[Toybox] [PATCH] file: parse JPEG files enough to pull out their size in pixels.

[scsijon]

On 3/1/24 12:23, enh wrote:
> On Tue, Dec 19, 2023 at 5:51 PM scsijon <scsijon at lamiaworks.com> wrote:
>>
>> On 19/12/23 08:05, toybox-request at lists.landley.net wrote:
>>>      1. [PATCH] file: parse JPEG files enough to pull out their size
>>>         in pixels. (enh)
>>> ----------------------------------------------------------------------
>>>
>>> Message: 1
>>> Date: Sun, 17 Dec 2023 16:26:56 -0800
>>> From: enh <enh at google.com>
>>> To: toybox <toybox at lists.landley.net>
>>> Subject: [Toybox] [PATCH] file: parse JPEG files enough to pull out
>>>        their size in pixels.
>>> Message-ID:
>>>        <CAJgzZoqp8f72kiX2R5okCURLhnS9tJysURiugmk7irX7VAp3RA at mail.gmail.com>
>>> Content-Type: text/plain; charset="utf-8"
>>>
>>> This doesn't handle refilling toybuf in the case where there's so
>>> much EXIF data before the header we're looking for that we hit the
>>> end of toybuf first. (In part because I couldn't decide whether to
>>> do that or switch to a scheme where we only ever lseek() around,
>>> reading big-endian shorts.)
>>> ---
>> Just wondering if this is going to work with compressed JPEG files or
>> are you going to see the compressed size instead?
> it's the size in _pixels_, so compression isn't relevant.
>
> if you mean "is this the _thumbnail_ size or the full image size?",
> no, this is the full image size. tbh, before reading the JFIF spec to
> implement this change, i had no idea that jpeg files could contain a
> thumbnail! i have no such jpegs on my system, so (a) nothing to test
> with and therefore (b) no obvious reason to bother to implement that
> functionality :-)

Thank you for clearing that up for me.

Also a single jpeg can contain a number of thumbnail images in a single 
full size jpeg image, there are a number of photo storage applications 
(mainly professional and costly) that do this instead of just the 
thumbnail in the matching full sized image, so you get a set of full 
images with the added thumbnail inside the image plus a single jpeg 
containing thumbnails of all in the set, just to make things more difficult.

>>>    toys/posix/file.c | 20 +++++++++++++++++---
>>>    1 file changed, 17 insertions(+), 3 deletions(-)
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regards

scsijon