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03-27-2017, 03:47 PM - 3 Likes   #1
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Hacking Exif with Exiftool

exiftool is an incredibly powerful metadata editor /extractor it works on more than just photographs. It's a commandline tool

essentially the important options are

exiftool -s yourphoto.dng or jpeg or whatever.

This produces a list of all tags and values in the file. in its prefered name for the tag.
if you don't use the -s switch you get expanded names
so say
FOV :43.6 degrees
becomes

field of view : 43.6 degrees

to read a singletag

exiftool -FOV yourphoto.dng

would return just the one tag.

exiftool -AFPointsInFocus .

This is a useful one for pentax users but first lets explain the . it means current directory and exiftool will run on every file it finds that it knows how to get exif information from. so cd into a folder with photo's and type that and you will get the focus points used.

Now thats quite handy there are 11 focus points on most Pentax camera's and if you put a rule of thirds grid on top of your photo
there is 1 at each intersection the middle rectangle has 1 halfway along each side if you draw a cross between the intersections you get the centre focal point and the two outside ones are level with the centre point and in the mid point of the 2 rectangles left and right of the centre rectangle.

It helps to know where the camera thought it had focus.
The interesting value is none. So not in focus at any focus point of course its a plain of focus so something might be on that plain but it's likely this is a bad shot unless you focused and recomposed.

exiftool -AFPointsInFocus . > focus.txt

this redirects the output of exiftool to a text file 'focus.txt' this text file you can run a find on later and e.g find none and you might get told it occured 4 times and you can go to the list and there will be a photo filename associated with each case of 'none'.

Ok writing

exiftool -LensType yourphoto.dng

Lens Type : PENTAX-F 28-80mm F3.5-4.5 or Sigma or Tokina Lens

ok now that isn't supported by dx0 but what module might be close?

exiftool -LensType="Sigma 24-70mm F2.8 IF EX DG HSM" yourphoto.dng

and now that is written to the dng

its the sametag switch as for a read but ="new tag value is wriiten in quotes"
if a tag is read only or incorrect for whats required exiftool will tell you.

And as it happens thats enough to convince dx0 that you were using that lens on that body. You can find a list of all lenses that Pentax writes to the exif file and in dx0 module manager which lenses they have a supporting module for.

Ok but now i'm getting to where I am a little stuck.

Light Levels

On a DA 18-55 lens i took a series of shots at different apertures the results went something like

exiftool -LightValue

15 15 15.1 14.6 15 15 which is pretty much the same as you would expect

but with a km lens with green button metering i got no light value


AEMeteringSegments : 14.0 12.9 11.8 12.2 13.2 11.4 11.0 11.4 12.5 12.6 13.0 14.2 13.1 11.8 12.1 13.6 11.5 11.1 11.1 12.9 14.5 13.5 14.2 13.9 13.0 12.0 13.6 13.2 10.6 12.4 12.6 14.0 13.8 13.6 13.8 13.4 12.5 14.2 14.2 13.5 14.4 12.9 13.2 14.8 13.1 13.4 13.5 14.0 16.0 14.8 13.0 13.4 13.5 13.4 13.8 13.8 14.1 14.0 13.0 14.0 14.5 14.2 14.0 14.9 13.8 13.8 13.9 14.6 14.1 12.5 14.5 14.2 14.6 14.2 14.9 14.1 14.0

might have the answer if i knew where they were (the final light value is 15.0 )

I just did a series of shots. Effective LV Shutter Speed
======== ./_IGP3570.DNG F2.8
Effective LV : 2.5
Shutter Speed : 1/8000
======== ./_IGP3571.DNG F4
Effective LV : 2.5
Shutter Speed : 1/8000
======== ./_IGP3572.DNG F5.6
Effective LV : 1.0
Shutter Speed : 1/3000
======== ./_IGP3573.DNG F8
Effective LV : 0.0
Shutter Speed : 1/1500
======== ./_IGP3574.DNG F11
Effective LV : -1.5
Shutter Speed : 1/500
======== ./_IGP3575.DNG F16
Effective LV : -2.5
Shutter Speed : 1/250
======== ./_IGP3576.DNG F22
Effective LV : -3.5
Shutter Speed : 1/125

I think I may have cracked it Effective LV is exposure compensation!

