Basic processing using The Gimp

This tutorial is intended to guide relative newcomers to digital imaging through basic post-processing. The final result will be an image ready for sharing on an internet gallery. Hopefully using these steps will provide a good foundation for learning more advanced post-processing methods and tools in the future.

We will start with a JPEG file created at the camera’s largest size and finest quality setting. The image below is ‘straight off of the camera’, only resized for the article.

Unfinished image of a Heerman’s Gull, CA, USA

1. Create a copy - always treat the original images from the camera like original slides or negatives. I have a separate directory structure for the originals, which I set to read-only. If you make a mistake, you can always start again. Also, as you get better with your Digital Technique, you may wish to go back and re-process older photos.

2. Open your copy of the image in The Gimp - and zoom the image to fill the available work area. (The Gimp: on the main window select a zoom amount from the drop-down list get most of the image visible without scrolling - usually about 25 or 33%).

 

 3. Adjust brightness and contrast - to correct any minor exposure or lighting issues in the image. The simplest method is to use the ‘Brightness and Contrast’ tool to adjust the overall brightness and contrast of the image. (The Gimp: select Tools-Colour Tools-Brightness Contrast…).

 

This will open the brightness/contrast dialog box which has very simple controls.

The brightness slider controls the overall brightness of the image, move it to the left and the images is made darker, to the right it is made lighter.

 

The contrast slider works in a similar way. Move the slider to the left and the contrast is reduced, to the right and contrast is increased.

Now it is clear that such large adjustments do not give very accurate or pleasing results, so you need to look at the image and decide what adjustment is needed.

Notice how the dark and bright areas in the images above are made solid black and/or white depending on the adjustment? This is the key thing to avoid when making this type of adjustment – avoid losing detail in the bright and dark areas.

A slight increase in brightness and contrast just gives this image a slight ‘lift’. However, different images require different amounts of contrast and there is no fixed rule for applying these settings. A dramatic landscape may benefit from higher contrast whereas an intimate portrait may not. If the light was soft and misty, you may wish to boost the contrast or reduce it if there was harsh side-light.

!	Note that the brightness is controlled in the camera by the exposure settings (shutter speed, aperture and ISO).
!	Note that the contrast can be adjusted in the camera before taking a photo using the Contrast setting within its set-up menus – refer to your camera manual for details on how.

4. Set the Colour Balance - to make the whites look white! The human brain can quickly adjust the colours that we see to make white look white irrelevant of any colour cast from the light source. For example, a white card would have a blue tint on a sunny afternoon and a yellow tint indoors when lit by a light bulb. We do not normally register these differences because our brain makes the change without us knowing, however the digital image may need some help to get the colours looking correct.

In The Gimp the colour balance tool allows you to adjust the colour levels in an image by adjusting sliders of opposite colours. (The Gimp: select Tools-Colour Tools-Colour Balance...)

Although this box looks a little daunting, it is actually very simple. By adjusting the sliders you adjust the colour cast on the image.

The examples above show the effect of applying too much red or yellow. In the example image, there is a slight blue cast to the whites causing the image to appear slightly cold. Adding a little red and yellow will often ‘warm’ up an image. Note that only very minor adjustments were made and were judged by eye, using image preview (by ticking the Preview checkbox).

!	Note that the Colour Balance is controlled in the camera by the White Balance setting. This can normally be set to ‘Auto’ where the camera will estimate any adjustment or set for the current conditions - for example ‘Cloudy’ when the sky is overcast.

5. Colour saturation - have you noticed how some of the more famous landscape photographs have very rich, deep colours? This is often because of the film used by the photographer – Fujifilm Velvia is a well known example for its saturation of colours. With digital photography, saturation is controlled directly, either by setting the camera or through post-processing. (The Gimp: select Tools-Colour Tools-Hue Saturation...).

Increasing the saturation slightly will often add a little ‘punch’ to the image. Be careful though, too much will look fake and add unwanted artefacts to the image. These may be colour specs or bands in uniform areas, such as sky, or just making a single colour look too strong - green is often the first to look odd. Reduce the saturation and the colours will fade.

For now, only adjust the Saturation slider. I prefer to type a value into the box, and rarely use a value above 25 and make sure that the "Master" button is selected in the middle of the colours. Try different values and see how they look - like most things in Photography, there is no single, perfect value and any single image may need a differing amount.

