# (API) Specify image channels
**URL:** https://discourse.itk.org/t/api-specify-image-channels/2009
**Category:** Engineering
**Created:** [July 3, 2019, 10:54am UTC](https://discourse.itk.org/t/api-specify-image-channels/2009 "2019-07-03T10:54:32Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@nm97](https://discourse.itk.org/u/nm97)
#### Post date: [July 3, 2019, 10:54am UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/1 "2019-07-03T10:54:32Z")
How can I specify image channels.
I have created image object.
```auto
using ImageType = itk::Image< uint8_t, 2 >;
ImageType::Pointer image = ImageType::New();
ImageType::SizeType size;
size[0] = width;
size[1] = height;
ImageType::IndexType start;
start.Fill( 0 );
ImageType::RegionType region;
region.SetSize( size );
region.SetIndex( start );
image->SetRegions( region );
image->Allocate();
```
I can specify size, start but could not find in documentation how to specify image channels.
---
### Author:  [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [July 3, 2019, 2:30pm UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/2 "2019-07-03T14:30:40Z")
You just have to define vector image type.
```auto
using VectorPixelType = itk::Vector; // 10 channel pixel
using ImageType = itk::Image< VectorPixelType, 2 >; // 2D image
```
Read more about it in the [software guide](https://itk.org/ITKSoftwareGuide/html/Book1/ITKSoftwareGuide-Book1ch4.html#x45-500004.1), section 4.1.6.
---
### Author:  [@nm97](https://discourse.itk.org/u/nm97)
#### Post date: [July 3, 2019, 2:57pm UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/3 "2019-07-03T14:57:27Z")
Thank you very much for quick response.
In that case I get an error in this line
```auto
rescaler->SetOutputMaximum( 255 );
rescaler->SetOutputMinimum( 0 );
```
```auto
error: conversion from ‘int’ to ‘itk::RescaleIntensityImageFilter, 2>, itk::Image, 2> >::OutputPixelType’ {aka ‘itk::Vector’} is ambiguous
180 | rescaler->SetOutputMinimum( 0 );
```
---
### Author:  [@tim-evain](https://discourse.itk.org/u/tim-evain)
#### Post date: [July 3, 2019, 3:09pm UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/4 "2019-07-03T15:09:42Z")
These methods expect arguments of same type as the pixels in your image. Since the pixel in your image is a vector (aka multi-channel image), you should input a vector.
Giving only one value is ambiguous, i.e. in which channel should the value goes ? first? all?
If it’s the latter, you could do something like:
```auto
itk::Vector MaxPixel, MinPixel;
MaxPixel[0] = MaxPixel[1] = MaxPixel[2] = 255;
MinPixel[0] = MinPixel[1] = MinPixel[2] = 0;
rescaler->SetOutputMaximum( MaxPixel );
rescaler->SetOutputMinimum( MinPixel );
```
HTH,
Tim
---
### Author:  [@nm97](https://discourse.itk.org/u/nm97)
#### Post date: [July 3, 2019, 4:50pm UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/5 "2019-07-03T16:50:27Z")
In that case I get an error in the last line
```auto
using VectorPixelType = itk::Vector; // 10 channel pixel
using ImageType = itk::Image< VectorPixelType, 2 >; // 2D image
using OutputVectorPixelType = itk::Vector;
// using OutputPixelType = float;
using OutputImageType = itk::Image< OutputVectorPixelType, 2 >;
using FilterType = itk::MinMaxCurvatureFlowImageFilter< ImageType, OutputImageType >;
FilterType::Pointer filter = FilterType::New();
```
```auto
invalid static_cast from type ‘itk::Vector’ to type ‘float’
185 | auto b = static_cast< T2 >( a );
```
---
### Author:  [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [July 3, 2019, 5:01pm UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/6 "2019-07-03T17:01:50Z")
I guess that `MinMaxCurvatureFlowImageFilter` does not support vector images. You might use [SplitComponentsImageFilter](https://itk.org/Doxygen/html/classitk_1_1SplitComponentsImageFilter.html) or [VectorImageToImageAdaptor](https://itk.org/Doxygen/html/classitk_1_1VectorImageToImageAdaptor.html) to split the image into constituent channels, apply some processing on each channel independently, and combine them using [ComposeImageFilter](https://itk.org/Doxygen/html/classitk_1_1ComposeImageFilter.html).
---
### Author:  [@blowekamp](https://discourse.itk.org/u/blowekamp)
#### Post date: [July 3, 2019, 5:28pm UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/7 "2019-07-03T17:28:51Z")
Here is a snippet of code generated in SimpleITK which run certain filters on a “per component basis”:
```auto
//
// Dispatched methods to call ExecuteInternal on each component of the VectorImage
//
template Image BilateralImageFilter::ExecuteInternalVectorImage ( const Image& inImage1 )
{
typedef TImageType VectorInputImageType;
typedef typename VectorInputImageType::InternalPixelType ComponentType;
typedef typename itk::Image ComponentImageType;
// we must define the input and output image types should be the
// same as the scalar execute internal method
typedef ComponentImageType InputImageType;
// Define output image type
typedef InputImageType OutputImageType;
// Get the pointer to the ITK image contained in image1
typename VectorInputImageType::ConstPointer image1 =
this->CastImageToITK( inImage1 );
typedef itk::VectorIndexSelectionCastImageFilter< VectorInputImageType, ComponentImageType > ComponentExtratorType;
typename ComponentExtratorType::Pointer extractor = ComponentExtratorType::New();
extractor->SetInput( image1 );
typedef itk::ComposeImageFilter ToVectorFilterType;
typename ToVectorFilterType::Pointer toVector = ToVectorFilterType::New();
unsigned int numComps = image1->GetNumberOfComponentsPerPixel();
for ( unsigned int i = 0; i < numComps; ++i )
{
extractor->SetIndex( i );
extractor->Update();
Image tmp = this->ExecuteInternal( Image( extractor->GetOutput() ) );
typename OutputImageType::ConstPointer tempITKImage = this->CastImageToITK( tmp );
toVector->SetInput( i, tempITKImage );
}
toVector->Update();
return Image( toVector->GetOutput() );
}
```
---
### Author:  [@nm97](https://discourse.itk.org/u/nm97)
#### Post date: [July 3, 2019, 5:44pm UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/8 "2019-07-03T17:44:30Z")
> [@dzenanz](#):
>
> ependently, and combine them
Ok, but in InsightToolkit-5.0.0/Examples/Filtering/MinMaxCurvatureFlowImageFilter.cxx in this example input image has 3 channels and still it does not specify the number of channels.
But still it filters image and writes in the file as a multichanel image
---
### Author:  [@nm97](https://discourse.itk.org/u/nm97)
#### Post date: [July 3, 2019, 6:02pm UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/9 "2019-07-03T18:02:22Z")
And one more question,
If `MaxCurvatureFlowImageFilter` does not support vector images how can I filter multi channel images without using vector? Because as I said in example there is no use of vector, but it filters fine
---
### Author:  [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [July 3, 2019, 6:34pm UTC](https://discourse.itk.org/t/api-specify-image-channels/2009/10 "2019-07-03T18:34:44Z")
Instead of `itk::Vector`, you might want to try `itk::RGBPixel`.
In the example you mentioned, the pixel type is scalar = grayscale (float). If color image is supplied, the IO mechanism in ITK converts it to grayscale and the example deals with that afterwards.