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What does a video card do? What is DirectX and X11?

 
 
darkknight
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      01-27-2006

Hi

What does a video card do?

The code being executed in the main CPU (not the video card) determines
what appears on the screen i.e. what the colour each of the one million
or so pixels on the screen should be. Where does it put this
information?

What does an expensive video card do better than a cheap video card?


What is DirectX, SDL or X11?

If a program in the main CPU wants to draw a red rectangle on the
screen, why can't it just write
for (x=100; x<=200; ++x) SetPixel(x,300,red);
for (x=100; x<=200; ++x) SetPixel(x,500,red);
for (y=300; y<=500; ++y) SetPixel(100,y,red);
for (y=300; y<=500; ++y) SetPixel(200,y,red);

What do you need DirectX for?

TIA
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nick
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      01-27-2006

darkknight wrote:

> Hi
>
> What does a video card do?
>
> The code being executed in the main CPU (not the video card) determines
> what appears on the screen i.e. what the colour each of the one million
> or so pixels on the screen should be. Where does it put this
> information?
>
> What does an expensive video card do better than a cheap video card?
>
>
> What is DirectX, SDL or X11?
>
> If a program in the main CPU wants to draw a red rectangle on the
> screen, why can't it just write
> for (x=100; x<=200; ++x) SetPixel(x,300,red);
> for (x=100; x<=200; ++x) SetPixel(x,500,red);
> for (y=300; y<=500; ++y) SetPixel(100,y,red);
> for (y=300; y<=500; ++y) SetPixel(200,y,red);
>
> What do you need DirectX for?



Program says "draw a rectanglle, then rotate it in 3 dimensions to 45
degrees then put a realistic texture on it"

This can be done by the central processing unit (Cpu) or the graphics
processing unit (Gpu), depending on the Gpu capabilites and the drivers
used. The Gpu is designed to do graphics functions very fast, the Cpu
will generally not be tuned to do those things fast, and also has to do
other stuff.

DirectX and SDL are doors to the video (and audio) hardware, they
provide a framework for accessing the fast stuff in the Gpu.

X11 is a graphics system which is common in unix systems (but also
works on windows, MacOSX, IRIX, Sparc and others). It is a client
server system, the server is the set of drivers that put stuff onto a
display. The client is the user program that displays on a server. The
client and the server are usually on the same machine, but don't have
to be, which means X11 can work over networks (a program on my work
computer can display on my home computer).

>
> TIA
> --


 
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darkknight
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      01-27-2006
nick wrote:

>
> darkknight wrote:
>
> > Hi
> >
> > What does a video card do?
> >

>
> Program says "draw a rectanglle, then rotate it in 3 dimensions to 45
> degrees then put a realistic texture on it"
>
> This can be done by the central processing unit (Cpu) or the graphics
> processing unit (Gpu), depending on the Gpu capabilites and the
> drivers used. The Gpu is designed to do graphics functions very fast,
> the Cpu will generally not be tuned to do those things fast, and also
> has to do other stuff.
>
> DirectX and SDL are doors to the video (and audio) hardware, they
> provide a framework for accessing the fast stuff in the Gpu.
>
> X11 is a graphics system which is common in unix systems (but also
> works on windows, MacOSX, IRIX, Sparc and others). It is a client
> server system, the server is the set of drivers that put stuff onto a
> display. The client is the user program that displays on a server. The
> client and the server are usually on the same machine, but don't have
> to be, which means X11 can work over networks (a program on my work
> computer can display on my home computer).
>



Thanks.

So do all video cards have the same interface and commands or does
DirectX have to know how to drive lots of different video cards?

Does X11 use the "extra" capabilities of the GPU or does it just send
basic SetPixel type commands to the GPU - or is there another layer
between X11 and the video card?

With an application such as Windows explorer, that just draws static
images, would the GPU be doing anything more than just accepting
SetPixel commands?

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nick
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      01-27-2006
You are probably getting to the limit of my technical expertise, but
I'll have a go. No warranty blah blah

video cards have different commands - thats why either the manufacturer
makes a driver, or someone reverse engineers one.

Whether X11 implements extra capabilities depends on the state of
driver development for that card. eg nVidia provide (closed source) X11
drivers that implement most of the features of their cards (2d and 3d
acceleration, XvMC, etc). Some other manufacturers will not release
enough specs, or put the effort in, to get it working. Also X11 is a
standard, there are more than one implementations. The popular open
source implementation is xorg-x11 and you will now find that in most
linux distributions. There are also commercial X11 implementations.

As for windows explorer, i am not sure about the answer to your
question. However it is the graphics subsystem that draws on the
screen, not explorer. explorer tells another part of the operating
system to draw something, and that is done in combination between the
cpu and the gpu, quite how the system knws what stuff can (and should)
be offloaded to the gpu, and which has to be crunched in the cpu, i do
not know. I suspect that explorer would be written to run everywhere,
and wouldn't be dependent on fancy graphics capabilities.

 
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Murray Symon
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      01-28-2006
On Sat, 28 Jan 2006 10:44:50 +1300, darkknight wrote:

> nick wrote:
>
>
>> darkknight wrote:
>>
>> > Hi
>> >
>> > What does a video card do?
>> >
>> >

>> Program says "draw a rectanglle, then rotate it in 3 dimensions to 45
>> degrees then put a realistic texture on it"
>>
>> This can be done by the central processing unit (Cpu) or the graphics
>> processing unit (Gpu), depending on the Gpu capabilites and the drivers
>> used. The Gpu is designed to do graphics functions very fast, the Cpu
>> will generally not be tuned to do those things fast, and also has to do
>> other stuff.
>>
>> DirectX and SDL are doors to the video (and audio) hardware, they
>> provide a framework for accessing the fast stuff in the Gpu.
>>
>> X11 is a graphics system which is common in unix systems (but also works
>> on windows, MacOSX, IRIX, Sparc and others). It is a client server
>> system, the server is the set of drivers that put stuff onto a display.
>> The client is the user program that displays on a server. The client and
>> the server are usually on the same machine, but don't have to be, which
>> means X11 can work over networks (a program on my work computer can
>> display on my home computer).
>>
>>

>
> Thanks.
>
> So do all video cards have the same interface and commands or does DirectX
> have to know how to drive lots of different video cards?


There is a driver layer between them. Each card has its own driver.
DirectX provides an abstracted API to the application program.
As far as I can recall, DirectX will emulate in software any functions
that are not supported in the card's hardware.

> Does X11 use the "extra" capabilities of the GPU or does it just send
> basic SetPixel type commands to the GPU - or is there another layer
> between X11 and the video card?


Some X11 drivers support different degrees of hardware acceleration.
Many support full 2D acceleration. X11 uses "DRI" for 3D graphics.
refer http://xfree86.org/ and http://freedesktop.org/Software/xorg

> With an application such as Windows explorer, that just draws static
> images, would the GPU be doing anything more than just accepting SetPixel
> commands?


Yes, 2D acceleration (in the GPU) can be used for bitblit operations as
many windows functions are operating on rectangular areas. e.g. it is more
efficient to get the GPU to move a rectangular window from one position to
another via a hardware-assisted bitblit than by getting the CPU to move
each pixel on an individual basis.
You can verify this by disabling Video Acceleration in your Windows
settings in control panel, and observing the difference (if any).

 
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Gordon
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      01-28-2006
On Fri, 27 Jan 2006 21:26:28 +1300, darkknight wrote:

> What does an expensive video card do better than a cheap video card?


Makes better (more complex) moving pictures.
 
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darkknight
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      01-28-2006


Thanks guys.

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