Tech bytes for tea?

The Levels command allows the user to adjust the three main tonal ranges of a photo – the shadows, midtones and highlights. The Curves tool, however, allows you to target any tone or tonal range in the photo to tweak their brightness and contrast.

Using the Curves command

With the photo open in GIMP, choose the Colors > Curves command (or Tools > Color Tools > Curves)  from the main menu. An “Adjust Color Curves” dialog box pops up showing a square grid with a straight diagonal line. The histogram of the photo can be seen in the background of the grid for reference.

As for the Levels command, an understanding and analysis of the histogram of the photo is the basis for using the Curves command.

The horizontal X-axis of the grid represents the initial brightness values – from zero (black) to 255 (white) – of the pixels in the photo before the Curves command is applied. The vertical Y-axis represents the brightness values that each pixel is to be mapped into after the adjustments have been applied. It also ranges from zero to 255.

The lower left corner of the grid represents the black point (for pixels with brightness value zero) while the upper right corner represents the white point (for pixels with the maximum brightness value of 255).

Adjusting brightness

To begin adjusting the photo, click anywhere on the diagonal line in the grid. An anchor point is added to the line. You can drag the anchor point around with the mouse.

Drag the point downwards to make the photo darker. The straight diagonal line turns into a curve passing through the anchor point and the black and white points. A faint straight diagonal line is still visible in the grid – it serves as reference for an unadjusted photo.

When the cursor is inside the grid, you can see the X (input) and Y (output) values of the cursor at the top left hand corner of the grid. Mouse-over the anchor point that you’ve just dragged. In the example, you can see that the brightness of any pixel with original value 128 (X-value) will be reduced to 160 (Y-value). Pixels with brightness close to 128 will also be darkened. You can eyeball the effects of the adjustments by looking at the photo itself in the image window.

To lighten the photo, drag the anchor point down below the faint diagonal guide. Now pixels with brightness 128 will be darkened to 90, while the pixels of similar brightness will be darkened as well.

To remove an anchor point, simply drag it off the side of the grid. You can add as many as 14 anchor points (excluding the original black and white points) to the curve. This allows you to target up to 16 specific brightness values in the photo for brightening or darkening. However, you seldom need more than a handful anchor points to get the job done.

Adjusting contrast

If the histogram is bunched up the towards the middle, drag the black or points inwards horizontally to where the histogram begins and ends respectively. This is similar to dragging the black and white points of the Levels command inwards to maximise the tonal range of the photo.

The steeper gradient of the line indicates that the contrast of the photo is increased.

A typical Curves adjustment that can be applied to most photos is the “S-Curve”. This curve tends to enhance most photos by increasing the overall contrast and making the photo “pop” with vivid shadows and highlights.

To apply an “S-Curve” adjustment, add two points on the Curve – the first to lower the brightness of pixels of brightness 64, the second to increase the brightness of pixels of brightness 192. The above numbers are just guides – drag the adjustment points around with the mouse while eyeballing the photo.

The S-Curve suppresses the shadows and highlights while increasing the contrast of the mid-tones, where the main subject usually is. Add a third point with brightness of roughly 128 to increase or decrease the brightness of the midtones.

In addition to these adjustment points, you can add additional adjustment points to tweak specific tones in the photo – such as skin tones.

With some practice, you will be able to control and enhance any particular tone that appears in your photo.

This basic adjustment command uses a rudimentary algorithm that treats all pixels equally and adjusts them to the same extent. It’s useful if all you need is a quick and dirty adjustment to the photo.

For almost the same effort, the Levels command provides better control and the algorithm yields better results adjusting pixels to different extents depending on the settings you make.

That is why for many free photo editors, more advanced tools like the Levels and Curves command are not available or are only available when you upgrade to a paid version. In GIMP, these tools available for free.

Using the Levels command

Pict 3: This photo of a hanging potted plant is slightly underexposed because the camera has been fooled by the bright background.

Pict 4: The red petals are now more vibrant and discernible after the photo has been enhanced with the Levels command in GIMP.

Last week, we looked at how to analyse a photo’s characteristics and problems by examining its histogram in GIMP.

Using the histogram as a basis, we’ll learn to use the Levels command to improve the tonal distribution of a photo such that the overall brightness and contrast of the photo is just right and the entire range of brightness values (from 0 to 255) available is fully utilised.