!	Note that Saturation can be adjusted in the camera before taking a photo using the Saturation setting within its set-up menus - refer to your camera manual for details on how.

6. Cropping - this is where you get a second chance at composing your image. For web publishing and small prints you have a fair amount of latitude for cropping the image. The process is relatively simple and can make a significant improvement to an image.

In The Gimp the Rectangular Select tool is used for cropping an image. It is normally selected from the top left of the tool palette or by pressing 'R' on the keyboard.

You can now click and drag a box to ‘select’ the area that you want to keep from your image. By clicking the mouse and then dragging the mouse to highlight the area you want before releasing the button, you can create the selection shown above – notice the dotted line indicating the selected area.

It is possible to move the selected area using the Move Selection tool (select the tool indicated above or press 'M' on the keyboard). Make sure the Move tab is selected in the tool options and the Transform Selection button selected, as indicated in the example. Click and 'drag' the selection to where you want it.

In The Gimp, selecting 'Image-Crop Image' would crop the image to the selected area. I personally prefer to give birds a little room to move, so tend to leave some space ‘in front’, with a selection similar to below. Again, there are many tips and tricks for composing an image – none are right or wrong just different preferences.

!	Note that Composition is controlled in the camera by the photographer’s selection of lens and shooting position in relation to the subject.

7. Save the image - now is a good time to save the image! I would save the image as a TIFF file rather than JPEG - I will leave the rather complex explanation of why TIFF for another time. Saving now leaves you with a new ‘master’ file. This is where you can start any future work on the image - creating a new web-sized image or re-sizing for printing.

8. Image size - this is different to cropping the image, which changes the composition, in fact it is exactly the reverse of enlarging the image in traditional processing. The idea is to display the picture at a comfortable size on a web-page. In The Gimp, open the image size dialog (Image-Scale Image…).

The dialog has two sections under Image Size, Width and Height and the X and Y resolutions. To resize a document for the internet, we are interested in the number of pixels per side of the image, rather than physical size in inches or centimetres.
In general, you will want the image to be about 700 to 800 pixels on the longest side. This means that you set the width or height depending on the orientation of the image. In this case the image is orientated to landscape, so the longest side is width. The height would be the longest side for a portrait orientation.

Under the Quality section, set Interpolation to “Cubic (best)”. In the width field, type the image size you want (in this case 720 pixels). The Gimp will automatically calculate the number of pixels for the Height.

After pressing Scale zoom 100% into the image.

9. Sharpening - the final improvement before saving. Now that all of the post-processing is complete the image is nearly ready. Whilst all of the steps above have improved the image tones, colour and composition they have unfortunately degraded the sharpness. Each step of the digital capture and processing introduces an amount of softness into the image, starting with the light capture by the camera sensor, through in-camera processing to each step documented above. Some amount of sharpening needs to be applied to each and every digital image, whether applied in-camera or during post-processing.

There are many methods to sharpen an image and many approaches. The most commonly used is the Unsharp Mask filter (in The Gimp, Filters-Enhance-Unsharp Mask…).

The settings used above are a good start point for web sized images (Radius: 1.0, Amount: 1.2, Threshold: 0 levels) but every image is different and requires more or less sharpening. Vary the amount and Radius in small amounts, increasing the values to increase the effect and decreasing to reduce. It is worth increasing the size of the dialog to get as much of the image into the preview window as possible. Also too much sharpening can cause some nasty side-effects.

Notice the high contrast on the edges of the bird and slight halo around the outside - too much sharpening.

Remember that this is not correcting any problems with the actual captured image - if it is out of focus or blurry due to motion then there is little that can be done to fix the image.

10. Save the image - by creating a new version. Saving the image as a jpeg will allow you to adjust the final file size using the Quality setting. In The Gimp, File-Save as… will open the following dialog.

Expand the Browse other folders and Select File type to reveal all of the options as seen above. Select the location you wish to save the image and select the JPEG file type. Press OK and this will open the JPEG dialog.

When you change the Quality setting, the size of the file will change where the higher the quality setting, the larger the file. The file size is estimated in the lower part of the dialog. In general a file should be saved less than 200K for use on web galleries. However, if you reduce the quality too far, you will get nasty effects in the image.

So, that’s all there is to it! Whilst this process seems complex and too much effort at first, it quickly becomes easy and second nature. These types of adjustment should take less than 2 minutes with practice and do give that extra bit of quality to your images.

Heerman’s Gull, CA, USA

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