Read a grayscale png.

[wizard29]

Hi, everyone.

After last update of IM I found a some weird behaviour of file opening. The grayscale image is opened as totally white image but it is wrong.

Before I used IM "6,9,0,7" version and Magick++ API under OS Windows 7 and everything works fine for png images.
Now I try to use the IM "6,9,1,3" version.

[glennrp]

I wasn't able to reproduce the problem on Ubuntu commandline interface with IM-6.9.1-3 downloaded today. What is your libpng version? run "convert -list format | grep PNG" to find out.

[wizard29]

libpng version is 1.6.12.

[wizard29]

convert.exe utility makes the same result (convert.exe grayscale.png grayscale.jpg).

[wizard29]

By the way, the same issue I have under Mac OS X 10.10.3.

[glennrp]

I relinked my IM-6.9.1-3 with libpng-1.16.12 but still could not replicate your result. I'm getting the expected scribbly drawing.
pngcheck doesn't show anything peculiar about the file:

• glenn.rp> pngcheck -v *7.png
  File: h777.png (12176 bytes)
  chunk IHDR at offset 0x0000c, length 13
  426 x 227 image, 8-bit grayscale, non-interlaced
  chunk pHYs at offset 0x00025, length 9: 2835x2835 pixels/meter (72 dpi)
  chunk iCCP at offset 0x0003a, length 792
  profile name = Photoshop ICC profile, compression method = 0 (deflate)
  compressed profile = 769 bytes
  chunk cHRM at offset 0x0035e, length 32
  White x = 0.31269 y = 0.32899, Red x = 0.63999 y = 0.33001
  Green x = 0.3 y = 0.6, Blue x = 0.15 y = 0.05999
  chunk IDAT at offset 0x0038a, length 11250
  zlib: deflated, 32K window, maximum compression
  chunk IEND at offset 0x02f88, length 0
  No errors detected in h777.png (6 chunks, 87.4% compression).

Perhaps there is some problem with the color managment chunks (iCCP and cHRM). During conversion
to JPEG format, the cHRM chunk gets discarded but the iCCP chunk is copied into the JPEG.

[wizard29]

Glenn,
Thank you for the response. I think you right. Looks like a conversion problem.

Could someone to clarify this issue?

[wizard29]

When I tried to make conversion from grayscale.png to grayscale1.png I get a warning message that looks like:
convert.exe: profile 'icc': 'GRAY': Gray color space not permitted on RGB PNG `g
rayscale1.png' @ warning/png.c/MagickPNGWarningHandler/1656.

And totally white grayscale1.png as result.

[wizard29]

if I try to convert:
convert.exe grayscale.tif grayscale1.png
everything works as expected. But I get the same warning exception.

[snibgo]

"identify -verbose" shows it contains an ICC profile. Depending on what you are trying to do, you might want to convert it to sRGB and strip the profile:

Code: Select all

convert grayscale.tif -profile sRGB.icc -strip out.tif

[wizard29]

Snigbo,
Thank you.

The problem is conversion from the grayscale.png. For other grayscale images everything works as expected. I have checked:
grayscale.bmp
grayscale.jpg
grayscale.png
grayscale.tif

All works as I expected except the grayscale.png. I make conversion from all grayscale.* images to the sRGB color space.

[wizard29]

I ran the "identify" utility for the grayscale.png. Result of color map looks weird. Perhaps it is not substantively:

png.txt
jpg.txt

[snibgo]

Your png.txt and jpg.txt both contain:

Code: Select all

    icc:description: Dot Gain 20%
    icc:manufacturer: Dot Gain 20%

So they contain embedded profiles and have NOT been converted to sRGB.

When you convert to bmp or other format that can't store colour profiles, the embedded profile is silently ignored.

So your bmp version looks different (on Firefox on my computer) to your png version. Firefox recognises the profile and converts it to sRGB.

[wizard29]

The "convert.exe grayscale.png -profile sRGB.icc grayscale1.png" gives expected result.

[wizard29]

Thank you,
Snigbo.