Color filters

Map to gradient

Map to gradient lets you drastically alter the colors of your image based on a gradient. Each pixel in the image is converted to a grayscale value, and that value is used as in index into the current gradient. You can select any of 8 gradients in the gradient panel, or make your own using the gradient editor.



Negative creates a negative of your image - much like a film negative. This differs from 'invert' under the buffer menu mainly in how alpha is handled. Like all tools under the filter menu, alpha is supported, where the buffer tools work on the whole buffer.

Invert value

Invert value works much like the negative filter, except it only operates on the value component of your image. Color information remains the same while the value is negated.  As you can see here, the green of the leaves and the blue of the sky remains the same, while the values are inverted.

Swap channels

Swap channel swaps the red, green, and blue channels for interesting color results.

Roll channels

Roll channels is similar to swap channels. It rolls the red, green, and blue channels, so that red becomes green, green becomes blue and blue becomes red.


Color/Eight bit dither applies 8 bit dithering techniques to the buffer. While the apparent color is reduced, the buffer is maintained at 24 bit depth. Optimized computes a palette of 256 colors from the image and maps to it. Ordered and random dither uses a fixed color space where each component is quantized to 6 steps.  Color dithering is useful for 'roughing up' a piece of artwork, to make it appear less smooth.  It can be useful also for creating the look of grain, or for simulating computer generated images of the old days when colors were limited to 256 or less.

Color/One bit dither applies 1 bit dithering techniques to the buffer. The random dithering is useful for generating a stipple effect. The buffer is maintained at 24 bit depth.


Color replacer

The color replacer, as its name suggests, lets you replace one color with another.  Say you wanted a person to have green eyes instead of blue, or a flower to be purple instead of yellow...

Click in the preview window to select the color you want to replace. The results will show up in the full sized buffer window. Use the hue, saturation, and value sliders to change the colors.

The key type dropdown list controls how the color is replaced. RGB difference is the default mode, and will give you adequate results in many situations, but other modes may be better for certain circumstances. In the case of the flowers above, the hue difference mode was used because the flowers have a distinctly different hue than the rest of the image, even though shading changes considerably.


Power curve

The power curve filter attempts to simulate the way human vision is less sensitive to color in low lighting. The formula for this function is gamma (Saturation * Value). Saturation is reduced more severely for lower values. It might also be useful for simulating old film or photographs, where the colors have faded non-uniformly.

The invert option makes higher (lighter) values less saturated.


The posterize filter reduces the number of colors in an image so that banding becomes evident. Internally, the image is always maintained at 24 bit depth, despite the appearance of color reduction.



The Solarize filter simulates the result of a photographic development process gone wrong, where the picture has been accidentally exposed to unwanted light.


The sepia filter simulates the look of old black and white photographs that have turned yellowish or over the years.

Fading” controls how much the photochemical paper has degenerated over the years. The lights become darker, and the darks become grayish.

Desaturate” controls how much color intensity has faded. Simulating old black and white photographs should be completely desaturated. Old color photographs may still have some color, but it may be somewhat faded.

Yellowing” controls how much the photo paper has yellowed.


The halftone filter simulates the appearance of old newspaper or comic book print. You can chose a black and white or color halftone.

A halftone pattern is also widely used in modern printing. Since black and white printers print only black ink on white paper, and color printers only have 3 to 7 colors available, a full color image is always reduced to a smaller number of colors.

In order to make the eye believe it is seeing millions of colors, or hundreds of levels of gray, a dither pattern is introduced into the image. The effect that the dots blend visually in order to give the appearance of many colors.

The halftone filter is also suitable for preparing photographs for print, when a halftone process is desired, in which case, your image resolution should equal your desired print resolution, ie, pixels = printer dots.

In the case of color halftones, the halftone pattern for each color channel is automatically rotated the recommended 45 degrees.

Halftone plus

In the case of the Halftone plus filter, some additional options are available, such as this stipple mode. Like the halftone filter, in each case, the image is converted into a 1-bit-per-channel image, with a different dither pattern for each option. This makes an image with up to 7 colors possible -- red, magenta, yellow, green, cyan, blue, and black. This is consistent with colors used by most printers.

Graphic halftone

The previous halftone filters generated a very natural looking halftone pattern that realistically simulates patterns created by screen printing. The Graphic halftone filter intentionally uses a very precise graphic rendering in order to produce dither patterns.

In this case, anti-aliased ellipses are used to produce halftone patterns. The final look is much more graphical and suitable for design purposes.


The duotone filter converts an image to monotone, where the range of values is mapped between two colors. It can be thought of as mapping an image to a gradient, where the gradient is a blend between two colors you define.


Somewhat similar to the Duotone filter, the tritone filter alters the color of an image using user defined colors.

However, the Tritone filter creates a gradient from three points set at the shadow/mid-tones/highlights levels in an image.