The Curves command (Image/Adjust) enables you to change the pixel values of an image. It allows you to focus with greater precision on certain tonal areas and is therefore more powerful than the Levels command (see Levels).
The "Curves" dialog consists of a tonal graph on a grid background. The tones of the image are altered by manipulating this graph in conjunction with the tonal range represented by a particular square or squares. The graph can take either of two forms: one which allows you to manipulate the graph by using points or markers, the other which enables you to draw a curve freehand. The two forms of curve can easily be converted into one another.
The "Curves" dialog contains the following options:
Channels: Choose to edit one of the individual channels - Red, Green or Blue - or all the channels combined - Luminosity.
Tone Graph: This is a diagonal straight line by default. It represents the spread of tones throughout the image. Pixel values are altered by manipulating this graph. The four-by-four grid against which the graph is set allows you to focus your changes on particular tonal areas. The horizontal bar at the bottom of the graph represents the gradual changing of the tonal range from black to white. So, if you would like to make the lighter tones darker you should concentrate on the squares towards the upper right-hand corner, but if you’d like to make the darker tones lighter concentrate on the squares towards the bottom lower left-hand corner. The square markers at the extreme ends of the graph represent the tonal end-points. It is not necessarily the case that the image contains these tones, so, for example, to alter the darkest tones in the image you may have to concentrate on a square that is further along the graph from the bottom left-hand square.
The tones can be changed by clicking on the graph in the square that contains the relevant tones e.g. shades, mid-tones, highlights. A marker will appear on the graph at this point. There are two functions of markers. The first is to make tones lighter or darker by dragging the marker upwards or downwards respectively. The straight line graph will adjust to form a curve, using the marker as its point of origin, and the image will dynamically darken or lighten to reflect the new graph. With only one marker in place the whole graph bends into a curve and therefore affects the whole tonal range. Since you will usually want to concentrate on a limited range of tones, the second function of the markers is to anchor the graph, therefore restricting the curvature whenever you reposition a particular marker. To do this put several markers in place along the graph at regular intervals and then move the relevant marker up or down as required. Only a small section of the graph will be affected and hence only a small part of the tonal range will be altered. Note that the graph will attempt to form the smoothest curve possible between the markers when the Smooth curve option is set (see below for the difference between Smooth and Free curves).
You can place up to fourteen markers on the graph. Reposition a marker by dragging it along the graph in the required direction. Remove a marker by dragging it horizontally till it disappears (dragging vertically alters the graph itself, of course). Usually you will only need to make small adjustments to the graph, anything greater altering the image tones radically.
Smooth and Free: The default setting is Smooth. This ensures that the curve is adjusted smoothly i.e. no "sharp corners", therefore causing the tonal alteration in the image to be smoother too. Markers can be added only to smooth curves. The Free option removes any existing markers, including the end-points, and allows you to use your mouse to draw a "curve" freehand, without the rest of the graph being adjusted to remove sharp corners. You can switch between these at will. Note, though, that switching back to Smooth from Free smoothes out the curve and adds ten markers at key intervals i.e. it doesn’t return the markers that you may previously added while in Smooth mode.
If you continually click on Smooth you will notice that the curve gradually straightens out, eventually returning to its original diagonal form.
Reset: This resets the graph to its original straight diagonal form and removes any markers, apart from those at the end-points. The image dynamically reverts to its original state.
Once you are satisfied click on OK to save the changes and exit the dialog; otherwise click on Cancel to exit without saving.
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