to debug it into the framework, FlexPrintJob, specifically.
> So here's what's happening.  When my app prints, I take full control over the paging.
 That is, I create a UIComponent of each individual page.  Internally, in FlexPrintJob there
is code to determine if your page can fit on a printed page and if it can't it creates extra
pages.  Since I am sizing my pages to be exactly the size of the printed page, in theory,
this extra paging code inside FlexPrintJob should never become active, but it is in a bad
way and I'll explain why.
> Some background.  When dealing with floating point calcuations everyone knows that you
can't depend on a value exactly equaling another value.  Depending on how the number was calculated
is may be off by a tiny epsilon, as is the case here.  So, imagine you do some complex calculation
and expect to get 10.0, but in reality you get 10.000000000000001.  Then you have another
calcuation where you do a Math.ceil on this.  We'll you should have gotten 10, but you wind
up getting 11, and it's unexpected. Likewise, if you use the mod (%) function and do x % 10
where x is 10.000000000000001, the result is 0.000000000000001, when 0 was expected.
> Well, in FlexPrintJob, this is what's happening.  When calculating the number of pages
in the horizontal and vertical directions, the code divides the passed in UICompoent size
by the page size.  In my case they should be equal giving one page.  However, because some
scaling had been involved in the calcuation, under some conditions, the values are "almost"
equal.  In my case, they were off by about 1e-14 or so.  This minor amount messed up the ceiling
calculation and subsequent modulo operation and caused the number of pages algorithm to be
2 instead of one, which is what caused the blank pages in between.
> Anyway, I addressed the problem by making a copy of FlexPrintJob and storing it locally
with my code.  I modified the ceiling calculation to substract a small epsilon prior to performing
the ceiling.  Likewise, when comparing the lastPageWidth and lastPageHeigth to zero, which
comes from the modulo, instead of comparing it against zero, I take it's absolute value and
make sure it's greater than epsilon.
> In my experience, I've found that this should be standard practice when dealing with
floating points.  I can't tell you how many times I used this same trick in other apps when
performing ceiling and floor and modulo.  Basically, you substract a small epsilon prior to
calling ceil and you add a small epsilon prior to calling floor.
> I'm attaching my updated code if anyone one is interested.  Please don't ask for me to
attach repeating test case, since it's very device specific.  The bottom line if anyone cares
is that ceil on Number is bound to fail at some point if you don't do this, so I think my
fix is sound.
> Hope this helps someone.

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