Legacy ImageMagick Discussions Archive

Hi,

I needed a simple thing, to rotate and write some png file in 1-bit depth, it didn't matter what format (gif, png, you name it). As it turned out, it's quite a problem in IM. My first approach was (<input> is 1-bit colormap PNG if it matters): as PNG nicely support palette-based format. No luck, stored as 8-bit. Second try: and Still 8-bit. Desperate try: Success! After some tests it seems PNG can be stored only as GrayScale when one wants 1-bit depth. Why? Palette-based format is perfectly valid => BUG here!

Another strange behavior: stores 8-bit as I write above. But stores 1-bit! Not only it's strange, it's also confusing! Aren't colors and depth redundant parameters? This "trick" seems to work for GIF, too but unfortunately, not for PNG

My version of IM is 6.6.4-10 2010-10-10 Q16 ...

I'd love to see I did something wrong... I tried also +antialias and +dither but no luck either.

try using gif as it supports bi-level (not sure PNG does)

convert input -monochrome -type bilevel result.gif

or

convert input -colorspace gray +dither -colors 2 -type bilevel result.gif

Well, my point isn't to get it working by some workaround. I did this with using .bmp + -colors 2. I'd like to know why PNG doesn't work, i.e. if it's bug or some unclear documentation issue.

see http://www.imagemagick.org/Usage/formats/#png_formats and http://www.imagemagick.org/Usage/formats/#png_write

ehm, yeah, right... this is exactly what doesn't work, that's the point i'm talking about here.

EDIT: ah, ok, I overlooked the notes. So it seems it's disabled for palette modes as a "feature"...

you can always post an enhancement request to the Developers forum - but no guarantee how quickly that might happen

OK, I see I messed things here. Basically, your links confirm expected behavior, i.e. indexed palette mode should support 1, 2, 4 and 8 color bit depth. However, when I do the most simple thing: it gives me (even with input.png being 1-bit colormap!) :

convert: Cannot write image with defined PNG:bit-depth or PNG:color-type.

Funny things is when I type: the result is 1-bit (correct) grayscale (incorrect!!!). To me it seems like there's hardcoded implication 1-bit => grayscale, totally forgetting palette-based mode.

Dear developers, there are three bugs:

1. Error in documentation

-define png:bit-depth {depth}

(should be png:bit-depth={depth})

2. Unclear relationship with -colors and -depth / png:bit-depth (documentation states under which condition png:bit-depth overrides -depth, fair enough, but no word about when one should (not) use -colors)

3. Actual bug in IM, command about must work or documentation is wrong.