[vtkusers] Is there any one know how to generate a JPEG file manually?

[Q Ma]

Hi, there,

Is there any one know how to generate a JPEG file manually?
I have an arrary of data, grey value for each pixel,and want to output into a
JPEG file.
I use vtkJPEGWriter, with following code:

vtkJPEGWriter *imagewriter=vtkJPEGWriter::New();
  imagewriter->SetFileName(imagefilename);
vtkStructuredPoints *im = vtkStructuredPoints::New();
  im->SetDimensions(1024,768,1);
vtkFloatArray *scalartemp = vtkFloatArray::New();	
im->GetPointData()->SetScalars(scalartemp);
for(int x=0;x<1024;x++)
	for(int y=0;y<768;y++)
		scalartemp->InsertValue(y*1024+x,(char)(xy[x][y][2]/5));
imagewriter->SetInput(im);
imagewriter->Write();

It does not work. And an error reported like this:
warning: In /var/tmp/BUILD-cedward1/VTK/IO/vtkJPEGWriter.cxx, line 186
vtkJPEGWriter only supports unsigned char input

Can anyone with some help on how to output JPEG file manually?
Thank you very much.

Regards,
Qi