What does quant size mean?

[Art Vandelay]

When people refer to pic. quality they refer to bitrate which I understand the higher the rate the better for pic. quality. They also refer to quant size. What does this mean?

Thanks in advance.

[anonymez]

Encoding at a constant quantizer ensures that the output, at least technically, will be of constant quality. The lower the quantizer, the better the quality, but larger the file size. Single pass, unpredictable file size.

[blutach]

Quant is the technical term that refers to compression factor. The higher the value, the more compressed an image is (and the lower the quality of the pic). Small quant means low compression and the greater the ability to compress it more.

In its purest sense, quantisation is a way of making a range of values equal a single value (and thus losing some information in the process).

Let's take an example - JPEG image compression. The data starts off as 24 bits per pixel (8 for each of three color components) and is processed using a mathematical transformation called a discrete cosine transform. By reducing the precision of the transformed values via a quant matrix, the number of bits needed to store the image is reduced (3 bits per pixel cf 24 in the original). Of course in a movie, the original started off life on film (analogue) and transforming it to even a BMP requires some quantisation.

To go any deeper into this we'd need to write a book.

Regards

[ffreezinmn]

Quant is the technical term that refers to compression factor. In its purest sense, quantisation is a way of making a range of values equal a single value (and thus losing some information in the process).

Thank you.

Art beat me to the question. I have searched for info on Quant since I first read about it in DVD-RB forum.

[blutach]

If you really wanna know about it, ask my good mate manono at Doom9 - he'll talk for hours about DCT and quant.

Regards