With the photo open in GIMP, choose the Colors > Levels command from the main menu. A complicated looking Levels dialog box pops up showing a histogram of the photo as well as many controls and input fields to control the histogram is to be adjusted.

The reason why many beginners hesitate to use the Levels command is the seeming complexity of the dialog box. But in most cases, all you need is to drag one to three of the triangular sliders directly below the histogram. Here’s how.

Darkening a photo

Pict 5: Busy bee working on Lavender flowers in Provence, south of France.

Pict 6: The photo after darkening using the Levels command in GIMP.

Pict 7: Drag the triangular slider from the left edge inwards towards the right.

The three boxes below the histogram shows the numeric brightness values of where the sliders are. It serves as a reference or when you need to key in specific numeric values to use. Most of the time, you should simply drag the slider inwards based on where the bulk of the histogram begins.

Dragging the blackpoint slider to 60 tells GIMP to darken pixels of brightness 60 and below down to zero. The other pixels in the histogram are correspondingly darkened as well. This also increases the overall contrast of the photo because a wider range of brightness value is now used in the photo.

To further finetune the brightness of the photo, drag the centre slider to the left or right to lighten or darken the mid-tones.

Lightening a photo

Pict 8: Drag the triangular slider from the right edge inwards towards the left.

For the slightly underexposed photo of the potted flower on the wall, the histogram stops short at around the brightness level 210. Drag the whitepoint triangular slider from the right edge inwards towards the left until it is at the point where the main bulk of the histogram ends (see Pictures 3, 4 and 8 ).

Dragging the whitepoint slider to 210 tells GIMP to set 210 as the white point – all pixels that are of brightness 210 and greater are increased to the maximum of 255. The other pixels are correspondingly lightened as well.

As before, the overall contrast of the photo is increased because the entire range of brightness values is now used in the photo.

Again, drag the centre slider to the left or right to lighten or darken the mid-tones further.

Adjusting all three sliders

The snapshot of the port of Marseilles was taken on an overcast evening and is not only too dark but sorely lacking in contrast, resulting in a dull and flat photo (See Pictures 1 & 2).

Pict 10: The histogram is now spread across the entire range of brightness.

The histogram in the Levels dialog box shows that all the pixels are bunched up in the mid-tones – there are now extreme white or black pixels – so the highlights are not bright enough while the shadows are not dark enough.

Pict 9: Drag the blackpoint and whitepoint triangular sliders from both edges inwards.

In this case, drag the blackpoint and whitepoint sliders inwards to where the bulk of the histogram begins and ends (see Picture 9). This darkens the shadows and lightens the highlights forcing the bunched up histogram to spread out across the entire range of brightness – thereby increasing the overall contrast of the photo.

As the photo is still too dark, drag the centre slider to the left to further brighten the photo.

The resulting histogram after the adjustment is now spread out across the entire brightness range (see Picture 10).

Make a poster or postcard from your favourite photo.

Ever wanted to make a postcard or a poster from your favourite photo? Here are some techniques using layers and layer masks in GIMP.

Add a grid of white dots

Open the photo in GIMP. From the Layers dialog, you can see it resides in the Background layer which is the only layer for the moment.

The original photo from morguefile.com.

A grid of white dots is overlaid onto the photo.

A new empty layer filled with white is added above the original photo in the Background layer.

Right-click on its thumbnail in the Layers dialog and select “Add Layer Mask” command from the pop-up menu. In the “Add Layer Mask” dialog box that pops up, select the “White (full opacity)” option and press the OK button. A layer mask filled with white is added to the layer – you can see its thumbnail beside the original thumbnail in the Layers dialog.

Use the Grid filter to create the grid of white dots.

From the main menu, select the “Filters > Render > Pattern > Grid” command. In the Grid dialog box that pops up, increase the horizontal width setting until the preview in the dialog box shows a grid of white dots of the desired size. Here I used a setting of 13.

The white layer now appears as a grid of white dots overlaid on the original photo.

Right-click on the upper layer in the Layers dialog and select “Apply Layer Mask” to merge the layer mask into the original solid white layer. The layer mask is removed and the layer becomes a transparent layer with a grid of white dots overlaid on the photo below.

Hiding the white dots from the heads and faces

Use a layer mask to hide the white dots from the heads.

Add a new layer mask on this layer, again selecting the “White (full opacity)” option in the  “Add Layer Mask” pop-up dialog box.

