调整JPEG大小而不会降低质量

Question

I am trying to work on a small webbased Image editor with .NET . I am having issue with JPEG image resize.As I understand there is no way to manipulate JPEG without loosing some quality and saw this as only way to do it . Resizing an image in asp.net without losing the image quality . In this sample target image is saved as a png .

So What I want to do is
1. When user uploads image save it as PNG
2. Manipulate it like resize rotate etc
3. Save it as JPEG

If I save that png as a jpeg will that loose quality as well ? Or should it be fine as its just saving it with other format and not really manipulating it.

Thanks.

Answer 1

If you don't want to lose quality, don't use jpeg. What's wrong with just keeping it png? When you resave the png as jpeg, yes you will lose some quality depending on the compression ratio you choose. That's the nature of the jpeg format.

Answer 2

PNG format is a lossless format, so as long as you are creating the PNG with the source image you are OK.

For reference:

JPEG - Lossy for most of the specifications (the new spec has a lossless format also, but not many libraries support it yet).

PNG - Lossless

TIFF - Lossless (RLE encoding)

BMP - Lossless (RAW encoding)

Answer 3

Yes, it will lose quality as well. The difficulty of resizing JPEG is due to the compression (aka encoding) algorithm itself, so it is irrelevant what image format is the source, because to encode the image you have to decode it to a raw format (bitmap, that is) first.

Also, it's good to remember, that if the image you're encoding has been a JPEG before, encoder will have harder time encoding it again after resizing. JPEG divides image into 8x8 pixel blocks to perform the encoding, and resizing moves the borders of these blocks. This is important note, because with higher compression ratios the borders between blocks become sharper -- and JPEG is especially bad in encoding irregular sharp edges in image (I'm skipping all the internal details, they're not important for now, comment if you'd like to know them anyway). This effect is not important for typical photo compression ratios, though.

Essentially: if you encode image as JPEG, you have to encode it at least once (obvious, isn't it? ;) ). And JPEG (ie lossy) encoding equals losing quality.