ImageMagick Reduces Colorspace to Gray

grayscaleimagemagickrgb

I convert RGB and CMYK TIFF images to RGB JPEGs using

convert a.tif -colorspace rgb a.jpg

If the TIFF image contains only gray pixels, the colorspace of the resulting JPEG is gray, not RGB.

How can I force ImageMagick to always use RGB?

Best Answer

Try this:

convert a.tif -colorspace rgb -type truecolor a.jpg

However, I have to ask: How exactly do you determine your verdict 'colorspace of resulting JPEG is gray, not RGB'?!?

The ImageMagick tool identify can look at the colorspace used by files. If you have convert, then you'll have identify too:

identify -format "%[colorspace]   <== %f\n"  *.png *.jpeg *.pdf *.tif

Example output:

 sRGB   <== fullsize-coalesce-originals.png
 Gray   <== tiffg4.tif
 CMYK   <== cmyk.pdf
 CMYK   <== photoshop.jpeg

Related Solutions

Convert PDF to image with high resolution

It appears that the following works:

convert           \
   -verbose       \
   -density 150   \
   -trim          \
    test.pdf      \
   -quality 100   \
   -flatten       \
   -sharpen 0x1.0 \
    24-18.jpg

It results in the left image. Compare this to the result of my original command (the image on the right):

(To really see and appreciate the differences between the two, right-click on each and select "Open Image in New Tab...".)

Also keep the following facts in mind:

The worse, blurry image on the right has a file size of 1.941.702 Bytes (1.85 MByte). Its resolution is 3060x3960 pixels, using 16-bit RGB color space.
The better, sharp image on the left has a file size of 337.879 Bytes (330 kByte). Its resolution is 758x996 pixels, using 8-bit Gray color space.

So, no need to resize; add the -density flag. The density value 150 is weird -- trying a range of values results in a worse looking image in both directions!

Recommendation for compressing JPG files with ImageMagick

I use always:

quality in 85
progressive (comprobed compression)
a very tiny gausssian blur to optimize the size (0.05 or 0.5 of radius) depends on the quality and size of the picture, this notably optimizes the size of the jpeg.
Strip any comment or EXIF metadata

in imagemagick should be

convert -strip -interlace Plane -gaussian-blur 0.05 -quality 85% source.jpg result.jpg

or in the newer version:

magick source.jpg -strip -interlace Plane -gaussian-blur 0.05 -quality 85% result.jpg

Source.

From @Fordi in the comments (Don't forget to upvote him if you like this): If you dislike blurring, use -sampling-factor 4:2:0 instead. What this does is reduce the chroma channel's resolution to half, without messing with the luminance resolution that your eyes latch onto. If you want better fidelity in the conversion, you can get a slight improvement without an increase in filesize by specifying -define jpeg:dct-method=float - that is, use the more accurate floating point discrete cosine transform, rather than the default fast integer version.

Best Answer

Related Solutions

Convert PDF to image with high resolution

Recommendation for compressing JPG files with ImageMagick

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