Indexed png transparency using /Mask

Thanks, but i'm not sure i understand. Do you have an example of what you mean?

I've followed through the logic of 3 seperate pdf libraries and all just strip the palette values and transparency values from a PNG's PLTE chunk and it's tRNS chunk and then apply them to the output to generate the pdf.

They all have the same logic to output the trns properties as a /Mask property on the image object

var trns = '',

for (var i=0; i < img['trns'].length; i++)

     trns += (img['trns'][i] + ' ' + img['trns'][i] + ' ');

out('/Mask [' + trns + ']');

Which converts the input of,

[255, 255, 192, 192, 160, 160, 128, 128, 96, 96, 64, 64, 32, 32, 0, 0]

to the output of

/Mask [255 255 255 255 192 192 192 192 160 160 160 160 128 128 128 128 96 96 96 96 64 64 64 64 32 32 32 32 0 0 0 0 ]

I appreciate your help, but if you don't understand PNG's, then you're probably not the best person to answer this question. It would take a while for me to fully explain the PNG spec to you.

Great, thanks a lot. I'd started to make inroads with this yesterday and slowly came to the same conclusion you've helpfully now confirmed!

I noticed yesterday that the majority of indexed PNG's i've come across usually only have an array of transparency values  consisting of a single 0 with the rest being 255. From looking at an existing  PDF generator, i realised that when that's the case, you can just pass a single value  (instead of the complete array) to add the /Mask to an image. With that value being the index of the position of the 0 within the PNG's transparency array. This works fine for the majority of indexed pngs that i've tested, as most only elect a single (pixel) colour within their palette to represent a fully transparent pixel and the rest are fully opaque.

So if the PNG's tRN's values are as follows,

[255, 255, 255, 255, 255, 255, 0]

then the /Mask would be set as

/Mask [6, 6]

After getting this to work, i'd started to think about how this might work with multiple values with varying transparency values. But then realised that it would probably just be easier to unroll it and create an image and sMask.

Out of interest, do you know where i might look to find an example or explanation of how to apply the non-trivial solution? Is there anything within the PDF reference?