I got this issue, too. Anyone can help us?

Hi, thanks for the question. Could you clarify a bit what you do expect here?
Do you want to have 4-channel image with separate alpha channel? Or you want to clear the input based on the alpha value?
Ultimately, to decode PNGs we use OpenCV and by default the alpha channel is discarded, I think. Probably, to implement this, we need a custom code path for PNGs with reading the alpha channel.

Hi.
i want to have 4-channel image with separate alpha channel

@naknakLEE - as @awolant said, we need custom code path to output the alpha channel from the decoder. Moreover, some operators may not work properly with this kind of input/require dedicated code path with the special treatment of this 4th channel - like color based operations (hue, saturation, brightness).
Adding this to our backlog.

Hi @naknakLEE,
In the meantime you can use an external library to load the image with 4 channels and use the external_source operator, please check parallel and prefetch_queue_depth parameters to make this process efficient.

Hi @awolant & @JanuszL,

I was wondering if there was any progress or changes regarding this issue, since it's been a while since this question was asked.

I'm trying to decode 4-channel (with alpha channel) images (.png or .bmp). As a work-around, I extracted the alpha-channels as 8-bit grayscale images to later reassemble them with the corresponding RGB images in my script, but it would be nice to not have to decode the alpha channels separately.

Thanks!

Hello @Renzzauw
There's a new operator, fn.experimental.decoders.image which handles alpha channel (sort of) and also higher bit depths.
When you specify img = fn.experimental.decoders.image(enc, output_type=dali.types.ANY_DATA), you will get the image with alpha. Currently there is no first-class support for alpha, so the channel order may vary - indeed, for PNGs they are returned as BGRA whereas for JPEG 2000 the channel order depends on the backend - CPU decoder returns BGRA and the GPU decoder returns RGBA. Despite those limitations, you should be able to do what you want.