Image - Invert, Invert At

The Invert and Invert At commands are closely related commands used to invert the color values in pixels in RGB or RGBa images. Invert simply switches 0 to 255 and 255 to 0 and likewise all values in between. The Invert At command divides all pixel channels into 255 levels and then subtracts the pixel value from 255 to get the new value, using an optional center point to shift the inversion.

Center	Center point of the inversion (by default 128).
Preview	Check to see effect in action.

This command is also available on the Transform Toolbar for images as the Invert and Invert At transform operators. However, when used from the transform toolbar the effect is immediate with no preview possible. The parameter for Invert At is used for the center point of the inversion.

In a simple black and white image, using 0 for black and 255 for white, a near-black pixel value of 5 will be converted to 250, or near-white. The result is a photographic negative image.

For RGB color images or any other image with more than one channel this process is applied to each channel. If a pixel has a high blue value and low red and green values (like the blue sky in our sample image), the blue value will end up low and the red and green values will be high to result in a yellow tone.

The Invert At command includes a slider bar that allows setting the center point of the inversion process. Leave it at 128 to achieve a normal, symmetric inversion as discussed above. Increasing the center point increases the brightness of the inverted image, while decreasing the center point reduces the brightness of the inverted image.

The above example shows default inversion with a standard center point of 128 as well as brightened and darkened inversions at center points of 192 and 64 respectively. The center point control is provided because negatives are often unexpectedly light or dark and require immediate adjustment to suit one’s taste. Using the Invert command is effectively equivalent to using an Invert At command with a center point of 128.

Comments

The difference between the two commands is subtler than simply providing a center control for Invert At. The difference between the two is that:

Invert always maps 0 to 255 and 255 to 0 (and, of course, inverts all other values in the middle), while
Invert At with a center of 127 maps 0 to 254 (not 255) and 255 to 0, and
Invert At with a center of 128 maps 0 to 255 and 255 to 1 (not 0).

The above may seem like splitting hairs, but when using Invert with surfaces the difference between 0 and 1 and 254 and 255 may be very significant.

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