2017

• Gamma Correction - To adjust the white point. Adjusting the white point controls
the brightness of the picture's display on screen. To use the Gamma Correction
dialog box, adjust the midtones by entering a new value in the Gamma field or
by dragging the slider. A higher value produces a darker picture, overall.
Although modifying gamma gives you some control over picture display,
differences between Windows and Mac OS may still cause issues. Windows uses
a higher gamma value (2.2) for display than Mac OS (1.8), so the same picture
will look darker on Windows.
• Desaturate - To convert a color picture to a black-and-white picture while leaving
the color mode and lightness value of each pixel unchanged. For example, it
assigns equal red, green, and blue values to each pixel in an RGB picture to make
the picture appear grayscale
• Invert - To invert the gray values of each channel in a picture. This effect is
recommended for 1-bit, grayscale, and RGB pictures. Because CMYK pictures
contain a black channel, this effect is not recommended for CMYK pictures. The
inversion of the black channel usually results in images that are mostly black or
mostly white.
• Threshold - To convert color pictures to black and white, without using gray.
Enter a value in the Threshold field or drag the slider; all pixels lighter than the
threshold value are converted to white and darker pixels are converted to black.
• Posterize - To modify the tonal levels for each channel in a picture to produce
special effects. To use the Posterize dialog box, enter a new value in the Levels
field or drag the slider. For example, choosing five tonal levels in an RGB image
results in 15 colors (five for each of the three primary colors)..
Blend Mode
Use the Blend Mode drop-down menu to select a blend to apply to the image.
Opacity
Use the Opacity drop-down slider to select a the opacity percentage to apply to the
image.
270 | A GUIDE TO QUARKXPRESS 2017
PICTURES