AE Metering Segments is pretty much identical for each shot example below.

AE Metering Segments : 10.4 10.6 11.2 11.1 12.6 13.9 14.2 14.1 13.9 13.6 13.4 10.4 11.2 11.2 11.6 12.8 14.1 14.4 14.2 13.9 13.5 13.0 11.2 11.2 11.0 11.6 12.6 13.9 14.0 13.8 13.4 13.0 12.4 9.8 10.0 9.4 10.1 11.0 12.2 12.1 11.9 11.8 11.4 11.1 8.4 9.0 8.5 9.5 10.2 10.2 9.6 8.8 8.6 8.2 8.2 8.1 8.4 8.4 9.2 9.6 9.9 9.0 7.5 7.6 7.8 8.0 7.6 8.0 8.8 9.1 9.2 9.1 8.2 7.1 7.1 7.4 7.6

So the camera is measuring the light with the aperture wide open, then when the green button is pressed it calculates exposure compensation for the closed down aperture. It wasn't quite fair to the body because it's too bright outside. But i think based on this it should be possible to calculate the aperture used on a given lens

M42 lenses will be harder to figure if you take a shot at maximum aperture and record the light level then the following shots will probably stepping down 1 EV a stop.

What do you think

03-27-2017, 04:59 PM - 1 Like   #2
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Thanks for sharing your experience. A few notes:
  1. Not all standard EXIF tags are read/write
  2. The MakerNotes section is always read-only Edit: This is not strictly true. You can write to this space, but doing so creates some risk.
  3. The MakerNotes section is not a friendly place
  4. ExifTool "composite" tags are computed from other tag values and are intended to be read-only. "Light Value", "Lens ID" and "Hyperfocal Distance" are all examples of composite tags.
  5. ExifTool MakerNotes and Composite tags are specific to ExifTool and software that uses ExifTool for metadata parsing.
  6. While ExifTool's parsing may be exhaustive, its output should not be deemed complete
Point #3 is important in several ways. Here are summary points:
  • The information in the MakerNotes is not set up as nice name value pairs. Data are in reliably consistent locations, but don't come with convenient labels.
  • The ExifTool developers (Phil Harvey and others) make their best guess as to the purpose of data at a particular file location and give it a purpose-appropriate name. Usually they guess right, but sometimes not.
  • A particularly MakerNotes data location may be "overloaded" meaning that it may be used for different purposes by different functions depending on camera configuration and operating status.
  • The above two points sometimes result in tags where what they mean may not be the same as what we think they mean. A good example might be the facial recognition tags where data are sometimes found despite the camera not being in live view. Some have suggested this is evidence of sophisticated image processing by the RGB meter sensors found the higher-end bodies. I did some testing and remain unconvinced. Still, the data are there and they mean something
  • Duplicate data are often found in multiple locations. This is often true with lens model id codes. ExifTool and other parsers do their best to find all the codes, but will often barf if they only find, say, 2 out of the 3 it expects. Lightroom does this frequently and ExifTool not so often.
  • Some output data (e.g. Lens friendly names) are specific to ExifTool and one should not expect spelling and abbreviations to be the same when using other tools
Again, thanks for posting your experiences.


Steve

Last edited by stevebrot; 03-30-2017 at 09:37 AM.
03-27-2017, 05:02 PM   #3
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QuoteOriginally posted by blackest Quote
but with a km lens with green button metering i got no light value
Light value is computed from shutter speed, aperture, and ISO and is equivalent to EV100 EVISO. The reason why there is no value for an M-series lens is because the body has no clue what aperture was selected.

