Articles on PIXLS.US

Darktable 3:RGB or Lab? Which Modules? Help!

Sun, 26 Jan 2020 00:00:00 +0000

Original post in French by Aurélien PIERRE , edited by the pixls community.

Darktable is slowly converging to a scene-referred RGB workflow. Why is that? What does it involve? How does the use of darktable change? Answers here…

This article begins with a 3 section introduction of the Lab space. You don’t need to understand it in detail in order to understand what happens next.

What is Lab?

The color space CIE Lab was published in 1976 by the International Commission on Illumination (CIE), in an attempt to mathematically describe the color perception of the average human being. Lab space aims to decouple the brightness information (L channel) from the chroma information (channels a and b) and takes into account the non-linear corrections that the human brain makes to the linear signal it receives from the retina. Lab space is derived from CIE XYZ space , which represents the physiological response of 3 of the 4 types of photo-sensitive cells in the retina (the cones).

Processing a nightscape in Siril

Tue, 16 Apr 2019 09:08:28 -0600

Siril is a program for processing astronomical photographs.

In this tutorial, I’ll show you how to process a nightscape in Siril 0.9.10.

It doesn’t intend to be comprehensive tutorial but rather to present a basic general workflow that is a good starting point for those who want to learn Siril.

For this purpose, I’m sharing the raw files I used for the image I presented here , except that for this tutorial I limited the number of frames for the sake of bandwidth and processing speed.

A Q&A with the CHDK Developers

Sun, 17 Feb 2019 14:11:42 -0600

Introduction

CHDK is a free, open source software add-on that runs on Canon PowerShot cameras and expands their functionality. Some of its features are:

Professional control: RAW files, bracketing, manual control over exposure, zebra mode, live histogram, grids, etc.
Motion detection: Trigger exposure in response to motion, fast enough to catch lightning.
USB remote: Simple DIY remote allows you to control your camera remotely.
Scripting: Control CHDK and camera features using uBASIC and Lua scripts. Enables time lapse, motion detection, advanced bracketing, and more.
PTP: Shooting control, live view, and file transfer from Linux and Windows.

I talked with the core team of developers to learn more about CHDK.

Create lens calibration data for lensfun

Thu, 15 Nov 2018 08:00:00 -0500

[Article updated on: 2019-12-09]

Introduction

All photographic lenses have several types of errors. Three of them can be corrected by software almost losslessly: distortion , transverse chromatic aberration (TCA) , and vignetting . The Lensfun library provides code to do these corrections. Lensfun is not used by the photographer directly. Instead, it is used by a photo raw development software such as darktable or RawTherapee. For example, if you import a RAW into darktable, darktable detects the lens model, focal length, aperture and focal distance used for the picture, and it then calls Lensfun to automatically correct the photograph.

From Russia with Love

Mon, 23 Jul 2018 16:03:11 -0600

Ilya Varivchenko is a fashion and portrait photographer from Ivanovo, Russian Federation. He’s a UNIX administrator with a long-time passion for photography that has now become a second part-time job for him. Working on location and in his studio, he’s been producing a wonderful body of work specializing in portraiture, model tests, and more.

He’s a member of the community here (@viv), and he was kind enough to spare some time and answer a few questions (plus it gives me a good excuse to showcase some of his great work!).

(NSFW) What Stefan Sees

Fri, 04 May 2018 14:11:42 -0600

Stefan Schmitz is a photographer living in Northern France and specializing in sensual and nude portraits. I stumbled upon his work during one of my searches for photographers using Free Software on Flickr , and as someone who loves shooting portraits his work was an instant draw for me.

Franzi Skamet by Stefan Schmitz

Khiara Gray by Stefan Schmitz

Profiling a camera with darktable-chart

Wed, 25 Apr 2018 19:00:07 -0500

[Article updated on: 2019-06-18]

What is a camera profile?

A camera profile is often a combination of a color lookup table (LUT) and a tone curve which is applied to a RAW file to get a developed image. It translates the colors that a camera captures into the colors they should look like. If you shoot in RAW and JPEG at the same time, the JPEG file is already a developed picture. Your camera can do color corrections to the data it gets from the sensor when developing a picture. In other words, if a certain camera tends to turn blue into turquoise, the manufacturers internal profile will correct for the color shift and convert those turquoise values back to their proper hue.

