I am trying to convert gray scale images to RGB using the imagemagick command-line tools.
It is working fine for PNG images, using:
convert image.png -define png:color-type=2 result.png
(taken from an answer to "How to convert gray scale png image to RGB from comand line using image magick")
Although checking with identify -format %r result.png will still return DirectClass Gray, I can see it worked using gdalinfo as there are now 3 bands / channels listed:
gdalinfo [successfully converted PNG]:
Driver: PNG/Portable Network Graphics
Files: result.png
Size is 567, 479
Coordinate System is `'
Image Structure Metadata:
INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left ( 0.0, 0.0)
Lower Left ( 0.0, 479.0)
Upper Right ( 567.0, 0.0)
Lower Right ( 567.0, 479.0)
Center ( 283.5, 239.5)
Band 1 Block=567x1 Type=Byte, ColorInterp=Red
Band 2 Block=567x1 Type=Byte, ColorInterp=Green
Band 3 Block=567x1 Type=Byte, ColorInterp=Blue
However, it seems the -define option is only working for PNG images.
My question: How can I achieve the same effect for JPG images?
When I try the above command for JPG, it does not work:
convert image.jpg -define jpg:color-type=2 result.jpg
gdalinfo [unsuccessfully converted JPG]:
Driver: JPEG/JPEG JFIF
Files: result.jpg
Size is 1500, 1061
Coordinate System is `'
Metadata:
EXIF_ColorSpace=1
EXIF_PixelYDimension=2480
...
EXIF_YCbCrPositioning=1
EXIF_YResolution=(300)
Corner Coordinates:
Upper Left ( 0.0, 0.0)
Lower Left ( 0.0, 1061.0)
Upper Right ( 1500.0, 0.0)
Lower Right ( 1500.0, 1061.0)
Center ( 750.0, 530.5)
Band 1 Block=1500x1 Type=Byte, ColorInterp=Gray
Overviews: 750x531, 375x266, 188x133
Image Structure Metadata:
COMPRESSION=JPEG
The PNG colour types do not apply to JPEG images, so you can't use the same technique. Try forcing the colorspace to sRGB, and/or setting the type to TrueColour so you don't get a palettised image:
convert input.jpg -colorspace sRGB -type truecolor result.jpg
来源：https://stackoverflow.com/questions/34874771/convert-jpg-image-from-grayscale-to-rgb-using-imagemagick