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peterhil
peterhil
With a PDF file you can also make the image crop box be exactly the reference image area, add crop marks and put the CC note and other info possibly inside or outside the crop area.
peterhil
For anybody wondering – the correct DPI is about 508 for some reason.
tingle
508 pixels/inch = 200 pixels/cm, exactly. The resolution unit in PNG is either the meter, or it is unspecified. 1 inch = 25.4 mm, 25.4 * 2 = 50.8. 508 is close enough to 600, and 600 dpi is a resolution commonly found in desktop printers.
If you've worked with PNG files under Photoshop, you may have noticed that a picture saved with exactly 72 dpi has 72.009 dpi when you close it and open it again. Using an integer multiple of 254 for the dpi prevents such a drift.
Gracana
It's a nice round metric measurement. 508 dots per inch = 20 dots per mm.
traceroute66
Its all very well saying "here's a free photographic reference, download it and print it". But I worry about the lousy job your average cheap home inkjet printer will do of getting it onto paper.
Some things are just better bought from a reputable manufacturer than bodging your own. Photographic reference charts may well be one of them.
hatsunearu
Printing middle grey with an uncalibrated printer is uh... kinda meaningless.
https://smile.amazon.com/WhiBal-White-Balance-Pocket-Card/dp...
I've been using this for my photo adventures; I try to keep it in my wallet at all times (I worry about it getting discolored by rubbing against other cards though) and whenever I notice that the white balance is gonna get fucky (esp when there are multiple light sources) I whip that out and take a picture of it in the same scene.
edit: Also people love to use grey cards for getting exposure right but it's complete bunk science; if you're using the raw workflow (if you're fine tuning white balance you almost definitely are) you should shoot as bright as practically possible and not too bright to prevent clipping and adjust exposure as you see fit in editing.
D13Fd
Unless you are doing extreme precision professional work--likely an extremely small subset of the people reading this, if any--a calibration target from a black and white laser printer or a photo printer is fine and will give great results.
Some photographers seem to worship color correction in a weird way. Other error factors, like reflected light hitting your calibration target, are likely to supercede any minor color imprecision from those kinds of printers.
zokier
Wouldn't practically all printers do middle grey with only black ink/toner/whatever, thus having very little chance of having any color cast? Considering how color temp adjustment is not very exact thing to start with, I feel that generic printout should be good enough for most uses. And for color critical work, you'll want a full color swatch (eg colorchecker) anyways instead of just middle grey.
incrudible
I don't know the impact of this, but:
- there is no such thing as perfectly black ink
- color printers will mix color ink into greyscale printouts
Doxin
A "fun" workaround would be to print a very high resolution black-and-white checkerboard. Printers generally have plenty of resolution to make that appear as gray even to the naked eye. Of course managing to print something like that without something in the pipeline trying to anti-alias stuff and introduce gray back is left as an exercise to the reader.
smodad
Thanks for posting the link, because I was about to ask. It seems like printing out the reference on a home printer would make the accuracy dependent on whether or not your printer was well calibrated (and then you're back to the same problem about color accuracy.)
doodlebugging
I was scanning satellite photos yesterday and came across what I think are ground reference points built to enable determination of exact ground distances using a series of concentric rings and oriented linear features. I believe knowledge of the exact diameter of the rings and length of the chords would allow a user to measure features in the photo with acceptable precision.
I know that early pilots had a series of markers that they could use to keep themselves on track as they flew their long distance routes. These things were oriented concrete markers, some of which are still accessible.
We all need a reference, a way to cross-check or ground truth our results and conclusions.
dcminter
After reading this article I noticed the reference on this BBC article: https://www.bbc.com/news/science-environment-57853537 (first picture)
Though I note that that one looks suspiciously like the contra-recommended laser printed output.
Baader-Meinhof phenomenon - or did reading that Beeb article inspire someone to find and post this one?
iseanstevens
Thanks, this is handy.
I’ve done some little experiments but having a standard-ish thing is better.
Would this integrate to give scale, in say, Meshroom? Or it’s manual calibration step someplace along the workflow?
I’m in it more for the reasonably accurate 2d/3d measurements, but approximately accurate color is nice as well.
snoopen
I'm curious if there are any good methods for correcting lens distortion. Like can I print a poster, take a photo and use that to create a correction map?
yboris
If I remember right, the professional software "DXO Optics Pro" was doing this. Since then, they bundled that functionality into their newer products:
https://www.dxo.com/technology/lens-distortions/
The way it works, iirc, is you can choose the model of your lens and the software fixes things. Works for professional lenses, unsure about others.
zokier
Opencv has basic distortion correction: https://docs.opencv.org/4.5.3/dc/dbb/tutorial_py_calibration...
For more advanced stuff, look up CAHVORE models and related models.
mrichards212a
Does anyone know of an opencv which can be used to calibrate calibrate using something like this?
neolog
Is a single gray really enough to calibrate color?
jedimastert
Neutral gray is normally used for exposure.
The theory behind modern light metering is that if you average the "brightness" of every pixel (or the average density of crystals in the case of film) is should "visually" be around 50% brightness or density, or neutral grey. Spot metering on something that's already neutral gray sort of short cuts that process
stavros
Not all color, but you don't need all color. You need color temperature and maybe luminosity, and it's good for that.
stared
> You need color temperature
For artificial light (or mixed sources, e.g. from reflections), it is not correct. The light source is characterized by its whole distribution (which can be anything) rather than one temperature parameter (for the Planck distribution).
An easy example is a fluorescent lamp spectrum (https://commons.wikimedia.org/wiki/File:Fluorescent_lighting...).
gsliepen
Unfortunately, cameras only capture RGB values, not a whole spectrum. It is impossible to perfectly color-correct the image unless you know both the spectrum of the light source and of the materials in the scene. A reference card with lots of colors still only provides you with the known spectra of the colors on that card, it still might not contain a color with the exact same spectrum as that flower you are trying to photograph for example.
Another issue is that even if you have the perfect photography + post-processing setup, when I see the result it might be on a non-calibrated monitor, or on paper but in a room with non-perfect lighting, and perhaps I was just looking at something else a second before and my brain's auto white-balance hasn't caught up yet. So I might still not perceive it as was intended. This is why that gold dress looks blue to some people.
The patch of grey on the reference scale + good lighting of the scene to begin with will get you 99% of the way, the remaining 1% is extremely hard to get right, if at all possible.
andreareina
You're going to have a hard time calibrating that without a whole bunch of patches with known reflectance/transmission profiles, in practice you just have to color-correct scenes under such light by eye and there will be things that look wrong. When I did this professionally I'd still use the grey as a primary reference along with skin, since we're very attuned to what those should look like.
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This seems very handy – however the reference image would be infinitely easier to print on the correct size if it was a PDF file. PNG image format does not retain the resolution IIRC.