Convert 8bit grayscale jpeg image to 24bit jpeg image

[abhilash.ar]

Using CommandLine approach of ImageMagick I wanted to convert 8bit grayscale jpeg image to 24bit jpeg image maintaining the same image density(dpi).

I used the below command, but there is no change in the output image properties and bitdepth is unaltered
“ImageMagick-6.8.8-Q16\convert.exe” “Input_Image_Path” -density 300 -type TrueColor “Output_Image_Path”

[snibgo]

What is the difference between "8bit grayscale jpeg" and "24bit jpeg"? As far as I know, jpegs can always store three channels, almost always 8 bits per channel.

[fmw42]

What is the difference between "8bit grayscale jpeg" and "24bit jpeg"? As far as I know, jpegs can always store three channels, almost always 8 bits per channel.

I suspect the OP wants to convert the colorspace to sRGB, but have it grayscale. I don't think IM will allow that, unless at least one pixel is changed from gray. IM knows when the channels are all the same and forces the colorspace or type to gray.

[magick]

Add -type TrueColor to your command line.

[fmw42]

Add -type TrueColor to your command line.

That does not seem to work for me. Afterwards, IM still reports colorspace gray and type grayscale


convert zelda3.png -colorspace gray zelda3g.png

Code: Select all

Image: zelda3g.png
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: DirectClass
  Geometry: 128x128+0+0
  Resolution: 72x72
  Print size: 1.77778x1.77778
  Units: Undefined
  Type: Grayscale
  Base type: Grayscale
  Endianess: Undefined
  Colorspace: Gray
  Depth: 16-bit
  Channel depth:
    gray: 16-bit

convert zelda3g.png -type truecolor zelda3g_rgb.png

Code: Select all

Image: zelda3g_rgb.png
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: DirectClass
  Geometry: 128x128+0+0
  Resolution: 72x72
  Print size: 1.77778x1.77778
  Units: Undefined
  Type: Grayscale
  Base type: Grayscale
  Endianess: Undefined
  Colorspace: Gray
  Depth: 16-bit
  Channel depth:
    gray: 16-bit

[magick]

Does it work for JPEG? If not, we'll investigate. If not for PNG, alert our PNG maintainer that its a bug!

[snibgo]

My first reply was wrong. Jpegs can have just one channel. As magick suggests, "-type truecolor" ensures it has 3 channels.

Code: Select all

F:\web\im>%IM%convert logo: l.jpg

F:\web\im>dumpjpeg /il.jpg
      Input: /i l.jpg
l.jpg: JPEG image is 640w * 480h, 3 color components, 8 bits per sample
JPEG process: Baseline
width 640, height 480

F:\web\im>%IM%convert logo: -monochrome l.jpg

F:\web\im>dumpjpeg /il.jpg
      Input: /i l.jpg
l.jpg: JPEG image is 640w * 480h, 1 color components, 8 bits per sample
JPEG process: Baseline
width 640, height 480

F:\web\im>%IM%convert logo: -monochrome -type truecolor l.jpg

F:\web\im>dumpjpeg /il.jpg
      Input: /i l.jpg
l.jpg: JPEG image is 640w * 480h, 3 color components, 8 bits per sample
JPEG process: Baseline
width 640, height 480

@abhilash.ar: please clarify what you are trying to do. Ideally, please post your input to dropbox.com or similar and paste the URL here.

[glennrp]

Does it work for JPEG? If not, we'll investigate. If not for PNG, alert our PNG maintainer that its a bug!

The PNG that is written is color-type 2, depth 16. When "identify" evaluates the image (not the input file!), it discovers that the pixels
are all gray and reports "grayscale". If you have to know about the stored PNG colortype, etc., don't use "identify" to find out. Use a third-party
application such as "pngcheck" instead. Probably the same goes for JPG files, too, but you'd need a different third-party application.

[snibgo]

As I did above with dumpjpeg.

[abhilash.ar]

Thanks all for your replies

I had included '-type truecolor' in my command line. But it is not affecting jpeg images

[fmw42]

Thanks all for your replies

I had included '-type truecolor' in my command line. But it is not affecting jpeg images

How do you know? Are you using IM identify? If so, that was the point of the comments above. You may need to verify using some other tool besides IM identify, such as Exiftool or one of the other tools mentioned. Apparently IM sees three identical channels as reports it as grayscale. But other tools will recognize it as 3 channels (RGB).

[abhilash.ar]

I used the below command to get the desired output
“ImageMagick-6.8.8-Q16\convert.exe” -density 300 -type bilevel -type TrueColor “Input_Image.jpeg” “Output_Image.jpeg”

Thanks for all your suggestions

[fmw42]

I used the below command to get the desired output
“ImageMagick-6.8.8-Q16\convert.exe” -density 300 -type bilevel -type TrueColor “Input_Image.jpeg” “Output_Image.jpeg”

Thanks for all your suggestions

Proper IM syntax in this case would be to read the input before any other command.

see
http://www.imagemagick.org/Usage/basics/#why