I thought it would be helpful and fun to discuss basic instructions for photo editing sketches for posting here on Cloudy Nights. This is just one method you could employ using the free image editing software GIMP, which is similar to Adobe Photoshop and available on desktop for PC/Mac/Linux. There are many great free and low cost image editing applications and apps available that can be used to do the same thing, and I welcome any and all suggestions for other software that you've used to edit and process your sketches.
In this case, I was interested in editing an image of a pencil sketch made on white paper and photographed with my iPhone, the goal being to invert and desaturate the image and then adjust exposure and contrast levels and finally scale and size appropriately for the Cloudy Nights Sketching Forum. From the get-go, GIMP looks a bit complicated if you haven't used it before, so I've included screen shots of each step I used to edit and process my sketch image. Once you get used to the interface, GIMP actually has almost all of the same tools and features as the full blown professional Adobe Photoshop and can be a very powerful image editing tool.
After importing the image from my phone to computer, I open it in GIMP and then use the cropping tool if necessary to crop out any unwanted parts of the image. My next step is to desaturate the image since it's pencil on white paper. I'm pretty lazy when it comes to photographing my sketches and quite often color casts from different light sources will be visible in an image, sometimes yellow or blue. And smart phones aren't perfect for establishing neutral color balance. One way to get around this is to make sure you are using the same type of lighting when photographing your sketches. Don't mix daylight and an indoor LED lamp for example. I photograph most of my sketches using soft indirect daylight during the daytime. Desaturating insures that you won't see any color in an image. Omit this step if your sketch is in color. I've photographed color sketches outdoors on overcast days and this works pretty well with minimal need for color correction.
Because my sketch is on white paper, the next step is to invert the image to get a light object and dark background.
The following step may be the most difficult to master using GIMP's Levels adjustments, but with practice it will be easier to do in time. I prefer Levels to the basic Contrast and Exposure adjustments because I can darken my sketch to simulate the actual sky brightness I experienced during observing. Simply slide the middle Input slider in the Levels Tool to darken or lighten the image to the desired amount. Then, adjust the far right Input slider to increase the background sky brightness slightly if needed. If the highlights are too bright, adjust the far left Input slider to darken. You may need to adjust the middle slider again after adjusting the other two.
Next, scale the image to the appropriate size for posting here on Cloudy Nights (if you are uploading directly to your post). Rule is 1600 pixels longest dimension (or 1600 x 1200 pixels at 72dpi), no more than 500kb in file size.
Finally, you'll need to Export your file as a compressed JPEG for posting. Note the estimated file size on the last GIMP screen shown. Adjust the quality level until the file size is below 500kb.
Good luck and I hope this helps!
