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).
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.
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.