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Main Window - Image | Batch Process - Batch Steps - Add | Image | Color Depth...



Use this window to modify the color depth of an image. For example, it will convert a color image to grayscale, or a 24-bit image to 8-bit, or just about anything to monochrome (bi-level).



Desired Color Mode. Select the color mode (number of colors or color depth) you desire in the final image.


Bi-level (1-bit). Selects a two-color (black & white) destination image. Each pixel requires a single bit.


Threshold. When you select bi-level as the desired color depth, you may enter a threshold value for the dividing point between black and white. This number must range between 1 and 764. The sum of red, green and blue values for each pixel is compared to this threshold; if below, the destination pixel becomes black; if above, it becomes white. Note that if dithering is selected, this value may not make as much difference, as any error is propagated through neighboring pixels.


Order. When using an ordered dithering pattern for converting to bi-level, this selects the size of the box used. You can specify 1 through 8.


Indexed (8-bit). Selects 8-bit per pixel mode with an indexed color map. Use the Colors field to specify how many colors will exist in the color map. Select a number between 4 and 256. For color counts between 4 and 16, 4 bits per pixel are required; for a count of greater than 16, 8 bits per pixel are required.


Grayscale (16-bit). Selects grayscale 16-bit per pixel mode.


RGB (24-bit). Selects a 24-bit destination image; each pixel requires 24 bits. (this color depth was formerly know as Truecolor).


RGB (48-bit). Selects a 48-bit destination image; each pixel requires 48 bits. This mode, while occupying more space in the image file, accommodates better color representation.





Palette. Unless converting to Truecolor, you must select the palette to use. Several fixed and optimized palette selections are available:


Optimized (5-bit median cut)

Uses the "median cut" algorithm to select a color palette that best matches the colors of the original image. Selects from a total of 32,768 possible colors (5 bits each for red, green and blue).

Optimized (6-bit median cut)

Uses the "median cut" algorithm to select a color palette that best matches the colors of the original image. Selects from a total of 262,144 possible colors (6 bits each for red, green and blue). In some cases, this will mean a better color selection than the 5-bit algorithm, but it runs somewhat slower.

Optimized (8-bit median cut)

Uses the "median cut" algorithm to select a color palette that best matches the colors of the original image. This algorithm selects from a total of 16 million possible colors (8 bits each for red, green and blue). In some cases, especially rasterizing drawing with solid areas or gradient fills, this option produces much better results that 5 or 6-bit quantization. However, it is quite a bit slower, and will use very large amounts of memory if the number of colors in the original image is high. Not suggested for photographic images.

Optimized (popularity)

Selects the n most-used colors in the original image.


Creates a fixed levels-of-gray palette, with n evenly spaced colors.

ThumbsPlus Color

Selects the same colors used in the ThumbsPlus thumbnails; this is also the palette used for the Quick Dithering option (see below).

Web (Netscape/IExplorer)

Selects the palette used by Netscape Navigator and Microsoft Internet Explorer.


Uses the 236-color Indeo palette recommended for video frames. This is also a very useful general palette, somewhat similar to the ThumbsPlus palette.


Uses the recommended 236-color Cinepak palette recommended for video composition.

Std. Windows Color

Uses the 16 standard Windows colors (yuck - but sometimes you need 'em).

Light Windows Color

Uses 16 standard Windows colors as implemented by some display drivers (the darker colors are lighter than in the standard palette).

Standard VGA

Selects the default 16 VGA palette colors.

6x6x6 Even Distribution

Creates a palette with 6 levels of red, 7 of green and 6 of blue, evenly spaced. (Not very pretty for most images, but okay for some dithering jobs.)

6x7x6 Even Distribution

Creates a palette with six levels each of red, green and blue, evenly spaced. This is similar to the 6x7x6 palette, but more useful for pictures with large amounts of gray.

3x3x2 Even Distribution

Creates a palette with 3 levels of red, 3 of green and 2 of blue, evenly spaced.

Pasted from Clipboard

Pastes a palette from the image on the clipboard. This is useful for adapting several images to a common palette.

Loaded from File

This option allows you to load a palette from an existing graphics file, or from a ThumbsPlus palette file (.PAL), which you may create using the File | Save Palette As option on the View window File menu.

Common Optimized

(Batch conversion only) This option creates a common optimized palette of the desired number of colors from all images being batch processed.


Identity palette. Checking this box will cause ThumbsPlus to adjust any 17 to 236 color palette to be a Windows identity palette, placing the 20 Windows system colors in the first 10 and last 10 color map positions.


Preserve web colors. Check this option to ensure that any web-safe colors in the image are in the generated palette.




Dither. When reducing the number of colors in an image, you may select a dithering method from those listed below:



No dithering. Colors are simply mapped to the closest available color in the selected palette.


The most popular error diffusion dithering method. Fairly quick with reasonable results.


Another error diffusion dither. Takes slightly longer than Floyd-Steinberg, and sometimes produces nicer results.


A fairly complex error diffusion dither. Slower than Floyd-Steinberg or Burkes, but usually the nicest results, especially for truecolor to 256-color conversions.

Jarvis, Judice, Ninke

Another fairly complex error diffusion dither with a somewhat different appearance from Stucki.


For use with bi-level color modes only. Note: Selecting this dither method will force the color mode to bi-level.


Reduce error propagation. Check this option to reduce error propagation through neighboring pixels.


Noise level (%). ThumbsPlus can randomize each pixel by a small amount to (sometimes) improve the appearance. Select a value here between 0 (no noise) and 50 (lots of noise). This is particularly useful in getting rid of patterns produced in solid areas by the error diffusion dithers.


Note: ThumbsPlus retains the last selections you made in this window so you can apply the identical changes to subsequent files, or during batch processing.