QuoteOriginally posted by blackest Quote
exiftool -LensType yourphoto.dng

Lens Type : PENTAX-F 28-80mm F3.5-4.5 or Sigma or Tokina Lens

ok now that isn't supported by dx0 but what module might be close?

exiftool -LensType="Sigma 24-70mm F2.8 IF EX DG HSM" yourphoto.dng

and now that is written to the dng
LensType is a Pentax-specific MakerNotes tag. The numeric codes are stored in the EXIF and the display name is done as a lookup from a table built into ExifTool. The actual data are two 8-bit integers and are write-only. If you want to write a lens name into a field that the Adobe or DxO will read, I have had good luck writing the display name from ExifTool into the EXIF: LensModel tag.*

QuoteOriginally posted by blackest Quote
AEMeteringSegments : 14.0 12.9 11.8 12.2 13.2 11.4 11.0 11.4 12.5 12.6 13.0 14.2 13.1 11.8 12.1 13.6 11.5 11.1 11.1 12.9 14.5 13.5 14.2 13.9 13.0 12.0 13.6 13.2 10.6 12.4 12.6 14.0 13.8 13.6 13.8 13.4 12.5 14.2 14.2 13.5 14.4 12.9 13.2 14.8 13.1 13.4 13.5 14.0 16.0 14.8 13.0 13.4 13.5 13.4 13.8 13.8 14.1 14.0 13.0 14.0 14.5 14.2 14.0 14.9 13.8 13.8 13.9 14.6 14.1 12.5 14.5 14.2 14.6 14.2 14.9 14.1 14.0

might have the answer if i knew where they were (the final light value is 15.0 )
My understanding is that those values are voltages. On bodies with RGB meter sensors, the equivalent values (different tag) may be reassembled into a photograph of what the sensor actually "sees".
Edit:
Shows what I know. The ExifTool Pentax tag documentation has a note:
QuoteQuote:
(measurements from each of the 16 AE metering segments for models such as the K10D, 77 metering segments for models such as the K-5, and 4050 metering segments for the K-3, converted to LV)
The part about assembling the RGB meter results into a JPG image is true though I don't remember how it is done.
/Edit


QuoteOriginally posted by blackest Quote
So the camera is measuring the light with the aperture wide open, then when the green button is pressed it calculates exposure compensation for the closed down aperture. It wasn't quite fair to the body because it's too bright outside. But i think based on this it should be possible to calculate the aperture used on a given lens

M42 lenses will be harder to figure if you take a shot at maximum aperture and record the light level then the following shots will probably stepping down 1 EV a stop.

What do you think
I don't think so. At very least, you would need to know the maximum aperture in order to calculate the corresponding the T-stop needed to do the back-calculation. To be honest, I don't think that those values are meaningful when the meter is in center-weighted average or spot modes when using non-A lenses.


Steve

* To be honest, I usually cheat at this. ExifToolGui has a nifty function that automates this task with the display name written into EXIF: LensModel tag, and maximum aperture and focal length written into EXIF: LensInfo.

Last edited by stevebrot; 03-28-2017 at 05:35 AM.
03-27-2017, 05:20 PM   #4
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QuoteOriginally posted by stevebrot Quote
[*]While ExifTool's parsing may be exhaustive, its output should not be deemed complete
Excellent post by Stevebrot, and the last point bears repeating. In fact this is true for any exif software. Since the makernotes are always manufacturer-specific, each exif software makes its own best guess at translating makernotes to something tangible for the rest of us. So they're always a "best guess". Some best guesses are better than others, but they rarely completely agree

03-27-2017, 05:59 PM   #5
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QuoteOriginally posted by stevebrot Quote
Light value is computed from shutter speed, aperture, and ISO and is equivalent to EV100. The reason why there is no value for an M-series lens is because the body has no clue what aperture was selected.


Steve
This is of course true but the light metering does a pretty good job, matrix metering is going to be a little questionable but spot metering should be close to reality. Why do we get a lower light level from a smaller aperture? Personally I think if you're not under a roof when its raining you get just as wet in the middle of a court yard as in a field. so would it make sense for the middle of the metering matrix to give a real light value?

