[Feature] Add support for images

[andrzejchm]

This PR adds support for reading and writing images in Excel (.xlsx) files. The implementation:

Key Features:

• Support for reading images from existing Excel files
• Support for adding new images to Excel files

Technical Implementation:

• Adds ImageCellValue for representing images in cells
• Creates dedicated _ImageCellParser for reading images from Excel files
• Creates dedicated _ImageCellCreator for writing images to Excel files
• Handles Excel's internal image storage format and relationships
• Properly manages Excel's drawing relationships and media files
• Supports EMU (English Metric Unit) conversion for accurate image sizing

Example usage:

// Reading an image from a cell                                                                                                                                                                                                                     
final imageCellValue = excel.sheets['Sheet1']!.cell(CellIndex.indexByString("A1")).value as ImageCellValue;                                                                                                                                          
final imageBytes = imageCellValue.bytes;                                                                                                                                                                                                            
final imageFormat = imageCellValue.format; // "png", "jpg", etc.                                                                                                                                                                                    
                                                                                                                                                                                                                                                    
// Writing an image to a cell                                                                                                                                                                                                                       
final imageBytes = File('image.png').readAsBytesSync();                                                                                                                                                                                             
final imageCellValue = ImageCellValue(bytes: imageBytes, format: 'png');                                                                                                                                                                            
excel.sheets['Sheet1'].cell(CellIndex.indexByString("A1")).value = imageCellValue;     

I've augmented excel_example.dart and excel_test.dart files with image support showcase/testing.

[falerr]

Really good concept and very necessary, but tried it with diferent images and their cell appears empty.
Tried using it inside a List<CellValue?> to use on a sheet.insertRowIterables(cellvalues, rowIndex);

Edit: Tried also like in example and nothing, newSheet.cell(CellIndex.indexByColumnRow(columnIndex: colIndex, rowIndex: rowIndex)).value.toString() looks like: [Image: jpg autox50]

Really hoping a fix to this

[Abbasi233]

this feature works very well

[falerr]

this feature works very well

How did you manage to make it work?

File gets bigger with the ImageCellValues but doesnt show any images even when editing the file

[Abbasi233]

this feature works very well

How did you manage to make it work?

File gets bigger with the ImageCellValues but doesnt show any images even when editing the file

I did not do anything special. I just followed the example codes.

[andrzejchm]

@falerr could you tell me which software you use for opening the excel file? I can see there are major discrepancies between Apple's Numbers and MS Excel from office 365 on Mac. Also, the solution proposed here is more of a proof-of-concept and is not suitable for HUGE number of images, as I didn't have time to introduce image reuse etc. I can post a more sophisticated approach I've built after testing it in my own app, probably as part of this PR

[falerr]

@falerr could you tell me which software you use for opening the excel file? I can see there are major discrepancies between Apple's Numbers and MS Excel from office 365 on Mac. Also, the solution proposed here is more of a proof-of-concept and is not suitable for HUGE number of images, as I didn't have time to introduce image reuse etc. I can post a more sophisticated approach I've built after testing it in my own app, probably as part of this PR

I've been using Microsoft 365 on android and LibreOffice on PC, both of them didn't show the pictures. Also thanks for your help and work

[Abbasi233]

There is a very strange problem. If I add multiple pictures to the file, it probably adds the biggest picture (this is my guess) to all the cells I have added. For example, a small car picture must be added to the cells I marked here, but the following map image shrinks.
I changed the lines of the codes in many different combinations. But the result was always the same. When I commented out the part I only added the map image, the car image seemed correctly.

[falerr]

Any news? Looking forward to them!