My image pdfstern.png, created from weihnachtsstern.pdf by calling gswin32c -sDEVICE=pngalpha, has an alpha channel with all values equal 255, i. e. no transparency. After converting weihnachtsstern.pdf to weihnachtsstern.eps (by pdftops), I created epsstern.png, where the outside of the star has alpha values of zero. The version 8.62 has the same behavior.
Created attachment 4641 [details] PDF source
Created attachment 4642 [details] created from weihnachtsstern.pdf
Created attachment 4643 [details] source created from weihnachtsstern.pdf
Created attachment 4644 [details] created from weihnachtsstern.eps
This does not seem to be a PDF writer problem. I don't see any use of the pdfwrite device here at all.
sorry, component is PDF Interpreter, not PDF Writer. The problem persists in 8.71.
Not a problem, but because it was assigned to me I'm afraid it has not had the attention it should have. I will reassign it to the proper owner.
The PDF file uses PDF transparency, which is implemented by rendering a part of the image into a buffer and blending it with the background. The low level raster device such as pngalpha receives the buffer that contains no information about marked and unmarked pixels. The EPS file has no transparency and pngalpha device can detect marked and unmarked pixels. You can switch transparency support off with -dNOTRANSPARENCY option and get file similar to the file generated from EPS. Unfortunately, tracking unmarked pixels in transparency subsystem is a big project that benefits few devices and we don't have resources to address it now.