Tech bytes for tea?

Pict 1: Metalwork lantern in French Provence. Exposure was well-balanced, as indicated by the histogram below.

Many digital cameras today can display histograms on their LCD screens in both the review and preview modes. The histogram helps the photographer to assess whether the exposure settings for an image are optimal. This is most useful when bright sunlight makes it difficult to judge – just by viewing the photo preview/review on the LCD screen – whether a photo is too bright or dark.

If the histogram indicates that the photo is too dark or bright, the photographer can adjust the exposure settings to get a better shot.

Pict 2: Different parts of a histogram. The photo of the metal work lantern was well-exposed.

What is a histogram?

The histogram is basically a graph of the brightness levels of all the pixels in a photograph – from pure black (brightness value zero) on the left edge to pure white (brightness value 255) on the right edge.

Pict 4: An overexposed photo has its histogram bunched up to the right.

Pict 3: Lavenders in the French Riviera - overexposed.

The number of pixels in each of the 256 levels of brightness are counted and plotted on the Y axis as a bar chart to provide an idea of the tonal distribution of a photograph and its possible problems.

The region around the left edge of the histogram represents the shadows (dark tones) in the photo while the pixels near the right edge represents the highlights (bright tones). The middle region represents the midtones.

Analysing a histogram

When a photo is opened in GIMP, you can view the photo’s histogram by selecting the Windows > Dockable Dialogs > Histogram command from the main menu. By analysing the histogram, you can better decide how to use the tools in GIMP to enhance a photo’s brightness and contrast.

Pict 5: Flowers at Gourdon - underexposed.

Pict 6: Port of Marseille on a dull and overcast day.

Pict 7: All bunched up in the middle - suggesting a photo with poor contrast.

A well-exposed photo usually has a histogram which looks like a bell-shape and which stretches from one end of the histogram to the other (see Pictures 1 & 2).

A photo that is overexposed (too bright) usually has a histogram that is bunched up on the right, while an underexposed photo will have a histogram that’s bunched up to the left (see Pictures 3 to 5).

A photo with poor contrast will usually have a histogram that is squashed up towards the middle. The photo appears dull and flat. The brightest pixels are not bright enough while the darkest pixels are not dark enough (see Pictures 6 & 7).

Exceptions

Note that there are exceptions under special photographic situations. For example, a photo that comprises mainly bright tones (such a scene comprising white snow against a light coloured sky) can have a histogram that is bunched up to the right, even though it is properly exposed (see Pictures 8 & 10).

Pict 8: Eagle perched atop Gourdon.

Pict 9: Stained glass window in Provencal church.

Pict 10: Bunching up to the right caused by bright background behind the eagle despite proper exposure.

Pict 11: Histogram is bunched up because of the predominance of dark tones - despite proper exposure.

Pict 1: The Todaiji temple as seen against a clear blue sky.

These selections allow us to do local adjustments – whatever adjustments we make are confined to only the pixels within the selection boundaries.

Sometimes, the object we want to select in the photo is against a background with uniform colour, such as the sky, where the colour comprises a narrow range of tones of the same colour (see Picture 1).

In these cases, it is easier to select the uniform background using the Fuzzy Select Tool and then invert the selection rather than try to select the object itself – especially if the object has a complicated outline.

Pict 2: The temple has been lightened up and contrast increased.

The tool works best when the background colour is markedly different from the object to be selected.

In the photo of Todaiji in Nara, the temple is slightly underexposed and flat against the bright sky. I want to lighten up the temple without whitening out the nice blue sky. So I need to select the temple and the foreground scene before applying the lightening (see Pict 2).

Fuzzy Select Tool

Activate the Fuzzy Select Tool from the Toolbox.

It is known more commonly in other photo-editing programs as the Magic Wand tool because of its ability to make a complex selection seem so easy – where large welts of relatively uniform colour are involved. In fact, the symbol and cursor for the tool in GIMP comes in the form of a magic wand.

Click on any part of the blue sky. Large parts of the blue sky are selected as indicated by the marching ants showing the complex boundary of the areas selected. What has happened is that the Fuzzy Select Tool has selected all pixels of similar colour to the pixel that was clicked by the tool.

How similar must a pixel be to the clicked pixel for it to be selected? This is determined by the Threshold slider in the Tool Options.

Changing the default value from 15 to zero means that only pixels whose colour is exactly the same as the clicked pixel is selected. Using the maximum value of 255 means all pixels in the photo will be selected.

The selection process

There is a shortcut to varying the Threshold value while using the Fuzzy Select Tool. When you first click in the area to be selected, don’t release the button just yet.

Drag the mouse to the right or downwards and the Threshold value will be increased. You can see the selected areas grow as you drag the mouse cursor. You can also see the Threshold slider in the Tool Options moving to the right.

Conversely, drag the mouse to the left or upwards to decrease the Threshold value. You will see the selected areas shrink as you drag the mouse cursor. The Threshold slider also moves to the left.

Pict 3: The bright blue sky is selected by the Fuzzy Select Tool.

Stop or drag the mouse cursor in the opposite direction when you see the selection spilling into the temple itself. Release the mouse button to confirm the selection (see Picture 3).

If there are areas in the sky, that are not selected, hold down the Shift key on the keyboard and click in those areas to add them to the existing selection.

Lighten up the temple

For the moment, the blue sky is selected while the temple and the foreground are not.

To invert the selection, press Ctrl-I or Select > Invert from the main menu. The temple and the foreground are now selected while the blue sky is unselected.

The temple and the foreground can now be lightened (for instance, with the Colors > Brightness-Contrast command) without blowing out the sky.