Feature optional JPEG alpha channel

[hhelwich]

I have a use case where big PDF files should be generated with many big JPEG files. The JPEG files must not be changed so that the Exif and other meta data stays intact. I found out that this is possible with PDFKit.

I have the additional requirement that the JPEG files can have an alpha channel. I found that PDFKit supports that for PNG files but not for JPEG files. I also found out that it's not difficult to add this feature to PDFKit. I added an extra option alpha which can be used to add a grayscale JPEG mask. When the parameter is not used the current behavior is retained. Example usage:

const doc = new PDFDocument();
doc.image('foo.jpg', 0, 15, { width: 300, alpha: 'foo_mask.jpg' });

I would be glad if you think this feature is of general interest and accept it. If you don't find this feature helpful feel free to decline the pull request. Nevertheless i would be glad to get some feedback. E.g. i increased the PDF version to 1.4 because transparency was not possible in PDF 1.3 but i am not sure if this is without side effects.

[devongovett]

Interesting. Why is this specific to JPEG? Can we have a mask option for all image types?

[hhelwich]

I did it specific for JPEGs because i only needed it there. But i think it's a good idea to have the mask for all image types.
PNGs can have alpha transparency included. What should we do if an additional alpha transparency mask is specified?

[hhelwich]

I added the alpha option also for PNG images. If the alpha parameter is not used, the original behavior is retained. If the PNG image has an alpha channel and also the alpha option is used, the alpha channel given by the alpha option is used instead of the alpha channel included in the PNG image.