PNG Compression Level

[simran]

Hi All, 

When i am outputting a png, it appears it gets automatically compressed. 

However, i'm not quite sure how to ensure it does not. 

The relevant bit of my code is: 

import (

"image/png"

        "github.com/nfnt/resize"

)

imgResized := resize.Resize(newImageWidth, 0, srcImage, resize.MitchellNetravali)

png.Encode(outfile, imgResized)

The full code is at: http://play.golang.org/p/StWSWZ_FXt

Would appreciate any help on setting the compression level to zero. 

thanks,

simran.

[Jan Mercl]

On Fri, Jul 17, 2015 at 10:38 AM simran <sim...@dn.gs> wrote:

> Would appreciate any help on setting the compression level to zero.

What's to reason? PNG compression is lossless.

--

-j

[simran]

That's what i thought, but the encoding is producing images that sometimes have a very slight colour shift... so just want to try without any compression to see what happens. 

--

-j

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[Jan Mercl]

On Fri, Jul 17, 2015 at 11:12 AM simran <sim...@dn.gs> wrote:

> That's what i thought, but the encoding is producing images that sometimes have a very slight colour shift... so just want to try without any compression to see what happens.

I'd suggest to write small program which compresses a bitmap to PNG, decompresses the result back to another bitmap and compare it to the original pixels one by one.

If you find an example which fail this test then please fill an issue with the full source code for the test program and the source picture attached.

--

-j

[James Bardin]

On Friday, July 17, 2015 at 5:12:24 AM UTC-4, simran g wrote:

That's what i thought, but the encoding is producing images that sometimes have a very slight colour shift... so just want to try without any compression to see what happens. 

Are you certain that the images with a color shift don't have an embedded ICC profile, or color palette? (or possibly even gamma and chromacity values)

[Carlos Castillo]

You can set the compression level by making a image/png.Encoder value, setting it's CompressionLevel field (NoCompression is -1), and using it's Encode method. (RTFM: http://golang.org/pkg/image/png/#Encoder)

That being said, as suggested by the other posts, Compression in PNG is done by zlib, which is a lossless file encoding, so changing it's level shouldn't affect pixel color in any way.

[Michael Jones]

http://koti.kapsi.fi/juanttil/profiletest/

simple test images

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