[BGRA] What's wrong with Resample Filters Lanczos2 and 4 ?

[jipété]

Hello,

I compare the rendering of the resampling filters and to do this, I use an image that is modified by the 11 classic filters (Box, Linear, etc.) and I noticed that when converting with The Gimp, always in the same way, the .png generated by the graphics tool to bmp's for the comparison tool, 2 files have an alpha channel (I don't know where it comes from) and the 9 other files don't have one :
9+2_layers.png

Here is what I use to convert the base image:     

Code: Pascal  [Select][+][-]

1.       img.Width  := Wd;
2.       img.Height := Hd;
3.       image.ResampleFilter := SelectedFilter; // image is a TBGRABitmap;
4. // SelectedFilter is coming from another procedure,
5. procedure TForm1.radiogroupResFiltersClick(Sender: TObject);// radiogroup Items host the 11 filters
6. begin
7.   SelectedFilter := TResamplefilter(radiogroupResFilters.ItemIndex);
8.   imgPaint(nil);
9. end;
10. //
11.       stretched := image.Resample(Wd, Hd) as TBGRABitmap;

That's all.
 
Of course, to watch the result, I then use

Code: Pascal  [Select][+][-]

1.       copy := image.MakeBitmapCopy(clNone);
2.       img.Picture.Assign(copy); // and img is a TImage
3.       copy.Free;

This gives me a different rendering depending on the presence or absence of the alpha channel, and depending on the display tool used, one version or the other:
4resample.gif
Lanczos3--Lanczos4(or2).png

Is it a bug or a feature ?

Thanks,

[circular]

Hi! I am not sure about the steps to reproduce the problem. Would you have a sample program?

[jipété]

Would you have a sample program?

Sorry but this afternoon I've changed the way sizes are set-up and problem has disappeared.

I apologize for the inconvenience and you can delete that discussion, if you want to free disk space.
Regards,

[circular]

I am glad the problem is solved. This confirms the appearance of the images that looks like a problem of pixel format.

Have a nice day