How to create camera noise profiles for darktable

Sun, 15 Apr 2018 19:00:07 -0500

[Article updated on: 2019-11-26]

What is noise?

Noise in digital images is similar to film grain in analogue photography. In digital cameras, noise is either created by the amplification of digital signals or heat produced by the sensor. It appears as random, colored speckles on an otherwise smooth surface and can significantly degrade image quality.

Noise is always present, and if it gets too pronounced, it detracts from the image and needs to be mitigated. Removing noise can decrease image quality or sharpness. There are different algorithms to reduce noise, but the best option is if having profiles for a camera to understand the noise patterns a camera model produces.

Simple Exposure Mapping in GIMP

Tue, 05 Dec 2017 09:17:37 -0600

There are many different approaches to blending exposures in the various projects , and they can range from extremely detailed and complex to quick and simple. Today we’re going to look at the latter.

I was recently lucky enough to attend an old friends wedding in upstate NY. Mairi got married! (For those not familiar with her, she’s the model from An Open Source Portrait as well as A Chiaroscuro Portrait tutorials.)

Mairi's chiaroscuro portrait.

I had originally planned on celebrating with everyone and wrangling my two kids, so I left my camera gear at home. Turns out Mairi was hoping that I’d be shooting photos. Not wanting to disappoint, I quickly secured a kit from a local rental shop. (Thank goodness for friends new and old to help wrangle a very busy 2 year old.)

Faces of Open Source

Tue, 17 Oct 2017 13:20:28 -0600

Recently, @houz posted about an amazing project by photographer Peter Adams called Faces of Open Source .

Peter really (ahem) throws a light on many amazing luminaries from not only the Free/Open Source Software community, but in some cases the history and roots of all modern computing. He has managed to coordinate portrait sessions with many people that may be unassuming to a layperson, but take a moment to read any of the short bios on the site and the gravity of the contributions from the subjects to modern computing becomes apparent.

RawTherapee and Pentax Pixel Shift

Fri, 24 Mar 2017 13:17:10 -0600

What is Pixel Shift?

Modern digital sensors (with a few exceptions) use an arrangement of RGB filters over a square grid of photosites. For a given 2x2 square of photosites the filters are designed to allow two green, and one each red and blue colors through to the photosite. These are arranged on a grid:

The pattern is known as a Bayer pattern (after the creator Bryce Bayer of Eastman Kodak). The resulting pattern shows how each RGB is offset into the grid.

A Masashi Wakui look with GIMP

Mon, 28 Nov 2016 13:25:21 -0600

This tutorial explains how to achieve an effect based on the post processing by photographer Masashi Wakui . His primary subjects appear as urban landscape views of Japan where he uses some pretty and aggressive color toning to complement his scenes along with a soft ‘bloom’ effect on the highlights. The results evoke a strong feeling of an almost cyberpunk or futuristic aesthetic (particularly for fans of Bladerunner or Akira !).

Aligning Images with Hugin

Fri, 04 Nov 2016 13:12:04 -0600

Hugin is an excellent tool for for aligning and stitching images. In this article, we’ll focus on aligning a stack of images. Aligning a stack of images can be useful for achieving several results, such as:

bracketed exposures to make an HDR or fused exposure (using enfuse/enblend), or manually blending the images together in an image editor
photographs taken at different focal distances to extend the depth of field, which can be very useful when taking macros
photographs taken over a period of time to make a time-lapse movie

For the example images included with this tutorial, the focal length is 12mm and the focal length multiplier is 1. A big thank you to @isaac for providing these images.

Highlight Bloom and Photoillustration Look

Wed, 12 Oct 2016 13:47:35 -0500

Over in the forums community member Sebastien Guyader (@sguyader) posted a neat workflow for emulating a photo-illustrative look popularized by photographers like Dave Hill where the resulting images often seem to have a sort of hyper-real feeling to them. Some of this feeling comes from a local-contrast boost and slight ‘blooming’ of the lighter tones in the image (though arguably most of the look is due to lighting and compositing of multiple elements).

