changing the bit depth of Tiff file from 24 to 8 bit

[rjilani]

Hi: Gurus, we have some scanned Tiff files with JPEG 7 compression. All these files are color and scanned as 24 bit per pixel. I wonder is it possible to change the depth of bit per pixel from 24 to 8 bit? If yes does some one know how to do it via ImageMagick?

Regards,
Rashid.

[fmw42]

This should do it assuming IM supports tiff files with JPEG 7 compression.

convert image.tiff -depth 8 -type palette newimage.tiff

Can you post a link to your input image so we can test?

As a simple test, I created a JPG compressed tiff

convert rose: -compress jpeg rose_cjpg.tiff

Code: Select all

Image: rose_cjpg.tiff
  Format: TIFF (Tagged Image File Format)
  Class: DirectClass
  Geometry: 70x46+0+0
  Units: PixelsPerInch
  Type: TrueColor
  Base type: TrueColor
  Endianess: MSB
  Colorspace: sRGB
  Depth: 8-bit

Then convert to 8-bit palette

convert rose_cjpg.tiff -depth 8 -type palette rose_cjpg_palette.tiff

Code: Select all

 Format: TIFF (Tagged Image File Format)
  Class: DirectClass
  Geometry: 70x46+0+0
  Units: PixelsPerInch
  Type: TrueColor
  Base type: TrueColor
  Endianess: MSB
  Colorspace: sRGB
  Depth: 8-bit

But it is still type Truecolor and not palette.

So I think you need to remove the compression first. So I tried

convert rose_cjpg.tiff -compress none -depth 8 -type palette rose_cjpg_cnone_palette.jpg
or reversing the order of -depth and -type
convert rose_cjpg.tiff -compress none -type palette -depth 8 rose_cjpg_cnone_palette.jpg

Code: Select all

Image: rose_cjpg_cnone_palette.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Class: DirectClass
  Geometry: 70x46+0+0
  Units: Undefined
  Type: TrueColor
  Endianess: Undefined
  Colorspace: sRGB
  Depth: 8-bit

But that is still type Truecolor.

So I converted to MIFF and then TIFF

convert rose_cjpg.tiff -compress none MIFF:- | convert - -depth 8 -type palette rose_cjpg_cnone_palette2.tiff

Code: Select all

Image: rose_cjpg_cnone_palette2.tiff
  Format: TIFF (Tagged Image File Format)
  Class: PseudoClass
  Geometry: 70x46+0+0
  Units: PixelsPerInch
  Type: Palette
  Base type: Palette
  Endianess: MSB
  Colorspace: sRGB
  Depth: 16-bit

Which is type palette but 16-bits. It seems to ignore the -depth.

Using IM 6.8.6.7 Q16 Mac OSX

EDIT: If the -depth and -type are reversed it does work when using MIFF intermediate, but does not work directly as shown above.

convert rose_cjpg.tiff -compress none MIFF:- | convert - -type palette -depth 8 rose_cjpg_cnone_palette3.tiff

Code: Select all

Image: rose_cjpg_cnone_palette3.tiff
  Format: TIFF (Tagged Image File Format)
  Class: PseudoClass
  Geometry: 70x46+0+0
  Units: PixelsPerInch
  Type: Palette
  Base type: Palette
  Endianess: MSB
  Colorspace: sRGB
  Depth: 8-bit

I am not sure why this should be order dependent. Seems like a possible bug in that regard and with respect to needing to use MIFF as an intermediate.

[fmw42]

Topic continued in Bugs forum. See viewtopic.php?f=3&t=23845

Note I made a mistake above when testing

convert rose_cjpg.tiff -compress none -type palette -depth 8 rose_cjpg_cnone_palette.jpg

I accidentally used jpg for the output.

It works if the output is tiff

convert rose_cjpg.tiff -compress none -type palette -depth 8 rose_cjpg_cnone_palette.tiff