Anyway since the km lens doesnt give an aperture the body steps down with the green button and calculates compensation. As a lens closes to the same amount each time the compensation for a given lens is likely to be constant. therefore it should be possible to calculate the aperture used by the amount of compensation applied.

does this work spot metering on a manual lens with max aperture of 2.8 gave a shutter speed of 1/8000 for f4 and 15 ev at iso 100
EV calc makes that 17EV hmm so lets think for the ev value to be measured then the camera has to assume an aperture ... need to try this on a duller day i think.

Ok now a little lightroom bonus i created a number of over lays for pentax camera's in lightrooms loupe they are in the attached zip file.
unfortunately the example i uploaded got changed to a jpeg and is useless an overlay but there are more in the zip file.
Attached Images
 
Attached Files
File Type: zip lr-overlay.zip (1.70 MB, 26 views)
03-27-2017, 08:20 PM   #6
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QuoteOriginally posted by blackest Quote
Anyway since the km lens doesnt give an aperture the body steps down with the green button and calculates compensation. As a lens closes to the same amount each time the compensation for a given lens is likely to be constant. therefore it should be possible to calculate the aperture used by the amount of compensation applied.
I think you are way overthinking all of this, but will admit to not having done any thinking about these metadata tags. I am going to do a bit of look-see with a couple scenarios and post my results.


Steve
03-27-2017, 09:00 PM   #7
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QuoteOriginally posted by stevebrot Quote
Light value is computed from shutter speed, aperture, and ISO and is equivalent to EV100. The reason why there is no value for an M-series lens is because the body has no clue what aperture was selected.



LensType is a Pentax-specific MakerNotes tag. The numeric codes are stored in the EXIF and the display name is done as a lookup from a table built into ExifTool. The actual data are two 8-bit integers and are write-only. If you want to write a lens name into a field that the Adobe or DxO will read, I have had good luck writing the display name from ExifTool into the EXIF: LensModel tag.*



My understanding is that those values are voltages. On bodies with RGB meter sensors, the equivalent values (different tag) may be reassembled into a photograph of what the sensor actually "sees".
Edit:
Shows what I know. The ExifTool Pentax tag documentation has a note:

The part about assembling the RGB meter results into a JPG image is true though I don't remember how it is done.
/Edit




I don't think so. At very least, you would need to know the maximum aperture in order to calculate the corresponding the T-stop needed to do the back-calculation. To be honest, I don't think that those values are meaningful when the meter is in center-weighted average or spot modes when using non-A lenses.


Steve

* To be honest, I usually cheat at this. ExifToolGui has a nifty function that automates this task with the display name written into EXIF: LensModel tag, and maximum aperture and focal length written into EXIF: LensInfo.
I hacked the exif in for dx0 to take it needed an F-stop value which I was able to give it I also learned my hmc Hoya 28mm is very sharp at f5.6 I think its a tokina lens according to one of the lens reviews here.

exiftool -s -LightValue -ExposureTime -FNumber -EffectiveLV

======== ./_IGP3570.DNG LightValue : 15.9 1/8000 f2.8 2.5
======== ./_IGP3571.DNG LightValue : 17.0 1/8000 f4 2.5
======== ./_IGP3572.DNG LightValue : 16.5 1/3000 f5.6 1.0
======== ./_IGP3573.DNG LightValue : 16.6 1/1500 f8 0
======== ./_IGP3574.DNG LightValue : 15.9 1/500 f11 -1.5
======== ./_IGP3575.DNG LightValue : 16.0 1/250 f16 -2.5
======== ./_IGP3576.DNG LightValue : 15.9 1/125 f22 -3.5

spot metered values for igp3572

12.6 13.9 14
11.1 12.4 12.2
10.1 10.4 9.5

the only thing is there was -2 ev set...

03-28-2017, 05:33 AM - 2 Likes   #8
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QuoteOriginally posted by stevebrot Quote
The part about assembling the RGB meter results into a JPG image is true though I don't remember how it is done.
You can do it simply by feeding the data from the 4050 metering segments recorded in the EXIF into something as simple as Excel, and using conditional formating to highlight the values in a way that lets you visualize the results, like so:



showing the yellow/green read out from the RGB metering chip in the K-1.