A Chiaroscuro Portrait

Wed, 27 Jul 2016 13:16:07 -0500

Introduction (Concept/Theory)

The term Chiaroscuro is derived from the Italian chiaro meaning ‘clear, bright’ and oscuro meaning ‘dark, obscure’. In art the term has come to refer to the use of bold contrasts between light and shadow, particularly across an entire composition, where they are a prominent feature of the work.

This interplay of shadow and light is particularly important in allowing the viewer to extrapolate volume from a flat image. The use of a single light source helps to accentuate the perception of volume as well as adding drama and dynamics to the scene.

Display Color Profiling on Linux

Thu, 09 Jun 2016 16:50:08 -0600

This article by Pascal de Bruijn was originally published on his site and is reproduced here with permission. —Pat

Attention: This article is a work in progress, based on my own practical experience up until the time of writing, so you may want to check back periodically to see if it has been updated.

This article outlines how you can calibrate and profile your display on Linux, assuming you have the right equipment (either a colorimeter like for example the i1 Display Pro or a spectrophotometer like for example the ColorMunki Photo). For a general overview of what color management is and details about some of its parlance you may want to read this before continuing.

HDR Photography with Free Software (LuminanceHDR)

Tue, 26 Jan 2016 14:57:59 -0500

I have a mostly love/hate relationship with HDR images (well, tonemapping HDR more than the HDR themselves). I think the problem is that it’s very easy to create really bad HDR images that the photographer thinks look really good. I know because I’ve been there:

Don't judge me, it was a weird time in my life...

The best term I’ve heard used to describe over-processed images created from an HDR is “clown vomit” (which would also be a great name for a band, by the way). They are easily spotted with some tell-tale signs such as the halos at high-contrast edges, the unrealistically hyper-saturated colors that make your eyes bleed, and a general affront to good taste. In fact, while I’m putting up embarrassing images that I’ve done in the past, here’s one that scores on all the points for a crappy image from an HDR:

Users Guide to High Bit Depth GIMP 2.9.2, Part 2

Wed, 02 Dec 2015 12:00:00 -0600

This is Part 2 of a two-part guide to high bit depth editing in GIMP 2.9.2 with Elle Stone. The first part of this article can be found here: [*Part 1*](/articles/users-guide-to-high-bit-depth-gimp-2-9-2-part-1/).

Using GIMP 2.9.2 for radiometrically correct editing

<ol>
<li><a href="#linearized-srgb-channel-values-and-radiometrically-correct-editing">Linearized sRGB channel values and radiometrically correct editing</a></li>
<li><a href="#using-the-linear-light-option-in-the-image-precision-menu">Using the “Linear light” option in the “Image/Precision” menu</a></li>
<li><a href="#a-note-on-interoperability-between-krita-and-gimp">A note on interoperability between Krita and GIMP</a></li>
</ol>

GIMP 2.9.2’s unbounded floating point ICC profile conversions (handle with care!)

Using GIMP 2.9.2’s floating point precision for unclamped editing

<ol>
<li><a href="#high-bit-depth-gimp-s-unclamped-editing-a-whole-realm-of-new-editing-possibilities">High bit depth GIMP’s unclamped editing: a whole realm of new editing possibilities</a></li>
<li><a href="#if-the-thought-of-working-with-unclamped-rgb-data-is-unappealing-use-integer-precision">If the thought of working with unclamped RGB data is unappealing, use integer precision</a></li>
</ol>

Looking to the future: GIMP 3.0 and beyond

Radiometrically correct editing

Linearized sRGB channel values and radiometrically correct editing

One goal for GIMP 2.10 is to make it easy for users to produce radiometrically correct editing results. "Radiometrically correct editing" reflects the way light and color combine out there in the real world, and so requires that the relevant editing operations be done on linearized RGB.

