Legacy ImageMagick Discussions Archive

Hi,

I'm trying to convert a 4-bit-Bitmap into an 8-bit-Bitmap. When I use the command

convert testimage.bmp -colors 256 -depth 8 -type Palette testimage2.bmp

the result is still a 4-Bit-Bitmap. Calling identify for both images returns

testimage.bmp BMP 1302x272 1302x272+0+0 4-bit sRGB 16c 178KB 0.000u 0:00.000
and
testimage2.bmp BMP 1302x272 1302x272+0+0 4-bit sRGB 16c 178KB 0.000u 0:00.000

What am I doing wrong?

This is the image I'm using: https://www.dropbox.com/s/xbhqdzcm9ikoc ... e.bmp?dl=0

I'm using ImageMagick 6.9.7-4 in a Shell on Ubuntu 18.04.

Thank you in advance for your help!

Don't use "identify" to find the structure of an image file, because identify will tell you the structure of the image after it has been decoded. Instead, use exiftool:

Code: Select all

f:\web\im>exiftool testimage.bmp

ExifTool Version Number         : 10.97
File Name                       : testimage.bmp
:
:
Bit Depth                       : 4
Num Colors                      : 16
Num Important Colors            : 16

I don't think there is a reliable method for telling IM to use a palette that is larger than it needs to be. The image has only 8 different colours, so IM uses a palette with 16 entries.

"-colors N" is a colour reduction method. If your image already has N or fewer colours, it has no effect.

Thank you very much for your answer and your comments! I will look for another tool to (always) get an 8-bit-Bitmap then.

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