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copy png to png expands palette
Posted: Fri Jul 25, 2014 5:21 pm
by dlang
If you copy an 8bit png which uses only a few colors, say 10 and copy using "ncvonert -o new.png old.png", your new image has a palette of 256 colors expanding the size of the image (bytes on disk, not image dimension). The size of the image is critical since I am placing them in an Android apk package and want the smallest possible png.
Re: copy png to png expands palette
Posted: Fri Jul 25, 2014 5:37 pm
by cday
dlang wrote:If you copy an 8bit png which uses only a few colors, say 10 and copy using "ncvonert -o new.png old.png", your new image has a palette of 256 colors ...
Please try adding the following code from the NConvert Help file:
-colors num : Convert in Indexed Colors (256, 216, 128, 64, 32, 16 or 8 )
So
Code: Select all
nconvert -colors 16 -o new.png old.png
Please post NConvert questions under the NConvert heading -- Everyone!
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Re: copy png to png expands palette
Posted: Mon Jul 28, 2014 1:13 pm
by dlang
Using -colors <numColors> is not practical when I am converting a bunch of images using a DOS "for" loop command.
for %f in (*.png) do nconvert -o out\%f %f
I also do not know the number of color used in each image.
I only determined that nconvert was expanding the palette because some of my images got larger (size on disk).
I used tweakPng to display the file details and noticed the palette size increased.
I am using nconvert to strip off some of the meta chunk sections (text, iText, and zText).
Re: copy png to png expands palette
Posted: Mon Jul 28, 2014 1:47 pm
by cday
dlang wrote:Using -colors <numColors> is not practical when I am converting a bunch of images using a DOS "for" loop command.
Code: Select all
for %f in (*.png) do nconvert -o out\%f %f
Is not practical because... ? It doesn't run, or some other reason?
I also do not know the number of color used in each image.
If you open an image in a GUI image editor such as XnView you should be able to see the image properties including the number of colours, or you could upload an example image.[/quote]
I used tweakPng to display the file details and noticed the palette size increased.
But that shows the number of colours??
When a png file is opened NConvert may convert it to a 24-bit image while preserving the actual colours used; when you use the
-colours option you would need to set a value equal to greater than the number of colours in the original image.
I am using nconvert to strip off some of the meta chunk sections (text, iText, and zText).
You should be able to do that using the following flag:
-rmeta : Remove Metadata (EXIF/IPTC/...)
Have you created a list of the available NConvert commands using -help ?
Re: copy png to png expands palette
Posted: Wed Jul 30, 2014 3:30 pm
by XnTriq
Re: copy png to png expands palette
Posted: Thu Aug 07, 2014 9:04 am
by xnview
yes i use only 16 or 256 palette colors
Re: copy png to png expands palette
Posted: Mon Sep 08, 2014 5:03 pm
by DOS386
XnTriq wrote:Bug confirmed for 8- as well as 4-bit PNGs
I would NOT consider this as a BUG ... NC is neither a hard-core PNG optimizer nor a generic file copy tool, so to me it seems to work well, despite somewhat unfortunate result is this special case. Try OPTIPNG or PNGOUT or TRUEPNG or similar.
http://newsgroup.xnview.com/viewtopic.php?t=23326
> and want the smallest possible png
PS: some other PNG creation tools save pictures always as 32-bpp RGBA, even if there are just 5 colours in ... so NC is NOT that bad :-\
Re: copy png to png expands palette
Posted: Tue Sep 09, 2014 2:52 pm
by cday
DOS386 wrote:[I would NOT consider this as a BUG ... NC is neither a hard-core PNG optimizer nor a generic file copy tool, so to me it seems to work well, despite somewhat unfortunate result is this special case.
A ´bug´becomes a ´limitation´when it is documented as such...
