PNG compression

[Jon Harrop]

I just added save-to-PNG to our software and noticed that WPF generates
enormous PNG files. Re-encoding them under Linux makes them 20x smaller.

Is it possible to enable compression under .NET or is PNG not properly
supported?

--
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?u

[Peter Duniho]

On Sun, 12 Oct 2008 04:20:42 -0700, Jon Harrop <jo*@ffconsultancy.com>
wrote:

I just added save-to-PNG to our software and noticed that WPF generates
enormous PNG files. Re-encoding them under Linux makes them 20x smaller.

Is it possible to enable compression under .NET or is PNG not properly
supported?

PNG is always compressed. But there's a myriad of parameters that can be
applied as part of the compression, and .NET doesn't offer any way to
control them. You're stuck with the defaults.

I'm intrigued that you have images that can be compressed 20x smaller.
I've never seen .NET's PNG perform so poorly compared to other
implementations. It sounds as though you are either a) using a utility on
Linux that does a broad search of the different parameters, testing to
find the best compression technique for the image, or b) you have an image
that just happens to be able to be compressed optimally by whatever
compression parameters are used on Linux (whether explicitly or as
built-in defaults).

I suppose a third possibility is that you are not actually saving the
images as PNG files in .NET even though you think you are. That would
certainly explain such a large disparity.

In any case, unless you post more details, including the exact code you're
using in .NET to save the PNG files, as well as a detailed description of
how you compress them under Linux, it's not clear to me that anyone could
offer any sort of specific advice.

Pete

[Jon Harrop]

Peter Duniho wrote:

On Sun, 12 Oct 2008 04:20:42 -0700, Jon Harrop <jo*@ffconsultancy.com>
wrote:

>I just added save-to-PNG to our software and noticed that WPF generates
enormous PNG files. Re-encoding them under Linux makes them 20x smaller.

Is it possible to enable compression under .NET or is PNG not properly
supported?

To answer my own question: PNG is not fully supported by .NET.

PNG is always compressed.

Here is an excerpt from the PNG specification about its support for
uncompressed images:

"It is possible to store uncompressed data by using only uncompressed
deflate blocks (a feature normally used to guarantee that deflate does not
make incompressible data much larger)." -
http://www.w3.org/TR/PNG-Rationale.html

For example, here is a concrete implementation in OCaml:

http://code.google.com/p/aihiot/sour...p/ocaml/png.ml

But there's a myriad of parameters that can be applied as part of the
compression, and .NET doesn't offer any way to control them. You're stuck
with the defaults.

Indeed.

I'm intrigued that you have images that can be compressed 20x smaller.
I've never seen .NET's PNG perform so poorly compared to other
implementations. It sounds as though you are either a) using a utility on
Linux that does a broad search of the different parameters, testing to
find the best compression technique for the image,

I am using image magick with its default settings.

or b) you have an image
that just happens to be able to be compressed optimally by whatever
compression parameters are used on Linux (whether explicitly or as
built-in defaults).

That appears to be the case. I have tested with several other images and
they are typically only 50% more bloated when generated by .NET but some
are much worse.

I suppose a third possibility is that you are not actually saving the
images as PNG files in .NET even though you think you are.

I have verified that these are genuine PNG images and that the binary result
after decompression is always identical to the original.

In any case, unless you post more details, including the exact code you're
using in .NET to save the PNG files, as well as a detailed description of
how you compress them under Linux, it's not clear to me that anyone could
offer any sort of specific advice.

I just wanted to confirm that .NET provides only very rudimentary support
for PNG encoding.

--
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?u

[Peter Duniho]

On Mon, 13 Oct 2008 19:01:14 -0700, Jon Harrop <jo*@ffconsultancy.com>
wrote:

[...]

>PNG is always compressed.

Here is an excerpt from the PNG specification about its support for
uncompressed images:

I guess I should have been more specific. My point was that .NET always
tries to compress PNGs by default. You'd have to intentionally disable
compression, and off the top of my head, I don't recall any codec settings
that you can use with PNG that would do that (there's an
EncoderValue.CompressionNone enumeration value, but AFAIK you can only use
that with TIFF files).

"It is possible to store uncompressed data by using only uncompressed
deflate blocks (a feature normally used to guarantee that deflate does
not
make incompressible data much larger)." -
http://www.w3.org/TR/PNG-Rationale.html

Sure, but .NET doesn't do that.

Of course, that point does raise the question: have you compared an
explicitly uncompressed PNG generated with software that _does_ give you
that level of control with the output from .NET? For uncompressed images,
the data in the file should be byte-for-byte identical (or nearly so, if
there are some header differences or something minor like that).

Or at the very least, compared the PNG output size from .NET to the
nominal uncompressed size of your data? That would give you direct,
reliable information about just how much, if at all, .NET is compressing
your image.

Since you haven't posted the code here, whatever I or anyone else might
write about what .NET is doing in your case is pure speculation. Garbage
in, garbage out.

[...]
I just wanted to confirm that .NET provides only very rudimentary support
for PNG encoding.

..NET provides only very rudimentary support for PNG encoding.

Pete

[Michael J. Ryan]

On 10/12/2008 4:20 AM, Jon Harrop wrote:

I just added save-to-PNG to our software and noticed that WPF generates
enormous PNG files. Re-encoding them under Linux makes them 20x smaller.

Is it possible to enable compression under .NET or is PNG not properly
supported?

use an external exe like pngcrush to compress the png, this will require you
to launch an external process, and probably hide it from the user.. if this is
a desktop application, it shouldn't be too bad... it would be a small-ish
exe... it's also available cross-platform.

--
Michael J. Ryan - tracker1(at)theroughnecks(dot)net - www.theroughnecks.net
icq: 4935386 - AIM/AOL: azTracker1 - Y!: azTracker1 - MSN/Win: (email)

.... FRA #261: A wealthy man can afford anything except a conscience.

[Jon Harrop]

Michael J. Ryan wrote:

On 10/12/2008 4:20 AM, Jon Harrop wrote:

>I just added save-to-PNG to our software and noticed that WPF generates
enormous PNG files. Re-encoding them under Linux makes them 20x smaller.

Is it possible to enable compression under .NET or is PNG not properly
supported?

use an external exe like pngcrush to compress the png, this will require
you to launch an external process, and probably hide it from the user.. if
this is a desktop application, it shouldn't be too bad... it would be a
small-ish exe... it's also available cross-platform.

Yes. Would anyone be interested in a C# translation of a real PNG codec as a
commercial product?

--
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?u