[SDL] RE: pleae help! making a transparant background using SDL_image??/ (Graveyard

[David Olofson]

On Saturday 28 February 2004 01.04, Graveyard Filla wrote:
> thanks a lot david. i  didnt know that .jpg was destructive or
> whatever. hell, i didnt even know what a destructive format is.

Well, if you lower the quality when saving a JPG, you'll get the 
idea. ;-) What's happening is very similar to what's happening when 
encoding an mp3 file: Information that's less important (by some 
"smart" definition, which is a major part of the trick) is removed to 
make the file smaller.

BTW, both mp3 and JPG are based on the idea of converting the data 
from the time/space domain (samples/pixels) to the frequency domain, 
sorting the resulting spectra somehow, removing the least significant 
ones and then storing the remaining data, possibly after compressing 
it using some non-destructive method.

Non-destructive formats OTOH, just rely on patterns and stuff in the 
data, to store it more efficiently. Every single bit is restored when 
you uncompress the data. Examples of formats that use this type of 
compression would be ZIP, bzip2 and RAR. These very algorithms, or 
very similar algorithms, are also used "inside" various image 
formats, such as GIF - and our friend, PNG.


> thanks for your help. so .png is the preffered file type for this
> kind of stuff?

It's definitely the most suitable format I know about, that is also 
widely available.

Formats like PSD (PhotoShop's native format) and XCF (GIMP's native 
format) can be really rather nice if you want advanced stuff 
(multilayer graphics, animations, easilly configurable blending and 
stuff), but those formats are normally uncompressed and perhaps more 
seriously, not really standardized. They tend to change between 
versions of their respective applications, breaking some or all of 
the third-party loaders.


> is it much smaller then .bmp?

Yes, especially compared to the usual uncompressed BMP format. (The 
RLE format isn't widely supported, AFAIK.)


> well, i guess ill find
> out for myself.

Yeah, just try it. File sizes depend on the image data, but generally, 
I don't think you can get better non-destructive compression than 
with PNG, except possibly if you put your data in bzip2 or RAR 
archives or something. (Those compress some 10-25% better than most 
other popular algorithms on most kinds of data.)

If you go destructive, all bets are off, though. There's basically no 
limit to how small files you can have, but it depends entirely on 
what kind of graphics you have, and what kind of quality is required.

Note that for simple images with large flat colored areas (as opposed 
to gradients and "noise"), nondestructive formats often beat JPG at 
both size and quality! That kind of images compress well with typical 
nondestructive algorithms, so there's no point in messing up the data 
with algorithms tuned for photo-like images.


> thanks man!!!

No problem... :-)


//David Olofson - Programmer, Composer, Open Source Advocate

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