Histogram
General
The histograms in PhotoPerfect are diagrams which show the distribution of the brightness values (tone values) of an image using finely stepped columns which give the appearance of a complete curve (there are 256 values for an 8 bit image). Each brightness value corresponds with one column. All columns together produces what is known as the tone value curve. The X-axis of the diagram shows the range of brightness, with black to the left and white to the right. The Y-values give the number of pixels of the given brightness within the image.
If an image is correctly exposed, the histogram will have brightness values across the whole range from 0 to 255. If it is underexposed there will be more tone values in the dark range and hardly any in the right range (e.g. values from 0 to 200). An overexposed image, on the other hand, will have values in the upper range, e.g. 50 to 255.
By selectively manipulating the histogram, i.e. the values of which it consists, it is possible to influence the color channels red (1), green (2) and blue (3) as well as the brightness channel (4) for the whole image. In other words, you can influence both the total brightness as well as the brightness of individual channels.
Manipulating the histogram
Black level (1)
This slider can be used to set the black level for the tone values. This means that all truly black pixels in the image are given this value. The overall effect on the image is that it becomes fainter, i.e. the contrast is reduced. The standard setting is 0, i.e. completely black.
Tone value range (2)
Here, the range of the tone values can be set. The range can be set at both ends - on the left the dark values, on the right the light values. Thus if an image is underexposed, the right hand slider can be moved to the left (and as necessary the left hand slider to the right) until the tone values are spread throughout the image and the brightness is optimized. In this example the values of the tone value range are set to 4 to 174.
To take account of the fact that the human eye reacts with greater sensitivity in the darker range than in the lighter one, the center slider (gamma) can be used to shift the tone values in a non-linear way. When values greater than 1 are set the darker tones are more strongly accentuated than the lighter ones. For values less than 1 the darker tones are reduced more than the lighter ones.
White level (3)
This slider is used to adjust the white level for the tone values, i.e. all purely white pixels are set to this value. Here, too, this manipulation reduces the contrast of the image. The standard setting is 0, i.e. completely white.
RGB-Max
This function causes an automatic distribution of each channel individually.
Y-Max
Spreads all channels by the same degree so that the brightness is extended to the greatest possible area. This function corresponds with the Automatic RGB Contrast.
The functions described above apply to the brightness channel and all color channels, whereby the color channels influence each other. For instance, if the brightness of the red channel is reduced, this has the effect of accentuating the green and blue colors. Thus it is possible to alter the colors very specifically.
If an image contains mainly homogeneous colors (e.g. a large amount of sky), the tone value curves in the individual channels appear rather flat. In order to get an optimal result nevertheless, you can click on the color channel and turn the mouse wheel in order to alter the scale of the diagram. If this doesn't produce enough differentiation, you can increase the rescaling effect by keeping the [Ctrl] key pressed at the same time.
Key assignment: [H]