Users Guide to High Bit Depth GIMP 2.9.2, Part 1

Sun, 01 Nov 2015 12:00:00 -0600

Introduction: high bit depth GIMP 2.9.2

    <ol>
    <li><a href="#purpose-of-this-guide">Purpose of this guide</a></li>
    <li><a href="#useful-links-the-official-gimp-website-builds-for-windows-and-mac-building-gimp-on-linux">Useful links: the official GIMP website, builds for Windows and MAC, building GIMP on Linux</a></li>
    <li><a href="#editing-in-srgb-vs-editing-in-other-color-spaces">Editing in sRGB vs editing in other color spaces</a></li>
    <li><a href="#a-note-about-the-gamma-hack-that-s-provided-for-many-editing-operations">A note about the “Gamma hack” that’s provided for many editing operations</a></li>
    </ol></li>

<li><a href="#new-high-bit-depth-precision-options">New high bit depth precision options</a>

    <ol>
    <li><a href="#menu-for-choosing-the-image-precision">Menu for choosing the image precision</a></li>
    <li><a href="#which-precision-should-you-choose-for-editing-">Which precision should you choose for editing?</a></li>
    <li><a href="#using-the-image-precision-options-when-exporting-an-image-to-disk">Using the image precision options when exporting an image to disk</a></li>
    </ol></li>

<li><a href="#new-color-management-options">New color management options</a>

    <ol>
    <li><a href="#gimp-2-9-2-automatically-detects-camera-dcf-information">GIMP 2.9.2 automatically detects camera DCF information</a></li>
    <li><a href="#black-point-compensation">Black point compensation</a></li>
    </ol></li>

<li><a href="#new-and-updated-algorithms-for-converting-to-luminance-lab-and-lch">New and updated algorithms for converting to Luminance, LAB, and LCH</a>

    <ol>
    <li><a href="#converting-srgb-images-from-color-to-black-and-white-using-luma-and-luminance">Converting sRGB images from Color to Black and White using Luma and Luminance</a></li>
    <li><a href="#decomposing-from-srgb-to-lab">Decomposing from sRGB to LAB</a></li>
    <li><a href="#lch-the-actually-usable-replacement-for-the-entirely-inadequate-color-space-known-as-hsv-">LCH: the actually usable replacement for the entirely inadequate color space known as “HSV”</a></li>
    </ol></li>

Introduction: high bit depth GIMP 2.9.2

Purpose of this guide

As announced on the GIMP users and developers mailing lists, the recent (November 26, 2015) GIMP 2.9.2 release is the first development release in the GIMP 2.9.x series leading to GIMP 2.10. The release announcement summarizes the many code changes that were made to port the old GIMP code over to GEGL’s high bit depth processing.

Freaky Details (Calvin Hollywood)

Mon, 31 Aug 2015 14:33:50 -0500

German photographer/digital artist/photoshop trainer Calvin Hollywood has a rather unique style to his photography. It’s a sort of edgy, gritty, hyper-realistic result, almost a blend between illustration and photography.

As part of one of his courses, he talks about a technique for accentuating details in an image that he calls “Freaky Details”.

Here is Calvin describing this technique using Photoshop:

In my meandering around different retouching tutorials I came across it a while ago, and wanted to replicate the results in GIMP if possible. There were a couple of problems that I ran into for replicating the exact same workflow:

Color Curves Matching

Tue, 04 Aug 2015 14:10:36 -0500

In my previous post on Color Curves for Toning/Grading , I looked at the basics of what the Curves dialog lets you do in GIMP . I had been meaning to revisit the subject with a little more restraint (the color curve in that post was a little rough and gross, but it was for illustration so I hope it served its purpose).

This time I want to look at the use of curves a little more carefully. You’d be amazed at the subtlety that gentle curves can produce in toning your images. Even small changes in your curves can have quite the impact on your final result. For instance, have a look at the four film emulation curves created by Petteri Sulonen (if you haven’t read his page yet on creating these curves, it’s well worth your time):

Basic Color Curves

Mon, 27 Jul 2015 10:26:49 -0500

Color has this amazing ability to evoke emotional responses from us. From the warm glow of a sunny summer afternoon to a cool refreshing early evening in fall. We associate colors with certain moods, places, feelings, and memories (consciously or not).

Volumes have been written on color and I am in no ways even remotely qualified to speak on it. So I won’t.

Instead, we are going to take a look at the use of the Curves tool in GIMP . Even though GIMP is used to demonstrate these ideas, the principles are generic to just about any RGB curve adjustments.

