Tech bytes for tea?

Pict 1: The Layers dialog in GIMP displays the layers in the image file and facilitates the manipulation of the layers.

Yesterday, we used GIMP’s automated adjustment tools to tweak the brightness and contrast of digital photos. We also tried out manual exposure adjustment using the Brightness-Contrast command. Today, we take a look at Layers and use Layer Modes as an alternative way of tweaking brightness and contrast.

Quick intro to Layers

When a digital photo file is opened in GIMP, it is placed in the Background layer. You can see the Background layer in the Layers dialog (see Picture 1). If you do any painting or make any brightness or contrast adjustments to the photo, pixels from the photo itself in the Background layer is altered permanently (unless you use the Undo command).

Pict 2: Layers dialog with duplicated layer. The Layer Mode and Opacity slider is at the top.

You can create additional image layers above the Background layer. The commands for manipulating layers can be found in the Layers dialog. The six icons at the bottom of the Layers dialog include commands to create a new layer, and to duplicate or delete a selected layer.

Inside the Layers dialog, click on a layer’s name to select the layer, double-click the name to rename it and toggle the eye icon to the left of the layer name to hide or show the layer’s contents. When a layer is selected, any painting or adjustments will be confined to the pixels in the selected layer.

Pict 3: The screen layer mode lightens the pixels below. Duplicate the layer twice for a stronger lightening effect.

The pixels in different layers interact with each other in different ways – depending on the Layer Mode for each layer. There is a total of 21 different Layer Modes. Select one using the “Mode” dropdown box right at the top of the Layers dialog.

The default mode for a layer is “Normal” – the pixels in the upper layer simply obscure the pixels in the layers below. Below the “Mode” dropdown box is the “Opacity” slider which determines how much of pixels in the layer are visible.

Below, we will use the three most commonly used Layer Modes to darken, brighten or increase the contrast of a photo.

Pict 4: Create a new layer from all the earlier layers and apply Contrast layer mode.

First duplicate the Background layer containing the photo by selecting the Layer > Duplicate Layer command from the main menu. You can also click fourth icon from the left at the bottom of the Layers dialog.

A new duplicated layer containing a duplicate copy of the photo is created above the Background layer. In the Layers dialog, click on the name of the duplicated layer to select it. Now click the “Mode” dropdown box at the top of the Layers dialog to change the Layer Mode of the duplicated layer.

To darken overexposed photos that are too bright, set the Layer Mode of the duplicated layer to “Multiply”. The photo is now darkened. If the darkening effect is too strong, reduce the opacity of the duplicated layer by dragging the Opacity slider (just below the “Mode” dropdown box) to the left (see Picture 2 above).

Pict 5: This photo was shot near the asylum in Saint-Rémy-de-Provence, France; where Vincent van Gogh stayed and convalesced for a year

Pict 6: The original overexposed photo has been darkened by duplicating the layer and setting its layer mode to "Multiply"

Making underexposed photos lighter

To lighten underexposed photos that are too dark, duplicate the background layer containing the photo and set the Layer Mode of the duplicated copy on top to “Screen”. As before, control the intensity of the lightening either by reducing the opacity of the duplicated layer, or by further duplicating the duplicated layer – or a combination of both (see Picture 3 above).

Pict 7: These beautiful roses in the courtyard of Monastère St. Paul-de-Mausole are underexposed and too dark.

Pict 8: The roses have been brightened up by applying the "Screen" layer mode to the two duplicated layers.

If the background layer containing the photo is the only layer in the Layers dialog, duplicate the background layer using the Layer > Duplicate Layer command and set the Layer Mode of the duplicated copy on top to “Overlay”. This will increase the contrast of the photo.

Pict 9: This photo of a forgotten statue at a corner of the asylum appears dull and flat.

Pict 10: An "Overlay" layer mode was used to increase the contrast of the statue.

Set the Layer Mode of the new layer on top to “Overlay” to increase the contrast (see Picture 4 above).

Tomorrow, we’ll look at how to remove unwanted colour casts from photos using GIMP.

Tags: GIMP, image editor, photo editor, tips

This entry was posted on Wednesday, January 5th, 2011 at 8:00 am and is filed under GIMP, GIMP Basic Series, Howtos, Software. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.