Twilight — the time of magic light before sunrise and again after sunset — is when "sunlight scattering in the upper atmosphere" partially illuminates the earth's surface. Twilight ends in the evening and begins again in the morning when the Sun reaches 18 degrees below the horizon, at which point "scattered light from the Sun is less than that from starlight". The mid-point of twilight is when the sun is Nine Degrees Below the horizon.
There are two approaches to learning anything: Learn theory and then try to apply it. Or (my preferred approach) start experimenting and worry about theory later.
This "Color Management Experiment Kit" has ICC monitor profiles to play with, experiments to try, and images to try them on, sort of like the digital darkroom equivalent of being handed a kid's chemistry set, except the only thing you'll risk blowing up is a few pixels . . .
When Marti Maria rewrote LCMS to produce LCMS2, his motivation wasn't just to accomodate V4 ICC profiles: he designed LCMS2 to work in "unbounded mode". Unbounded mode ICC profile conversions eliminate interim clipping from color space encoding limitations. And when using linear gamma profiles at 32-bit floating point image precision, along with an appropriate output format, magic happens . . .
I run Linux and of course I use Gimp. But mostly I've used Gimp for resizing images for the web, which I've come to realize is somewhat akin to shooting flies with a canon. So when I was approached about writing a review of 'The Book of Gimp: A Complete Guide to Nearly Everything', I agreed, thinking I might learn a thing or two. Turns out there is a whole lot about Gimp that I didn't know, that I really needed to know to get the most out of using Gimp. If you don't know Gimp as well as you'd like to, read 'The Book of Gimp' . . .
This guide shows how to build and install multiple versions of Gimp — Gimp 2.9 from git, Gimp 2.8, and Gimp 2.6 — on 64-bit OpenSuse 12.2, with each version of Gimp in its own separate prefix. When you are finished, you can run all three versions of Gimp, even at the same time if you want to.
Specific information on this page applies to 64-bit openSUSE 12.2 and is current as of January 21, 2013. The general process should be valid for most flavors of Linux, for the reasonably foreseeable future . . .
This raw processor review focuses on unenhanced (radiometrically correct) raw processor output. Most raw processor reviews compare the results of processing raw file(s) using each raw processor's default settings, which means the resulting image has not only been interpolated (top image), but also enhanced (bottom image). So in effect the usual raw processor review is more a review of the raw processor's default image enhancement algorithms than its actual raw processing capabilities . . .
Has digiKam or some other DAM software left your image metadata in a mess? Exiftool can clean it up. Although this article focuses on removing unwanted metadata written by digiKam, the process can be adapted to clean up your image metadata regardless of which DAM software made the mess in the first place . . .
Technically speaking, RGB color mixing should always be done in a linear gamma color space. When you blur or use the normal blend mode to mix colors in the regular sRGB color space, the resulting color is darker than it should be and sometimes acquires a noticeable color cast.
This article is the first of a planned series of articles on editing in a linear gamma color space. Upcoming articles will examine channel mixing, converting a color image to black and white, applying curves, and using different blend modes . . .
I compared 15 sRGB (matrix) profiles used by a variety of open source image editing programs and found quite a bit of variation from one sRGB variant to the next. In this article I explain the differences between the sRGB profiles variants and I point out the practical, digital darkroom consequences.
Even though Hewlett-Packard and Microsoft proposed the sRGB color space as a way to eliminate color management, today you need color management just to manage the proliferation of sRGB profiles . . .
LCDs and CRTs — color-wise, they are very different beasts. If you are using sRGB as your LCD monitor profile, you might want to reconsider: sRGB was designed for use with CRT monitors. If you have an LCD monitor, sRGB is not likely to be a good enough match . . .
What's that funny horseshoe thing? It's all the colors in the world . . . You use color space profiles every day in your digital darkroom. If you'd like to know more about what a color space profile is, read on . . . Our specimen color space profile will be the ubiquitous and historically interesting sRGB . . .
Big changes are taking place in open source color management, including whether, how, and what happens when a system monitor profile is set. The monitor profile your editing software uses determines what you see on your screen. So now is a very good time to learn more about monitor profiles.
If you are not really sure what the difference is between calibrating and profiling a monitor; if you've heard the words "vcgt tag" but don't know what it means; or if you'd like to know what a system monitor profile is, this article is a place to start.
If you are a software developer and you mistakenly think that making and using more than one monitor profile is just plain stupid (the esteemed Bruce Fraser would have disagreed with you), then by all means, please read this article . . .
See Open Source Photography Articles & Tutorials for links to all the articles and tutorials on open source photography.