The 4050 number understates what is recorded in the EXIF - in total there are 6750 data segments recorded by the chip: 4050 segments for the RGB (ie 3 X 1350 data segment blocks, one for each primary colour group) + 1350 segments for flash metering + another 1350 segments for slave flash metering. All are visible in EXIFTOOL.

So it's not just 'resolution' but [colour] depth too.

The K-3/K-3II/645Z/K-1/K-P metering sensor 'resolution' is 45x30 (3:2) segments - ie 1350 segments in a 45x30 matrix, like so, for each RGB measurement:


It's all a lot more than the old 77 (7x11 matrix) segments of the K-5, 645D, K-S2 etc meter:
03-28-2017, 05:41 AM - 2 Likes   #9
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03-28-2017, 05:46 AM   #10
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QuoteOriginally posted by stevebrot Quote
I think you are way overthinking all of this, but will admit to not having done any thinking about these metadata tags. I am going to do a bit of look-see with a couple scenarios and post my results.


Steve
You may be right about that. However it makes sense that for a given lens that each stop down from max aperture a particular exposure compensation be applied. With that hoya it does 1/2 stops i think across its aperture range. It's a little odd it is a positive value and goes negative. I will have to try different lenses and see if its the same set of values if it is then that's even better than I am hoping for.

if max aperture = 2.8 then compensation should be something like

2.8:0
4.0:-1
5.6:-2
8.0:-3
11:-4
16:-5
22:-6

Because each f-stop should be reducing the aperture area by 1, physically there may be some variation, how accurate are the actual apertures.

For practical use it doesn't matter a deal because once a lens is profiled we have a lookup table. which can be used in a script and then it should be possible to batch convert km lens exif it probably would be a script per lens or per max aperture.
If its per max aperture it would be even better. for example at the moment the photographer name I have set in my camera is Tamron SP90 I could change it to 28mm F2.8 or perhaps just 2.8.

Then I can run a script to read that stored value set the max aperture then use the exp compensation to give me the used aperture and write that in as f number. Which would then mean no more more F-- in the exif

Ok it's a bit OCD but I like it

Then there are m42 lenses but that is a different case to solve for and may not be possible without a diary.
maybe you could just set the photographer to f8 or f5.6 since we can identify an m42 lens from Exif.
03-28-2017, 05:46 AM   #11
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Just an addendum:

With cameras like the K-x, you could also use [the late, great] PhotoMe to produce a JPEG showing a summary of the LV measured in each metering area:

03-28-2017, 06:18 AM   #12
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QuoteOriginally posted by rawr Quote
You can do it simply by feeding the data from the 4050 metering segments recorded in the EXIF into something as simple as Excel, and using conditional formating to highlight the values in a way that lets you visualize the results, like so:



showing the yellow/green read out from the RGB metering chip in the K-1.

The 4050 number understates what is recorded in the EXIF - in total there are 6750 data segments recorded by the chip: 4050 segments for the RGB (ie 3 X 1350 data segment blocks, one for each primary colour group) + 1350 segments for flash metering + another 1350 segments for slave flash metering. All are visible in EXIFTOOL.

So it's not just 'resolution' but [colour] depth too.

The K-3/K-3II/645Z/K-1/K-P metering sensor 'resolution' is 45x30 (3:2) segments - ie 1350 segments in a 45x30 matrix, like so, for each RGB measurement:


It's all a lot more than the old 77 (7x11 matrix) segments of the K-5, 645D, K-S2 etc meter:
Wow, did you actually do that? I kind of did something along those lines a lot cruder than that using the metered values and comparing them with the actual image. There is a 77 segment overlay in the zip file i uploaded. I put the metered values into 7 rows of 11 and it seems to be fairly close low ev values where the scene was dark the sky had the highest values on the left where a tree was the top third values dropped and the lowest values were on the ground as you would expect but your spreadsheet is amazing. I was looking to see if the order of the metered readings matched going top left to bottom right 1 row at a time. I'm kinda pixilated actually there's a thought.

