[vtkusers] vtkImageActor and scaling

[David Gobbi]

Hi Mark,

The underlying OpenGL mechanism for vtkImageActor is completely different
from vtkImageMapper, so if all other things are equal, any differences in
speed are due to how fast the GPU and its drivers can do glTexImage2D (for
ImageActor) versus glDrawPixels (for ImageMapper).

You can make the comparison between them more fair by making certain that
each of them uses the same number of pixel components.  By default,
vtkImageMapToColors will output an RGBA image.  Use
MapToColors->SetOutputFormatToLuminance() to ensure that it produces 8-bit
greyscale images instead.

  David


On Tue, Nov 30, 2010 at 9:56 AM, endlosschleife1 <
endlosschleife1 at googlemail.com> wrote:

> It's possible that I'm misinterpreting the performance results of my
> implementation and that applying the window level lookup table is actually
> not what makes the difference. I'm comparing a vtkActor2D/vtkImageMapper
> based and a vtkImageActor based implementation of something that shows a
> series of images one by one with a specific window/level.
>
> If the window/level is not the issue, how much of a difference in
> performance drop for the vtkImageActor implementation would be expected?
>
> I have 700 images (512 x 512, 12 bits stored), rendered into a 512 x 512
> viewport). vtkActor2D/vtkImageMapper takes about 8 seconds, vtkImageActor
> about 26 seconds to render the whole series. If I additionally resample my
> data to add a scale to best fit to the vtkActor2D/vtkImageMapper
> implementation I get to 18 seconds (that was my starting point to look for
> accelarated scaling). There is no visible difference when I add scaling to
> the vtkImageActor. That means that I'm getting much faster scaling for the
> vtkImageActor based implementation, but that overall it still performs
> slower. Unless there is some other bottleneck in my implementation.
>
> Mark
>
>
>
> 2010/11/30 David Gobbi <david.gobbi at gmail.com>
>
> On Tue, Nov 30, 2010 at 7:10 AM, endlosschleife1 <
>> endlosschleife1 at googlemail.com> wrote:
>>
>>  I was actually looking for accelerated scaling, so this sounds like
>>> vtkActor2D is not be the right thing for that.
>>>
>>
>> For accelerated scaling, you need to use vtkImageActor.
>>
>> I'm not sure, but it looks like my implementation probably doesn't use any
>>> GPU acceleration (it seems to be pure image data filtering). It still seemed
>>> to be not as performant as I expected (compared to an implementation where I
>>> only used VTK for the imaging pipeline, but did the LUT application
>>> and visualization separetely).
>>>
>>
>> The vtkImageMapper doesn't have accelerated window/level either.  It does
>> all the computations on the CPU and then uses glDrawPixels() to render the
>> image.
>>
>>
>>>  I create a vtkWindowLevelTable, set the window and center, and then
>>> call build on the lut. Then I used it with a vtkImageMapToColors instance:
>>>
>>> vtkImageMapToColors *color = vtkImageMapToColors::New();
>>> color->SetLookupTable(lut); // lut is the vtkWindowLevelTable
>>> color->SetInput(...) // some vtkImageData
>>>
>>
>> Yes, that looks typical.  Make sure that you call table->SetRampToLinear()
>> because by default, a VTK lookup table uses a curved ramp instead of a
>> linear ramp (I was shocked when I discovered this for the first time).
>>
>>   David
>>
>>
>
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