mogrify modifies TIFF images when not requested

[yecril71pl]

Code: Select all

convert -size 32x32 canvas:black canvas:gray +append canvas:silver +append canvas:white +append \
  -type Palette c4.tiff &&
mogrify c4.tiff &&
cmp c4.tiff in.tiff

c4.tiff in.tiff differ: char 6, line 1

Mogrify with no operation should create a copy of the original, especially when the original has been created with IM.

[magick]

We believe its the date and modify metadata that is changing. Try deleting these properties and your should get the original image. Type
identify -verbose myimage.tiff to see the properties and their values.

[yecril71pl]

I inspected the images with tiffinfo; the original image is palette and the mogrified image is grayscale.
I tracked down the problem to MagickReadImage; MagickGetType for this image returns Grayscale.

Workaround:

Code: Select all

MagickSetType (a_wand, PaletteType);

This is important because a 4-colour Palette image grows 4 times when converted to Grayscale.

This problem does not occur when the palette contains colours, as in

Code: Select all

convert -size 32x32 canvas:red canvas:blue +append canvas:green +append canvas:yellow +append \
   -type Palette in.tiff

Non-Workaround:

Code: Select all

MagickSetImageChannelDepth (a_wand, GrayChannels, 02);

This workaround does not work with subtle intensity differences, as lighter shades turn white:

Code: Select all

convert -size 32x32 canvas:black canvas:#e0e0e0 +append canvas:#f0f0f0 +append canvas:white +append \
   -type Palette in.tiff

The converted image ends up as 2-bit Grayscale DirectClass while the original is 2-bit PseudoClass (which is what is needed)