bug converting png to bmp (results in broken images)

[phildo]

the most basic conversion of a simple png image to a bmp results in an image osx refuses to open (in preview, pixelmator, etc...). also, some hack software I wrote a while ago fails to open it, thinking it's a bmp v4 (it only accepts bmp v5) (this pipeline used to work on older versions of imagemagick, or osx, or some combo).

the command I use is

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convert 0.png ~/Desktop/0.bmp

png used is
bmp result is

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convert --version

yeilds

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Version: ImageMagick 7.0.8-8 Q16 x86_64 2018-07-23 https://www.imagemagick.org
Copyright: © 1999-2018 ImageMagick Studio LLC
License: https://www.imagemagick.org/script/license.php
Features: Cipher DPC HDRI Modules 
Delegates (built-in): bzlib freetype jng jpeg ltdl lzma png tiff xml zlib

using macos high sierra (10.13.6)

[phildo]

hmm... I now see that chrome correctly renders the bmp in chrome when served from my web site. but if I save-as that image to my desktop, OSX refuses to open it.

[fmw42]

It fails for me also on Mac Sierra with IM 6.9.10.9 Q16 and 7.0.8.9.

This will not open:

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convert 0.png 0.bmp

These two produce black results:

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convert 0.png BMP3:0.bmp
convert 0.png BMP2:0.bmp

Perhaps it is the transparency in the PNG. The PNG is bilevel with 8-bit transparency. This may be a PREVIEW problem not being able to handle bilevel bmp with 8-bit transparency.

I can open it fine on my Mac in GraphicConverter. However, Photoshop gives only a black image

I have limited understanding of BMP format and only know from https://www.imagemagick.org/Usage/formats/#bmp

[phildo]

I found a workaround-

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convert 0.png PNG32:0.png
convert 0.png 0.bmp

produces a functional bmp (renders/opens on osx, including the bespoke software I had that only accepted a rigid bmp format).

I think the problem was that the png was considered grayscale + alpha. so the "real" bug here is:

"convert fails to convert grayscale pngs to bmps"

[magick]

The conversion appears to be working properly. It creates a BMP v4 image. Some viewers may not properly handle BMP v4 images. We did check with a few BMP viewers and half of them returned expected results and half failed and returned a black image.