Imagemagick automatically lowering depth too far.

[Aroymart]

Hello! I've just started using ImageMagick so I appologize if I'm missing something simple.

I'm working on a project that requires indexed pngs with a Bit Depth of 2. I'm creating the images and converting them using

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convert [image location 1] -depth 2 [new image location]

This works perfectly for images with at least 3 colors, however with an image that uses only black and white, it gives it a bit depth of 1. I've confirmed this by running it on several different images and going to http://regex.info/exif.cgi to check the BitDepth.

My question is, is there any way to force a bit depth of 2 regardless of whether or not there are only 2 colors?

I'm using

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 ImageMagick 6.9.3-10 Q16 x86_64 2016-05-04

on the command line.

Thank you in advance

[fmw42]

What platform? What output image format? Did you use PNG8:output.png?

IM typically tries to output the smallest depth that fits the data?

Did you try using

-define png:bit-depth {depth}

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

[Aroymart]

I'm using Windows 7 but through Cygwin. I'm outputting as png. I just tried it and that sets the depth to 8. There doesn't seem to be a PNG2: option though.

yea, I'm very limited by the other software.

I just tried

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 convert src/gfx/test.png -define png:color-depth=2 -depth 2 src/gfx/test4.png

and it's still bit depth of 1

[fmw42]

try

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convert src/gfx/test.png -define png:color-depth=2 PNG8:src/gfx/test4.png

But does your input image have more colors that 2? If so, you need to reduce the colors to black and white for example by

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convert src/gfx/test.png -colorspace gray +dither -colors 2 -threshold 50% -define png:color-depth=2 PNG8:src/gfx/test4.png

[Aroymart]

Thank you for the suggestions

The line

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convert src/gfx/test.png -define png:color-depth=2 PNG8:src/gfx/test4.png

results in a png of bit-depth 8

for images that start with 3-4 colors, converting the image to a bit depth of 2 works. Sometimes it groups greys that are too close together as the same color though so I'll experiment with those options to see if I can make it more strict, Thank you!

[fmw42]

The PNG developer may have other ideas to suggest. You can also try -monochrome to convert to binary if you have other colors than black and white.

[glennrp]

Don't use "PNG8:" because that forces a bit-depth of 8. Try this:

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convert test.png -colors 4 \
              -define png:color-type=3 \
              -define png:bit-depth=2 \
              -define png:exclude-chunk=bKGD \
              test5.png

(excluding the bKGD to prevent your image from having 5 colors, that won't fit in a 2-bit palette)

[Aroymart]

The PNG developer may have other ideas to suggest.

Oh sorry, I know there are multiple ways to define/talk about bit depth. I and the website are talking about 2 bits per pixel, so up to 4 different colors at once. I'd like to have my images indexed with a bit depth of 2, which works when I feed it 3 or 4 colors.

Don't use "PNG8:" because that forces a bit-depth of 8. Try this:

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convert test.png -colors 4 \
              -define png:color-type=3 \
              -define png:bit-depth=2 \
              -define png:exclude-chunk=bKGD \
              test5.png

(excluding the bKGD to prevent your image from having 5 colors, that won't fit in a 2-bit palette)

That worked! Thank you so much! I did change one thing though (for future reference if anyone needs anything as specific as this ). With

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-define png:color-type=3

the resulting image came out negated and gave me the warning

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convert: Wrote palette index exceeding num_palette `src/gfx/music2.png' @ warning/png.c/MagickPNGWarningHandler/1656.

It's probably just me misunderstanding how the input file should have been formatting, but removing it worked so I'm content.


Thank you all very much!