[vtkusers] How to get the framebuffer

[Ken Martin]

In current VTK I cannot see how GetPixelData can return null unless new
returns null due to being out of memory. The code boils down to

  unsigned char* ucdata = new unsigned char[width * height * 3];
  vtkRecti rect(x_low, y_low, width, height);
  this->ReadPixels(rect, front, GL_RGB, GL_UNSIGNED_BYTE, ucdata, right);
  return ucdata;

So if the return value is nullptr then "new" failed. So check around maybe
something else funny is going on.


On Wed, Jul 25, 2018 at 10:24 AM, Mathieu Westphal <
mathieu.westphal at kitware.com> wrote:

> Hi,
>
> It should work then.
>
> Take a look into :
> VTK/Rendering/OpenGL2/Testing/Cxx/TestRenderToImage.cxx
> VTK/Rendering/Core/vtkWindowToImageFilter.cxx
>
> Both are doing exactly what you are trying to do.
> The first one (the test) may be very useful to you.
>
> Best regards,
>
> Mathieu Westphal
>
> On Wed, Jul 25, 2018 at 4:20 PM, ling Hai <qyaoza at gmail.com> wrote:
>
>> Hi Mathieu,
>> Thank you for the response, I am already calling renderWindow->Render().
>> I tried reading the front, back, left and right buffers but am still unable
>> to get any data from the call. If it helps, I am not using shaders, color
>> points, or opacity the blending mode is composite, it is rendered by the
>> gpu.
>>
>> Linghai
>>
>> On Wed, Jul 25, 2018 at 3:52 AM, Mathieu Westphal <
>> mathieu.westphal at kitware.com> wrote:
>>
>>> Hi LingHai
>>>
>>> Make sure you have rendered into the renderWindow first.
>>> (renderWindow->Render())
>>> And try to read from the front and the back buffer.
>>>
>>> Best regards,
>>>
>>> Mathieu Westphal
>>>
>>> On Tue, Jul 24, 2018 at 7:57 PM, ling Hai <qyaoza at gmail.com> wrote:
>>>
>>>> Hi,
>>>> I am new to vtk and I am trying to get the front framebuffer from the
>>>> renderwindow like this:
>>>> int * dim = renderWindow->GetSize();
>>>> unsigned char* pixels = renderWindow->GetPixelData(0,0
>>>> ,dim[0]-1,dim[1]-1,true);
>>>>
>>>> but in pixels there is only a null value. I am using this on a 3d
>>>> render if that makes a difference the above segment of code called after
>>>> rendering the volume. What am I missing? Any help is appreciated thanks.
>>>> Linghai
>>>>
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>>
>
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-- 
Ken Martin PhD
Distinguished Engineer
Kitware Inc.
101 East Weaver Street
Carrboro, North Carolina
27510 USA

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