[vtkusers] How get pixel data in VTK?

Thu Apr 24 13:44:13 EDT 2008

I can be sure the image data is 3 component, but I don't know how to  
get the type those component. the pngwriter doesn't need to be worried  
about, this is  my old method which I could get the image data from  
png writer, and it did works very well. but now I want to directly get  
the pixel data, so that I don't have to pass the data through pngwriter.
PS: the image on the render window is what I expect to see, even I  
store it into a png file. it is not all black.

my render window is 560X560, and the ms in "nsigned long ms =  
id->GetActualMemorySize();" is 1041.

Yenan

Quoting Mark Wyszomierski <markww at gmail.com>:

> You have to remember to reply to the vtk users board as well, don't direct
> the emails to just me.
>
> How big is your render window, you're only sampling the first 1041 / 3
> pixels there, and I wouldn't be surprised if they were all black. You should
> also verify that the image data is 3 component unsigned char. I would also
> make sure that your png actually contains something by just writing it to
> disk.
>
> Mark
>
>
> On Thu, Apr 24, 2008 at 1:15 PM, <yenanqu at scs.fsu.edu> wrote:
>
>>
>> Here is my code:
>> ======================================
>> renWin->Render();
>> vtkWindowToImageFilter * w2i = vtkWindowToImageFilter::New();
>> vtkPNGWriter * writer = vtkPNGWriter::New();
>> w2i->SetInput( renWin );
>> writer->SetInput( w2i->GetOutput() );
>> w2i->Update();
>> vtkImageData * id = w2i->GetOutput();
>> id->Update();
>> //int* siz = id->GetDimensions();
>> //unsigned long ms = id->GetActualMemorySize();
>> //int ms1 = id->GetScalarSize();
>> unsigned char* pPix;
>> pPix = (unsigned char*)id->GetScalarPointer();
>>
>> for(int ee=0;ee<1041;ee++){
>>   int pp = pPix[ee];
>>   cout<<" "<<pp;
>> }
>> ===============================================
>> The thing I can be sure is there is no problem with id (vtkImageData)
>>
>>
>> Yenan
>>
>>
>>
>> Quoting Mark Wyszomierski <markww at gmail.com>:
>>
>>  Are you sure you actually set any of the RGB components in the first
>>> place?
>>> You could just be allocating the image array, all elements are set to
>>> zero,
>>> and you'll just get a zero for every element you access.
>>>
>>> Mark
>>>
>>> On Thu, Apr 24, 2008 at 12:49 PM, <yenanqu at scs.fsu.edu> wrote:
>>>
>>>  when I do
>>>> "unsigned char *pVtkPix = (unsigned char *)pImgData->GetScalarPointer();"
>>>> it doesn't seem to get the scarlar pointer, I have already do all the
>>>> update thing before this, and I also could get dimensions from pImgData.
>>>> but
>>>> when I output all the RGB value with your method, they are all 0.
>>>> this stuff really kills me~~~
>>>>
>>>>
>>>> Yenan
>>>>
>>>>
>>>> Quoting Mark Wyszomierski <markww at gmail.com>:
>>>>
>>>>  I think the RGB pixel data is usually allocated like:
>>>>
>>>>>
>>>>>   RGBRGBRGB etc....
>>>>>
>>>>> so you have numPixels * 3 actual elements in your image data array.
>>>>>
>>>>> You get a pixel pointer as usual and cast it to (unsigned char*):
>>>>>
>>>>> // Get a pointer to the actual pixel data.
>>>>> unsigned char *pVtkPix = (unsigned char *)pImgData->GetScalarPointer();
>>>>>
>>>>> // These are the r,g and b values for the first pixel:
>>>>> int rr  = pVtkPix[0];
>>>>> int gg = pVtkPix[1];
>>>>> int bb = pVtkPix[2];
>>>>>
>>>>> // For the second pixel:
>>>>> rr  = pVtkPix[3];
>>>>> gg = pVtkPix[4];
>>>>> bb = pVtkPix[5];
>>>>>
>>>>> and so on. Make sure you don't have RGBA data, if you do, then you just
>>>>> have
>>>>> an extra index per pixel which is the alpha component. Let me know if
>>>>> that
>>>>> helps,
>>>>>
>>>>> Mark
>>>>>
>>>>>
>>>>> On Wed, Apr 23, 2008 at 7:08 PM, <yenanqu at scs.fsu.edu> wrote:
>>>>>
>>>>>  Hi, Mark, I did have the data that is three component. Your example
>>>>> only
>>>>>
>>>>>> shows how to get pixel data that is one component which I never got in
>>>>>> my
>>>>>> code before. Do you have any idea how to retrieve this three component
>>>>>> data?
>>>>>> I have try bunch of methods, but none of them is working. any help will
>>>>>> be
>>>>>> greatly appreciated.
>>>>>>
>>>>>> Yenan
>>>>>>
>>>>>> Quoting Mark Wyszomierski <markww at gmail.com>:
>>>>>>
>>>>>>  Hi,
>>>>>> >
>>>>>> > It's definitely possible to get the scalar value from a vtkImageData
>>>>>> > object. You need to know the type of data your object is stored in
>>>>>> > though, for example you'll have difficulty getting the scalar value
>>>>>> if
>>>>>> > your data is three component RGB and you're trying to get a single
>>>>>> > pixel scalar pixel value.
>>>>>> >
>>>>>> > I have an example of getting the pixel value that the mouse moves
>>>>>> over
>>>>>> > in an image here:
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>>
>>>>>> http://devsample.org/index.php?option=com_content&task=view&id=48&Itemid=27
>>>>>> >
>>>>>> > which may help.
>>>>>> >
>>>>>> > Mark
>>>>>> >
>>>>>> >
>>>>>> > On Thu, Mar 27, 2008 at 1:37 PM,  <yenanqu at scs.fsu.edu> wrote:
>>>>>> >
>>>>>> > > Hi, all,
>>>>>> > > I found bunch of the VTK methods to retrieve pixel data from vtk
>>>>>> > > object, such as
>>>>>> > > GetPixelData, GetRGBACharPixelData, etc, but none of them is
>>>>>> working,
>>>>>> > > Does anyone know how to get pixel data from VTK object? Thanks in
>>>>>> > > advance.
>>>>>> > >
>>>>>> > > Yenan
>>>>>> > >
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