Reading/Writing to an IBuffer

Question

I would like to read/write to the WriteableBitmap::Pixels property which is an IBuffer.  IBuffer only has two properties, Capacity and Length.  I would like to get access to the buffer pointer's contents.

MSDN points says IBuffer is used by the IInput/OutputStreams.  I cannot seem to find a concrete implementation of these classes in the WinRT.  In C#, you simply do a buffer.AsStream().

Thanks for any help!

Answer 1

Unfortunately, that's not supported at the moment. See this thread (similar but in the C# forum) :

http://social.msdn.microsoft.com/Forums/en-US/winappswithcsharp/thread/39b3c702-caed-47e4-b7d3-b51d75cbca9b/

 

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http://blog.voidnish.com

Answer 2

Thanks Nishant.  The reply by Wes Haggard in that thread makes it sound like it is possible to read/write the pixel data...just there is no Render() method (which is ok for me right now).

 

 

Answer 3

Wes Haggard's approach suggests using an extension method that returns a .NET Stream. That's not very useful from a C++ perspective.

What makes this trickier is that the extension methods do not have any code in them (in the assemblies). They just return false (or are empty). It seems the C# compiler or perhaps the CLR dynamically patches up these functions at runtime. Else we could at least have tried to emulate in C++ what those extension methods do to get the underlying data.

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http://blog.voidnish.com

Answer 4

I'm not 100% on this, but by looking at the .NET assemblies in reflector, there seems to be an unpublished interface for getting the buffer:

[ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("905a0fef-bc53-11df-8c49-001e4fc686da")] internalinterfaceIBufferInternal { byte* Buffer { [SecurityCritical] get; } }

Answer 5

[I posted this a few minutes ago but for some reason the post vanished.]

Good catch there. That seems to be basically what the C# extension method does too. Obviously not so useful from a practical perspective, although it's good to know what happens there.

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http://blog.voidnish.com

Answer 6

Ok, I think I figured it out...though I'm not sure if its the "right" way..but surely its the undocumented way :)

Here's the code:

#include "MainPage.xaml.h"
#include <Windows.h>
#include <INITGUID.H>

using namespace Windows::UI::Xaml;
using namespace Windows::UI::Xaml::Controls;
using namespace Windows::UI::Xaml::Data;
using namespace Application1;
using namespace Windows::UI::Xaml::Media::Imaging;
using namespace Windows::Storage::Streams;

//905a0fef-bc53-11df-8c49-001e4fc686da
DEFINE_GUID(IID_IBufferInternal, 0x905a0fef, 0xbc53, 0x11df, 
    0x8c, 0x49, 0x00, 0x1e, 0x4f, 0xc6, 0x86, 0xda);
struct IBufferInternal : IUnknown
{
	STDMETHOD(GetBuffer)(unsigned char** c) = 0;
};

MainPage::MainPage()
{
    InitializeComponent();
	
	wb = ref new WriteableBitmap(320, 240);
	 
	auto buffer = wb->PixelBuffer;

	IUnknown* pUnk = reinterpret_cast<IUnknown*>(buffer);

	IBufferInternal* pBufferInternal = nullptr;
	HRESULT hr = pUnk->QueryInterface(IID_IBufferInternal, (void**)&pBufferInternal);

	unsigned char* pBuff = nullptr;
	pBufferInternal->GetBuffer(&pBuff);
}

Answer 7

Nice work :-) [highly risky to try and use that in actual code though since you can't be sure the internal interface will remain in later builds].

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http://blog.voidnish.com

Answer 8

Agreed!  Though one can say the same about the entire pre-release package :)

Answer 9

Just curious, what use is a WritableBitmap that you cannot actually either write or inspect the contents of?  I ask that in all seriousness -- is there a useful usage scenario for such an object?

Answer 10

This issue has been identified, and it should be a more straightforward task in the future. I could not locate a customer bug on this however is anyone would like to make their voice heard on this subject.

Thanks,

-David

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Answer 11

So, what's the deal now, 7 months later?  Can we read / write to the pixels?  How?

Answer 12

Yes. The Consumer Preview introduced the IBufferByteAccess interface to expose the bytes in the IBuffer. There are a number of threads discussing this. You can also load the buffer into a DataReader with DataReader.FromBuffer

--Rob