Press “D” on the keyboard to reset the Foreground colour to black. Activate the Paintbrush Tool from the Toolbox and paint on the layer mask in the image window. Paint around the heads and faces of the wedding couple to hide the white dots in that area.

From the main menu, select the “Layer > New from Visible” command to create a new layer that combines all the visible layers in one layer.

Adding the green arc

In the Layers dialog, click to select the layer below the new combined layer. This is so that any new layers created will be added BELOW the combined layer.

Zoom out until the photo is smaller than the image window.

Set the Foreground colour to green (or your preferred colour). Add a new layer by clicking the icon at the bottom left corner of the Layers dialog. In the “New Layer” dialog box that pops up, select the radio button for “Foreground color” option under Layer Fill Type and press the OK button.

A new layer filled with green is added just below the combined layer.

Right-click the combined layer in the Layers dialog to add a layer mask to the layer, using the “White (full opacity)” option. Press the minus key (“-”) on the keyboard to zoom out the view in the image window.

As the photo becomes smaller than the image window, you will see white space between the edges of the photo and the edges of the image window.

Adding a green arc at the bottom of the poster.

Activate the Ellipse Select Tool from the Toolbox and drag an elliptical selection around the couple’s heads. You can begin dragging in the white space between the photo and the edge of the image window so that the elliptical selection covers most of the photo except for a small arc at the bottom of the photo.

Remember you can adjust the size and position of the elliptical selection by dragging the side or corner control handles of the selection outline. Once happy with the selection, use the “Select > Invert” command from the main menu to invert the selection.

Press “D” on the keyboard to reset the Foreground colour to black. Select the “Edit > Fill with FG Color” command to fill the arc with black (in the layer mask).

An arc of green is revealed below the photo.

Adding the white highlight

Click on the green layer in the Layers dialog to select it.

Add a white highlight band in the green arc.

Add another new layer, this time selecting the radio button for “White” option under Layer Fill Type. The new white layer is added above the green layer but below the combined layer.

Add a layer mask to the white layer, selecting the “Black (full transparency)” option in the pop-up dialog box. A layer mask filled with black is added to the layer.

Press “D” to reset the Background colour to white. Using the Rectangle Select Tool from the Toolbox, select a vertical band and use the “Edit > Fill with BG Color” command from the main menu. A white band is added to the layer mask such that a white band runs through the green arc.

Adding the text captions

GIMP's Layers dialog.

Click the combined layer (the topmost layer) in the Layers dialog to select it, so that any text captions added will be added above it.

Activate the Text Tool from the Toolbox. In the Tool Options below the Toolbox, set the font, size and colour settings for the Text Tool. Click in the white band in the arc with the cursor and type the word “Wedding”. Click in the green portion to add the word “Kiss”.

After adding the captions, you can still change the font, size and colour setting for these captions by selecting them first with the Text Tool and then modifying them in the Tool Options.

Add edge shadow

Add a new empty layer right at the top of all other layers. Activate the Blend Tool from the Toolbox. Press “D” to reset the Foreground colour to black and the gradient to “FG to Transparent”.

Again, zoom out the photo (by pressing the minus key on the keyboard) until the photo is smaller than the image window. Drag a black-to-transparent gradient from outside the right edge of the photo (but inside the right edge of the image window) to slightly inside the right edge of the photo.

Save the file in .xcf format so that you can come back to tweak the design in the future. Save as .jpg to send the file for print at the studio.

Composite two photos seamlessly together using GIMP.

Now that you have learnt about Layers and Layer Masks, there are many tasks and effects that you can do using them. I had some questions from readers about how to apply certain effects to their photos so that they can use them in their websites.

Morning dew on flowers outside our hotel in Provence, France

Fading to white allows captions to be added.

Fading a photo to white (or any other colour)

This is easy. After opening the photo in GIMP, activate the BlendTool in the Toolbox.

The Blend, Move and Paintbrush tools are in the Toolbox.

In the Tool Options below the Toolbox, click on the rectangle beside the “Gradient” label and from the dropdown list of options, select “FG to Transparent” gradient.

Click on the Foreground Swatch in the Toolbox and select white from the “Change Foreground Color” dialog box that pops up. If you want the photo to fade to black, red or any other colour instead of white, simply select that colour and set that as the Foreground colour.

Now move the cursor to the image window and drag it from the left side of the photo to the right. A gradient going from pure white to nothing is overlaid onto the photo so that it appears as if the photo is fading into white. If you are not satisfied with the fading, press Ctrl-Z to undo and try dragging again. Trial and error is the best way to get your ideal fading effect.

