Question

How to get RGB values using JMagick(a wrapper of imagemagick) ?

Answer 1

If you want to get the “red” value of a specific image, on the commandline, you could use the following syntax. For the JMagick API, just look up how you’ll have to translate this into API calls:

 identify  -format "%[fx:s.p{111,111}.r]"  input.jpg
   0.427451

• fx: is ImageMagick’s special effects image operator that allows you to query all sorts of properties from an image, use them in a mathematical expression and apply them on the image;

• s. tells ImageMagick to select the current image in the sequence for applying the fx operation;

• p{111,111}. tells fx to use the pixel at column 111, row 111 for this operation (if you skip the pixel coordinates, fx defaults to p{0,0}, which is the pixel in the top left corner;

• r queries the red channel of the selected pixel;

• 0.427451 is the result: it means it is 42.7451% of the maximum value of the channel (255 for 8-bit, 65535 for 16-bit).

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 identify  -format "%[fx:s.p{111,111}.z]"  input.jpg
   8

• z queries the channel depth used for the pixel. Result is 8.

You can also query the Hue, Saturation and Lightness values for a pixel after converting the image to the HSL color system:

identify  -colorspace hsl  -format "%[fx:s.p{111,111}.hue]"  input.jpg 
   0.538012

identify  -colorspace hsl  -format "%[fx:s.p{111,111}.saturation]"  input.jpg 
   0.53271

identify  -colorspace hsl  -format "%[fx:s.p{111,111}.lightness]"  input.jpg 
   0.790196