Batch Resizing BMP files Increases Colour Depth

[Fellwalker]

I have searched but cannot find any post about this.

I have some 24 bit BMPs, which I want to be 8 bit.

Using Irfanview 4.27, I can easily convert each one, but whilst the batch converter allows me to set all of these options, and goes through the motions of converting the files, it actually does nothing.

Further investigation shows that if I batch convert an 8 bit BMP file to an 8 bit BMP it actually increases the bit depth to 24.

Is it a bug, an option that ought to be greyed out in the options, a feature or what? Can it be done at all?

[Bhikkhu Pesala]

No, its not a bug.

Resizing (using resample option) the image is what increases the bit depth.
Resize in one operation, then reduce the bit depth in another, or don't resample the image.

[Mij]

Indeed many of the editing options in batch conversion require that the bitmap is first converted back to 24 bit color depth, and there is no user control over the order in which selected processes are applied.
It may seem that the logical order would be to apply the conversion to 8 bit color last, after other edit operations are complete, but other users in the past have deemed other orders to be more sensible. There probably is no optimum order. It really does depend on what you are trying to achieve.
The best solution for you, Fellwalker, is to do the job in 2 passes. Make any other adjustments you need in the first pass and then do a second with just the 8 bit color conversion selected and nothing else.

[Fellwalker]

Thanks but it hasn't helped

Thanks guys.

Along the way, before posting, I did try each operation singly. I have been unable to get batch to resize a bmp up, but it will resize down. I have been unable to get batch to reduce the color depth to 8 bit.

I tried going via an intermediate file type. Starting from either the original 24 bit or my manually created 8 bit bmp, Irfanview will increase it to 32 bit if i choose ico as the file type to save, but then if I use that as a source, and ask it to convert to an 8 bit bmp, it goes to 24.

BTW, the reason for this is to use POIs from PocketGPSWorld in my satnav - Sygic MultiMap 2010 on my HD2 phone. They are a fantastic resource, but whilst most of their icons are 8 bit bmps, there are a few that are 24 bit. 24 bit icons crash the program, which is designed for 8 bit or 32 bit. I may have to change the files to ico, making them 32 bit, then change the extension back to bmp.

Windows 7 professional 64 bit. Latest version of Irfanview with latest plug-ins.

[Mij]

I have not tried ICO files. Will have a look this evening if I get time.

Reducing BMPs to 8 bits worked OK though. I have attached before and after screen shots as well as the Batch settings screen.

[Fellwalker]

Sussed it!

Well that was fun. I compared your images to my workflow, and found that I had a tick in the box that says "Auto adjust colors". Never thought about it - cannot remember setting it and didn't even notice it.

Untick that, and it all works! Resizes up and changes bit depth to 8!

Thanks ever so much guys. You've been a help and an encouragement to keep looking.

It looks as if that option needs greying out when playing with BMPs, or a warning that it does not work.

[Fellwalker]

Goshq

just noticed I am a newbie - I am that to the forum, but have been using and recommending Irfanview for at least 10 years. I just found an early downloaded file from July 2000 for installing Irfanview 3.21!