Device.Rasterizer.GetViewports must have "out" parameter

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I found that GetViewports must be used like this :

            Viewport[] Viewports = new Viewport[0];
            Context.Device.Rasterizer.GetViewports(Viewports);

I think it would be better like that :

            Viewport[] Viewports = null;
            Context.Device.Rasterizer.GetViewports(out Viewports);

Can we know the incoming viewports count before the call ? I think not. So, 
Viewports parameter must be created inside the GetViewports by slimdx.

Sorry if I am thinking wrong. Please share your ideas with me.

Original issue reported on code.google.com by OnurER@gmail.com on 5 Jul 2009 at 8:45

[GoogleCodeExporter]

You set the viewports, therefore you know how many you set. The native API 
requires
you to pass the count as well:

http://msdn.microsoft.com/en-us/library/bb173610(VS.85).aspx

We abstract this away by using the count of the array you provide to the managed
wrapper. Offhand, I don't know if the native API can be used with a null second
parameter to retrieve the count, which would be required for doing this via an 
out
parameter. However, it's not documented behavior so I'm wary on relying on it, 
even
if it works.

I will look into it; but from a design perspective this would also need to 
maintain
parity with the other getter methods that return arrays in the same interface, 
and
that may not be viable.

Original comment by josh.petrie on 6 Jul 2009 at 2:45

[GoogleCodeExporter]

hmm... 

1) we can't know the active viewports count every time.

I am developing a 3d library on slimdx. the application developer who use my 
library may set the viewports and my library 
doesn't track many direct3d methods. My library don't care viewports.

But In some point, a method in my library, stores the active viewports + set a 
viewport for its use + does its work + restores 
the stored viewports. This method doesn't know the active viewports count while 
storing them.

2) as you know from the directx documantation :

NumViewports 
[in, out] Number of viewports in pViewports. 

I don't know the purpose of [in] but I think the purpose of [out] is to get the 
active viewports count from the DX. it is absolutely will give the count and 
slimdx can init the array which is defined as "out" parameter in GetViewports 
method.

what do you think mate ? please share it.

Original comment by OnurER@gmail.com on 6 Jul 2009 at 4:23

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[deleted comment]

[GoogleCodeExporter]

Calling RSGetViewports with zero as the initial count does cause it to fill the 
count out with some value that is 
probably the bound viewport count. However that behavior is not documented, and 
until it is (or we know it will 
be) we can't support it. Promit's looking into whether or not it is safe 
behavior. 

Original comment by josh.petrie on 7 Jul 2009 at 3:02

[GoogleCodeExporter]

you know, microsoft (or many API developer) generally implements Get and Set 
methods with a pointer and a count parameters pair since there is no 
real array in C++. a Get method DOES HAVE to give the count while giving the 
Array pointer. Otherwise it would be useless :)

The error in the documentation is the [in] modifier of Count parameter of 
GetViewports. There is no need and no scenario to pass a array count to 
a get method. its purpose is to only [out] :)

Dear friend, I don't want to argue. just sharing my ideas :) hope you like it.

see you 

Original comment by OnurER@gmail.com on 7 Jul 2009 at 5:50

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I verified that passing NULL for the second parameter is acceptable. Go forth!

Original comment by promit....@gmail.com on 7 Jul 2009 at 2:54

• Changed state: Accepted

[GoogleCodeExporter]

I believe that native directx function (GetViewports) would ignore the current 
value of second parameter when we 
pass as not-null value and would override the old value with the new pointer 
that is allocated by it self. This 
results in a memory leak of memory array that is passed to GetViewports. I 
don't think GetViewports would 
deallocate or would use it to fill in, when we pass a not-null value.

The only use of second parameter must be [out] and always be passed [in] as 
null.

does Verified mean that you will fix it ? hehe I am happy to convince you :)

Original comment by OnurER@gmail.com on 7 Jul 2009 at 5:38

[GoogleCodeExporter]

No, that's not how the API works at all. Read the documentation carefully; it 
expects a preallocated array as input. If it worked the way you just said, it 
would 
take a D3D10_VIEWPORT**.

I believe jpetrie has a fix that will allow you to find out how many viewports 
are 
set.

Original comment by promit....@gmail.com on 7 Jul 2009 at 5:48

[GoogleCodeExporter]

It means he checked with people via his MVP status to confirm the counting 
behavior
is intended -- without that kind of confirmation we can't implement things 
based on
*undocumented* behavior.

RSGetViewports() does not allocate -- it writes into (count) elements of the 
provided
viewport array. The array can only be null if the first parameter points to an
integer that is zero, in which case the function will write nothing to the 
output
array and write the viewport count to the first parameter. Otherwise the 
function
writes (count) elements to the viewport array and writes the viewport count to 
the
first parameter.

An overload such as you are proprosing requires us to make an extra allocation, 
so we
won't replace the existing GetOutputs() method, but rather add an additional 
method
that uses an out parameter.

Original comment by josh.petrie on 7 Jul 2009 at 5:50

[GoogleCodeExporter]

This issue was closed by r1177.

Original comment by josh.petrie on 8 Jul 2009 at 1:02

• Changed state: Fixed

[GoogleCodeExporter]

Thank you :)

Original comment by OnurER@gmail.com on 8 Jul 2009 at 5:42