To fade in more gradually, drag further. You can of course drag from right to left or drag vertically or diagonally – depending on how you want the fading effect to appear.

The disadvantage of this quick-and-dirty method is that the gradient is added directly onto the photo. Once you’ve saved and closed the file, you can’t adjust the fading effect to reveal more of the original photo anymore.

Applying the gradient on a separate layer

Here’s a non-destructive approach that allows the fading effect to be tweaked six months down the line.

Instead of applying the gradient directly to the photo in the Background layer, apply it to a new empty layer above the Background layer. This way, none of the pixels in the original photo in the Background layer is changed or “destroyed”. The fading effect can be changed anytime in the future by replacing or modifying the colour and gradient in the upper layer.

Here are the steps.

Apply the gradient to an empty layer above the photo.

Add a new layer by clicking the icon at the bottom left corner of the Layers dialog. In the “New Layer” dialog box that pops up, select the radio button for “Transparency” option under Layer Fill Type and press the OK button.

A new empty layer is added above the original photo in the Background layer.

Use the Blend Tool to add the gradient to the new empty layer.

The result is that the photo appears to fade into white. You can further finetune the overall effect by using the Move Tool from the Toolbox to move either the photo in the Background layer or the gradient in the upper layer.

Simply activate the Move Tool from the Toolbox, click to select the layer you want to move in the Layers dialog, and drag in the image layer to make the move. You may find it easier to first change the default option of the Move Tool to “Move the active layer” to better control which layer you want to move.

Merging two photos into each other (non-destructively)

Little house on the prairie? More like ruins in a field of lavender.

This intelligent Spaniel named Bugis lives on the lavender fields of the Valensole plateau.

Pleasant memories from our drive through Valensole in Provence

To merge two photos into each other, simply place one photo as a layer above the other photo. Add a layer mask to the upper layer and use the Blend Tool to apply a gradient to the layer mask. The two photos will appear to blend into each other.

Here are the steps.

Open the first photo in GIMP. The photo resides in the Background layer which is the only layer in the Layers dialog for the moment. Bring in the second photo as a new and separate layer by using the “File > Open as Layers” command from the main menu. The new layer sits above the first photo in the Background layer.

Right-click the image thumbnail for the upper layer in the Layers dialog. In the pop-up menu, select “Add Layer Mask”. In the Add Layer Mask dialog box that pops up, press the Add button – any of the options in the dialog box for initialising the layer mask is fine since we will be modifying the layer mask immediately after creating it.

Press “D” on the keyboard to reset the foreground and background colours to black and white respectively.

Activate the Blend Tool from the Toolbox using any of the three “FG to BG” gradients in the Tool Options. Because the layer mask is active, dragging the Blend Tool in the image window will add the gradient to the layer mask instead of the white layer itself. The layer mask hides part of the photo in the upper layer based on the gradient in the layer mask. The result is that the two photos appear to fade into each other.

Again, none of the pixels in the two photos are altered or “destroyed”. The fading effect can be changed anytime in the future by modifying the gradient in the layer mask of the top layer.

I've modified the layer mask by painting with the Paintbrush Tool.

You can further modify the blending by using the Paintbrush Tool from the Toolbox to paint either black or white into the layer mask. Remember – white reveals the pixels in the upper layer while black hides. Shades of gray makes the pixels in the upper layer translucent.

As before, you can also use the Move Tool to move either of the photos around.

In the photo of the landscape waterfall, the rocks around the artificial waterfall are too dark because they are lying in the shadows of the surrounding trees. I want to lighten the rock features. Download the before photo and follow along.

This tranquil landscaped lake was shot in the Nijojo Castle in Kyoto, Japan.

I want to lighten only the rocks around the waterfall without affecting the trees.

Lightening the rocks has lightened everything else as well, making the photo too harsh.

Brighten the duplicated copy by using Colors > Brightness-Contrast command from the main menu. You can see that in addition to the rocks, the other parts of the photo are lightened as well. That is the problem with global adjustments – when you lighten the shadows, the highlights get blown out.

A Layer Mask is added to the duplicated layer in the Layers dialog.

Black areas in the mask make the corresponding areas in the layer transparent/invisible while white areas make the layer opaque/visible. Grey areas make the corresponding areas in the layer translucent or partially visible.