[SDL] A couple questions on SDL

[Lilith Calbridge]

>>> On 7/12/2006 at 8:56 PM, in message <44B5A845.4090209 at freeuk.com>,
darkmatter at freeuk.com wrote:
> Lilith Calbridge wrote:
> 
>> I'm getting my feet wet with SDL and, in a great sense, video
graphics
>> in general.  In looking for functions or combination of functions to
do
>> what I need I've come across some things that it would help me to
>> understand better. 
>> 
>> First, what is YUV?  More explicitly what is the purpose of a YUV
>> overlay?
> 
> YUV is a colour space format, as is RGB, but instead of storing the 
> amounts of red, green and blue per pixel, it stores the brightness
and 
> then the blue and red amounts in relation to the brightness. It more

> accurately models the human perception of colour and is mostly used
in 
> realtime video, for example in MPEG.
> 
> http://en.wikipedia.org/wiki/YUV 
> 
> A YUV overlay is a hardware method of displaying this data. Typically

> conversion from YUV->RGB is handled in hardware, although SDL can if

> neccesary do this in software.
 
>>  I've tried to find information through Google but pretty much
>> anything that discussed it did so from the view point that the
reader
>> knew what it was.
 
> In all honesty, YUV is only really used for motion video compression

> such as MPEG, DIVX, etc. If you're not playing back movies then it's
not 
> really of any relevance.

That's good to know.  Less I need to spend time on.  :-)
 
>> Second, does SDL provide a function that I'm overlooking that will
copy
>> the pixels of a small rectangle of a large surface into the pixels
of a
>> smaller storage surface.  My purpose is to restore the background
under
>> a blitted surface to it's original content before redrawing the
object
>> at a displaced position.  I don't mind doing the coding myself but
if
>> SDL supports this functionality it's probably more efficient than
what
>> I'd come up with.
> 
> Yes, SDL_Blit() will do this given the appropriate rect's.

That'll teach me to leave my thinking cap back at the office.  I was
thrown by the statement in the documentation that only the x and y
values of the source rectangle are used.  I should have figured that the
blit would use the w and h values of the destination rectangle to define
what to transfer from the source surface.

One more question if I could.  The SDL_CreateRGBSurfaceFrom
documenation says "The pixel data is not copied into the SDL_Surface
structure so it should not be freed until the surface has been freed
with a call to SDL_FreeSurface."  Does this mean that the surface
structure is set to point to the pixel data presented in the function
argument and doesn't create a pixel array of its own?

Thanks again.

Lilith