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2000.11.17 08:10 "Re: Has something changed with error detection in group4 data?", by Randall Myers
Joris Van Damme wrote:
> "Myers, Randall" wrote:
> > the worst
> > case (no errors detected when they clearly exist)
>
> Is this the worst case?...
> ... Therefore, in my opinion, if the
> library can go on decoding the image, a warning should be emitted
> instead of an error...
I'm sorry, I was not clear. I was not actually referring to
a value returned by a library function (which, as you point out,
has never revealed decoding errors which the library detects
but doesn't stop for). I use the TIFFErrorHandler callbacks
to detect what I referred to as "errors". Furthermore, I am
in fact relying on warnings rather than error reports. I had
not made this distinction because I treat them equally for my
purposes, but it is actually true that the conditions I most
often encounter end up generating a warning rather than an error.
And of course, by "worst case" I meant that case which was worst
for me - no indication rather than just fewer than normal
indications of a problem condition.
So to clarify: the behavior that has changed is that the library
reports fewer (or no) "Fax4Decode" warnings in revisions from
3.4beta036 forward than it did in previous revisions, at least for
a class of images I have trouble with.
And since my previous posting, I think I have determined that a
relevant code change was made to tif_fax3.h in 3.4beta036:
# diff tiff-v3.4beta035/libtiff/tif_fax3.h \
> tiff-v3.4beta036/libtiff/tif_fax3.h
1c1
< /* $Header: /usr/people/sam/tiff/libtiff/RCS/tif_fax3.h,v 1.31 1996/06/04 19:0
4:41 sam Exp $ */
---
> /* $Id: tif_fax3.h,v 1.33 1996/08/22 18:17:39 sam Exp $ */
510,514c510,516
< /* expect a final V0 */ \
< NeedBits8(1,eof2d); \
< if (!GetBits(1)) \
< goto badMain2d; \
< ClrBits(1); \
---
> if (RunLength + a0 < lastx) { \
> /* expect a final V0 */ \
> NeedBits8(1,eof2d); \
> if (!GetBits(1)) \
> goto badMain2d; \
> ClrBits(1); \
> } \
As an experiment, I substituted the older code fragment in the current
version of libtiff and found this reinstated the decode warning behavior
that I am used to seeing.
I don't know what condition this change was intended to address, but since
I've never relied on it, I feel pretty safe in reverting to the old code
as a temporary or custom solution to my particular problem. Does anyone
know what I'd be giving up?
Regards,
Randy Myers