How do you convert an image to monochrome using ImageMagicks's convert? Well it's not for the lack of options, but not all of them produces a file that is actually encoded as a 1 bit per pixel B/W monochrome file. According to the documentation we have:
And if your output is a png file then there is also:
Which purportedly must be used together with one of the other options.
Let us start by taking a look at the first one:
the image type.
Normally, when a format supports different subformats such as grayscale and truecolor, the encoder will try to choose an efficient subformat. The -type option can be used to override this behavior. For example, to prevent a JPEG from being written in grayscale format even though only gray pixels are present, use.$ convert bird.png -type TrueColor bird.jpg
Similarly, use -type TrueColorMatte to force the encoder to write an alpha channel even though the image is opaque, if the output format supports transparency.
Use -type optimize to ensure the image is written in the smallest possible file size.
From that description it surely sounds like that this should create a true monochrome image, right?
$ convert magick:wizard -type Bilevel type-bilevel.bmp $ identify type-bilevel.bmp type-bilevel.bmp BMP 480x640 480x640+0+0 8-bit sRGB 922KB 0.000u 0:00.002
8-bit sRGB? That doesn't sounds like monochrome to me. But at least the image is dithered:
depth of the image.
This the number of bits in a color sample within a pixel. Use this option to specify the depth of raw images whose depth is unknown such as GRAY, RGB, or CMYK, or to change the depth of any image after it has been read.
Doesn't says anything about whether it will have any influence on the output format. But let us try it:
$ convert magick:wizard -depth 1 depth-1.bmp $ identify depth-1.bmp depth-1.bmp BMP 480x640 480x640+0+0 8-bit sRGB 256c 177KB 0.016u 0:00.014
Nope, that didn't work either, but this time it got changed into a 256 entry palette based image apparently. It is also still a 3 channel rgb image, wich probably isn't surprising as we didn't tell it to change the colorspace:
Set the image colorspace.
Choices are:CMY CMYK Gray HCL HCLp HSB HSI HSL HSV HWB Lab LCHab LCHuv LMS Log Luv OHTA Rec601YCbCr Rec709YCbCr RGB scRGB sRGB Transparent XYZ YCbCr YCC YDbDr YIQ YPbPr YUV
To print a complete list of colorspaces, use -list colorspace.
So let us try with "Gray" colorspace:
$ convert magick:wizard -depth 1 -colorspace Gray depth-1-colorspace-gray.bmp $ identify depth-1-colorspace-gray.bmp depth-1-colorspace-gray.bmp BMP 480x640 480x640+0+0 8-bit sRGB 922KB 0.000u 0:00.001
So what about
-monochrome? Surely that should create a true monochrome image, right?
transform the image to black and white.
That was some beautifully useless documentation. So what happens this time?
$ convert magick:wizard -monochrome monochrome.bmp $ identify monochrome.bmp monochrome.bmp BMP 480x640 480x640+0+0 8-bit sRGB 922KB 0.000u 0:00.002
Not this either. But at least it decided to dither it this time:
So what about the last one?
Set the preferred number of colors in the image.
The actual number of colors in the image may be less than your request, but never more. Note that this a color reduction option. Images with fewer unique colors than specified by value will have any duplicate or unused colors removed. The ordering of an existing color palette may be altered. When converting an image from color to grayscale, it is more efficient to convert the image to the gray colorspace before reducing the number of colors. Refer to the color reduction algorithm for more details.
Nah, that only changes the number of colors, not the actual number of bits used… right?
$ convert magick:wizard -colors 2 colors-2.bmp $ identify colors-2.bmp colors-2.bmp BMP 480x640 480x640+0+0 1-bit sRGB 2c 38.5KB 0.000u 0:00.001
… 1-bit 2 colors?? Wat… How… I don't even… So, um, I didn't specify it should be greyscale, so what does it look like?
$ convert magick:wizard -colors 2 -colorspace Gray colors-2-colorspace-gray.bmp $ identify colors-2-colorspace-gray.bmp colors-2-colorspace-gray.bmp BMP 480x640 480x640+0+0 8-bit sRGB 922KB 0.000u 0:00.002
Whaaaaaaaat? It didn't turn it black/white either which may not be that surprising (but contrary to the behavior when we used
$ convert magick:wizard -colorspace Gray -colors 2 colorspace-gray-colors-2.bmp $ identify colorspace-gray-colors-2.bmp colorspace-gray-colors-2.bmp BMP 480x640 480x640+0+0 1-bit sRGB 2c 38.5KB 0.000u 0:00.000
At least that made some kind of sense. The image still looks identical to the one above though. But that leads us to the conclusion that
-monochrome -colors 2 probably should work:
$ convert magick:wizard -monochrome -colors 2 monochrome-colors-2.bmp $ identify monochrome-colors-2.bmp monochrome-colors-2.bmp BMP 480x640 480x640+0+0 1-bit sRGB 2c 38.5KB 0.000u 0:00.000
Wow. That actually worked:
That was… pretty braindead behavior, but at least there was a way to do it. Oh, and btw., don't attempt to convert the non-monochrome
-colors 2 images to png files with ImageMagick – the output you get is this:
Yup, that is a completely blank image…