Legacy ImageMagick Discussions Archive

I'm using the Commandline version of ImageMagick on my server running GNU/Linux and Apache. According to ImageMagick I'm using version "ImageMagick 6.3.7 04/15/09 Q16."

I'd like to convert a supplied 32-bit PNG (meaning 8-bits per channel over 4 channels: red, green, blue, and alpha. 4 * 8 = 32) to an 8-bit (meaning 2 bits per channel) and 4 bit (1-bit per channel) PNG. There's no way this is impossible, but I'm finding LOADS of trouble accomplishing it on the internet, and having a fair amount of it myself as well.

Here is what looked most promising:

http://www.imagemagick.org/Usage/formats/#png_write

See where it says "-define", well there's a lot more information there than on the main ImageMagick -define description. I thought I'd found exactly what I needed when I saw the table for "-define png:color-type". I thought I'd simply set my color type to 3 for indexed, which allows a bit depth of 1, 2, 4, or 8, then set my bit-depth to 2. Well, no go, since define will only work if you're not performing a lossy conversion, since 32-bit to 8-bit is lossy, it refuses to do anything at all. I also tried "-colors 256" because 8-bit images have 256 colors. Well, it reduced the number of colors to 256....but made it show up as a 64-bit image (16 bits per channel)!!! That's not right.

What the heck am I doing wrong? By all logic, there should be a simple function to reduce the bit-depth of an image, but it seems impossible. Someone, please help me!

EDIT: Here are some example images...

I applied this command to both of these examples:

convert {input image} -colors 256 {output image}

This is my main image, the one with the alpha, this is my main goal, to get this image to 8-bit (total) depth, here is the original:

http://infectionist.com/images/misc/ext ... on_256.png

And here is after applying the command:

http://infectionist.com/images/misc/ext ... _256_8.png

Here is my another test I did, using a screenshot from a game cutscene, original:

http://infectionist.com/images/misc/Cinematic1_0818.png

And after the command:

http://infectionist.com/images/misc/Cin ... 0818_8.png

As you can see, the colors ARE reduced, but the originals are showing up as 32 and 24-bit respectively (because the second image has no alpha, it's only 8-bit-per-channel * 3 channels), and the converted images are showing up as 64- and 48-bit respectively, obviously something is wrong here.

Don't know if IM can help for all your request. But use -depth 8 to force to 8-bits per channel on Q16 otherwise, you get 16-bits per channel. Also see -type palette or palettematte to set to use a color palette for 8 bits total (with alpha for matte)

Also see http://www.imagemagick.org/Usage/formats/#png_formats for info about PNG output formats supported by IM.

Also your 6.3.7 is very old (over 200 versions old). you may want to consider an upgrade.

Shared hosting, upgrade isn't in my control. I'll send an email to my host though.