In the above example, the result is darker than the original, because the channels were separate as linear gray and subsequently combined as linear color. For example,Ĭonvert myimage.png -separate myimage_channels_%d.pngĬonvert myimage_channels_*.png -combine myimage2.png When separating and recombining channels, with potential intermediate processing, it is important to identify the colorspace used, especially during the recombination. For example, the following produces non-linear grayscale channels.Ĭonvert myimage.png -set colorspace RGB -separate myimage_channels_%d.png However the same concept can be applied, if it is desired to keep the separate channels as non-linear grayscale. Normally, the conversion to separate each channel of an sRGB color image produces separate linear grayscale images. The same concept is needed when separating channels. For example, the following produces a non-linear grayscale result.Ĭonvert myimage.png -set colorspace RGB -colorspace gray myRGBimage.png
If you prefer to keep the conversion to non-linear grayscale, set the colorspace of the input to linear RGB so that -colorspace gray does not apply the gamma correction during the conversion process. This means that the conversion of a color image to grayscale via -colorspace gray looks darker relative to previous versions of ImageMagick (note that desaturating to convert to grayscale does not convert the image to linear grayscale). This change is appropriate, since many types of processing requires a linear colorspace. The second major change treats any grayscale image as linear rather than non-linear, as it was previously. In order to properly have the JPG output know that it is linear RGB, include an appropriate color profile. Therefore, if the above command is run with a JPG or TIF output format, the verbose information for the colorspace still shows sRGB. This only works on image types containing meta data that distinguishes between linear RGB and non-linear sRGB, such as PNG and GIF. To declare that an image is linear RGB rather than sRGB, you can use the set option:Ĭonvert myimage.png -set colorspace RGB myRGBimage.pngĪfterwards, the verbose information for the output file lists the colorspace as RGB. For example,Ĭonvert myimage.jpg -colorspace RGB -resize 200% -colorspace sRGB mybigimage.jpg Most image processing algorithms assume a linear colorspace, therefore it might be prudent to convert to linear color or remove the gamma function before certain image processing algorithms are applied. ImageMagick supports color profiles, however, for images without a profile or a declaration of colorspace, ImageMagick assumes non-linear sRGB. With the completion of the changes, RGB now means linear color and sRGB means non-linear color in terms of their respective colorspaces. In earlier versions, RGB really meant non-linear sRGB. The first major change was to swap -colorspace RGB and -colorspace sRGB. 2-bit images, produced by ImageMagick, are completely unusable.Īll of above was observable with antialiasing and without.Color management has changed significantly between ImageMagick version 6.7.5-5 and 6.8.0-3 in order to better conform to color and grayscale standards. requesting 8 colors produces png with 9 colors) images have severe artifacts, compared to same images, processed by pngquant. Unfortunately, results of quantizing are very bad, compared to pngquant. If the image is sufficiently small, manually realigning lines to pixel boundaries may greatly improve the quality of result.ĮDIT: I have got ImageMagick to work as well. Note, that images with lots of details (lines with non-integer width, lines thiner than 1px, fine curves) may not be represented with 2-bit palette without extreme quality loss, so experiment with colors count if you are not satisfied with result.Īlso note, that some editors (Inkscape by default, Illustrator with some settings) won't pixel-align image elements, so quantizing will be less efficient due to the way multiple pixels are used for representing single "original" image pixel. It can reduce the image to palette with desirable number of colors, up to 2-bit, with or without added antialiasing. Gimp isn't exactly lightweight and I have never gotten ImageMagick to do that properly, so I used a tool, called pngquant.
If your editor does not natively support saving as indexed PNG, you will have to perform a destructive (lossy) operation, called "quantizing".
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