10.4 10.6 11.2 11.2 12.6 13.9 14.2 14.1 13.9 13.6 13.4
10.4 11.2 11.2 11.6 12.9 14.1 14.4 14.2 13.9 13.5 13.0
11.1 11.2 11.0 11.6 12.6 13.9 14.0 13.8 13.5 13.0 12.4
09.8 09.9 09.4 10.1 11.1 12.4 12.2 12.0 11.9 11.6 11.2
08.4 09.1 08.5 09.6 10.1 10.4 09.5 08.9 08.5 08.4 08.4
08.1 08.4 08.4 09.2 09.6 09.9 09.0 07.6 07.6 07.8 08.0
07.8 08.0 08.8 09.2 09.2 09.1 08.2 07.1 07.1 07.4 07.6

Ok so these are the values for the AEMetering Segments in an 11 by 7 Matrix the attached photo was scaled to 13 px by 9px and then zoomed by 1600 in Gimp and then I took a screen shot there's a tree on the left and the bottom left corner is my blue car. there's a wall running left to right. Can you see it more importantly how do the light values compare with the metering matrix above, the outside pixels are not metered.
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03-28-2017, 07:17 AM - 1 Like   #13
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QuoteOriginally posted by blackest Quote
Can you see it
Sort of. This is how Pentax themselves compare 77 segment metering to 86k metering:


QuoteOriginally posted by blackest Quote
Wow, did you actually do that?
Yes, a while ago. Took a while to get it to work, but it certainly can produce a decent image overview (if you squint) - eg from a 645Z sample image:



This is a colour-chart shot with the K-1, with a full readout of the three RGB data arrays:

03-28-2017, 08:03 AM   #14
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QuoteOriginally posted by blackest Quote
You may be right about that. However it makes sense that for a given lens that each stop down from max aperture a particular exposure compensation be applied.
You are making the unwarranted assumption that EffectiveLV is influenced by the light meter and may be used to back-calculate the aperture. To evaluate, I did some test exposures.

K-3, M-mode, XR Rikenon 50/2 (no mount contacts), f/5.6, 1/30s, ISO 3200, EC = 0, M-mode

Case 1: evenly-lit blank wall, metered exposure determined using DOF preview in M-mode
EffectiveLV = 23.5

Case 2: evenly-lit blank wall, non-metered exposure after power cycle from Case 1
EffectiveLV = 23.5

Case 3: lens cap on after power cycle from Case 2
EffectiveLV = 23.5

Case 4: same as for Case 3 except changing the shutter speed to 1/40s
EffectiveLV = 23.9

Exif F Number = 0 for all four cases
Actual LV for all but Case 4 should have been 5.0 (f/5.6 @ 1/30s @ ISO 3200)*
Back-calculated aperture for 23.5 LV = f/3557**

Conclusions:
  • EffectiveLV does not change in relation to exposure metering or light intensity
  • EffectiveLV does vary by shutter speed (did not test for ISO)
  • EffectiveLV represents the camera's exposure settings (i.e. is calculated from aperture, shutter speed, and ISO)
  • EffectiveLV calculation by the camera apparently fails over to use an arbitrarily large f-number for the case when actual aperture is unknown (i.e. could not be set by the body)
  • The assumption the EffectiveLV may be used to calculate aperture when that value is unknown to the camera is false.


Steve

* Confirmed with a Pentax-A 5/1.7 mounted with the same exposure settings.
** SQRT((2^28.5)/30)

Last edited by stevebrot; 03-28-2017 at 08:23 AM.
03-28-2017, 08:26 AM   #15
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QuoteOriginally posted by blackest Quote
Then there are m42 lenses but that is a different case to solve for and may not be possible without a diary.
I did an M42 lens too and got the same results as above. The cases are the same. Sorry that your quest for a "hack"* was unsuccessful.

I suggest that field notes and writing the aperture and EffectiveLV into the EXIF before PP may be your best option. There is some risk that the metadata may be inconsistent, but that is probably inconsequential to most tools.


Steve

* an inelegant but effective solution to a computing problem
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