A Q&A with Photographer Riley Brandt

Wed, 15 Jul 2015 08:47:30 -0500

Riley Brandt is a full-time photographer (and sometimes videographer) at the University of Calgary . He previously worked for the weekly (Calgary) local magazine Fast Forward Weekly (FFWD) as well as Sophia Models International , and his work has been published in many places from the Wall Street Journal to Der Spiegel (and more ).

He recently announced the availability of The Open Source Photography Course . It’s a full photographic workflow course using only free, open source software that he has spent the last ten months putting together.

A Blended Panorama with PhotoFlow

Fri, 26 Jun 2015 11:31:39 -0500

The goal of this tutorial is to show how to create a sort-of-HDR panoramic image using only Free and Open Source tools. To explain my workflow I will use the image below as an example.

This panorama was obtained from the combination of six views, each consisting of three bracketed shots at -1EV, 0EV and +1EV exposure. The three exposures are stitched together with the Hugin suite, and then exposure-blended with enfuse . The PhotoFlow RAW editor is used to prepare the initial images and to finalize the processing of the assembled panorama. The final result of the post-processing is below:

Basic Landscape Exposure Blending with GIMP and G'MIC

Tue, 09 Jun 2015 10:34:49 -0500

Photographer Ian Hex had previously explored the topic of exposure blending with us by using luminosity masks in darktable . For his first video tutorial he’s revisiting the subject entirely in GIMP and G’MIC .

Have a look and let him know what you think in the forum. He’s promised more if he gets a good response from people - so let’s give him some encouragement!

An Open Source Portrait (Mairi)

Mon, 18 May 2015 12:04:49 -0500

This is an article I had written long ago (originally published in 2013). The material is still quite relevant and the workflow hasn’t really changed, so I am republishing it here for posterity and those that may have missed it the first time around.

The previous post for this article went over the shoot that led to this image.

If you’d like to follow along with the image of Mairi, you can download the files from the links below.

Skin Retouching with Wavelet Decompose

Mon, 20 Apr 2015 11:47:07 -0500

Skin retouching is a delicate art.

Effective skin retouching can feel like a black art.

There have been various methods detailed in the past for ways to “smooth” skin in GIMP . Those methods ranged from disappointing at best to downright ridiculous at worst. The disappointing methods were simply a product of the best methods available at the time. The ridiculous ones seemed to be due to a lack of subtlety.

Luminosity Masking in darktable

Tue, 06 Jan 2015 13:41:08 -0500

Luminosity Masking, the ability to create selections of your image based on its specific tones for ultra-targeted editing, is a relatively recent concept favoured by landscape photographers the world over. In this article, we will explore how to create and use Luminosity Masks in the F/OSS RAW editor darktable , so that you can make adjustments on your RAW files to isolated tones.

What is Luminosity Masking?

Luminosity Masking is a technique developed in the last 10 years or so primarily by American Southwest landscape photographer Tony Kuyper over at goodlight.us . Tony provides extensive writing and information on Luminosity Masking and how to create Luminosity Masks; in this article I’ll be primarily focusing and creating and using the masks in darktable, but if you want to really understand the basics I highly recommend giving Tony’s guide a good read over first.

Digital B&W Conversion (GIMP)

Tue, 16 Sep 2014 13:36:26 -0500

Black and White photography is a big topic that deserves entire books devoted to the subject. In this article we are going to explore some of the most common methods for converting a color digital image into monochrome in GIMP .

What We are Trying to Achieve

There are a few things you should focus on in regards to preparing your images for a B&W conversion. You want to keep in mind that by removing color information you are effectively left with only tonal data (and composition) to convey your intentions.

Building PIXLS.US

Mon, 01 Jan 0001 00:00:00 +0000

This is just a log of reference material for actually building this site. It’s mostly for my own reference and edification. If you’re reading this, good luck making sense of my notes…

Static Website with Node.js and Metalsmith

I decided to build this site as a static website. This means that I’m generating all of the material on my local machines, and then compiling them into static webpages that are then uploaded to the server for serving. While this does sound like a pain in the ass, there are static site generators that make this job much easier.