Re: How to incrementally compress jpeg?
Re: How to incrementally compress jpeg?
- Subject: Re: How to incrementally compress jpeg?
- From: Phi Le <email@hidden>
- Date: Fri, 20 Nov 2009 14:20:04 -0800
I am coming from a .NET C# background, so I have some expectations
that might not apply to Cocoa development. In .NET, I can
incrementally compress an image like this:
A = new Image(data)
B = A.save(JPEGType, 0.5)
C = B.save(JPEGType, 0.5)
where the sizes A > B > C and the deltas are significant. I can do
this if B is saved to disk and reload back.
So if I am given an NSImage, but not the backing NSData directly, can
I ask the NSImage, NSImageRep or something else to give me information
about the original data, filetype, compression factor ?
Phi
On Fri, Nov 20, 2009 at 2:05 PM, Jens Alfke <email@hidden> wrote:
>
> On Nov 20, 2009, at 1:47 PM, Phi Le wrote:
>
> I have an jpeg image and I want to be able to incrementally compress
> it using Cocoa/Core Image/Core Graphics. For example, I have A.jpg
> (3MB), I compress A and get B.jpg (1MB), compress B and get C.jpg
> (400KB), and so on till the image can't be compressed anymore.
>
> What you're doing doesn't make sense; image compression doesn't work that
> way. If you JPEG-encode a pixmap, then load it back into an image, you have
> another pixmap. Compressing the resulting pixmap isn't going to result in
> smaller data just because you decompressed it from a smaller block of data.
> If you want to experiment with progressively greater and greater compression
> levels, you have to start with the same source pixmap every time and
> gradually decrease the compression factor. (For example, I did this in iChat
> to compress the user's buddy icon down to under 2K to fit in a Bonjour
> record. There's a loop that keeps increasing the compression factor until
> the data size is small enough.)
> —Jens
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