This is where you are getting it backwards. You can take your 3 megapixel camera, take a photo and compress it with PNG to get a X Mb file.

I can then then go get a 4 megapixel (oh how I wish), take a photo and also compress it to X Mb.

But the JPG may actually have more information, because it had more pixels to analyse while is was compressing. It could make a choice about with pixels to drop. However your 3 megapixel camera has already performed a lossy compression by throwing out a random 1 million pixels.

Hence, we call JPEG "lossy" and PNG "lossless". The accepted terminology is really quite simple.

The terminology is also quite deceiving. Of course if you feed digital data like an executable into both formats, JPG will corrupt it and PNG won't. But that's not what we are doing. We are feeding a digital representation of analog data into the formats. You have to consider the system as a whole.

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Jeremy
I didn't believe in evil until I dated it.

The terminology is also quite deceiving.

When you start questioning terminology that is widely used and generally accepted without a bit of controversy, it's time to wonder whether the misunderstanding is on your part or that of the majority who remain unconfused.

Of course if you feed digital data like an executable into both formats, JPG will corrupt it and PNG won't. But that's not what we are doing.

Uh... that's exactly what we are doing.

We are feeding a digital representation of analog data into the formats.

What the digital data represents is entirely irrelevant. In fact, erroneously taking it into account has badly biased your argument. The fact is, you don't know where the data is coming from or whether it is in fact a representation of analog data. For example, what if it is entirely created in The Gimp?

You have to consider the system as a whole.

No. You don't. Just as you can talk about the properties of a car's tires without mentioning the engine, you can talk about the properties of compression algorithms without discussing the camera that uses them.

-sauoq
"My two cents aren't worth a dime.";

When you start questioning terminology that is widely used and generally accepted without a bit of controversy, it's time to wonder whether the misunderstanding is on your part or that of the majority who remain unconfused.

Many facts are generally accepted, and provably wrong. I tend to be immediately suspicious when I hear a 'proof by popularity' argument. The entire body of scientific knowledge has been summed up by a wag as "Ideas that everyone knew were wrong, right up to the point where everyone knew they were right."

In this case I am trying to say that the fact is "right, with infinite resources". Since you can't have infinite resources, it's not quite right.

What the digital data represents is entirely irrelevant In fact, erroneously taking it into account has badly biased your argument.

You brought up the contents of the image in a previous post when you made an example of satellite images. It's not nice to switch horses midstream. Or are you trying to ride both horses at the same time?

The subject matters because any measurement, including a photo, has a margin of error. CCDs smear borders and have non-linear responses to hues and luminence, just like film. When the height of a mountain is calculated from imagery, it has a error of plus or minus x meters, where x depends on the measuring device. It is quite reasonable to loose data in this case, provided that x doesn't get too big.

Just as you can talk about the properties of a car's tires without mentioning the engine, you can talk about the properties of compression algorithms without discussing the camera that uses them.

Of course this is true - we certainly can talk about the properties of the tire without considering anything else. And yet, if you don't consider the engine, you run the risk of having the tire malfunction.

If you want to continue your tire analogy, this is like an engineer insisting on using a brand of rubber called 'superior rubber' because it has better shock absorbtion, while the other engineers attempt to tell him that tire will disintegrate when an engine is connected to it because the rubber is too soft. The engineer's main argument is "It's superior. Everyone else in the world understands the word superior. Why can't you?"

PNG itself is lossless, just like transmitting the original data is lossless. But in a practical application with limited bandwidth, tradeoffs always have to made, and the surrounding system must be considered. If we could ignore the surrounding system, there would be no need for compression - we could just assume infinitely fast machines with infinite memory and always work with the uncompressed original.

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Jeremy
I didn't believe in evil until I dated it.