Just one of several video files is shared by both projects. In project one no file is compressed. In project two most files are to be compressed (I haven't burnt this project yet). The rendered settings are identical for all video stream files in both projects (Sony AVC, Blu-ray 1920x1080-50i, 16Mbps; the settings are compliant with the source footage). The only "structural" difference is that some media of project two includes chapters and subtitles, whereas none of project one does. Could that be the reason?

What is the total size of your troublesome project? Are you sure that it will fit on a 25 GB disc? (Look at the total bytes of your source files using right/click Properties on the folder, and allow for menus.)

Your answer is contained herein:
Avioding Recompression in DVD Architect

The SCS Knowledgebase is one of the best-kept secrets on the internet. If more people would look there first, redundant questions on these forums would be cut in half.

@musicvid I agree that SCS are sort of shy about their knowledegebase. But I think it's because they don't care to update it. Anyhow it's true about the link you pointed to, which does not say a word about Blu-ray making.
@PeterDuke my project one (the one without compression) was estimated as 18.2GB, whereas project two that requires compression was 22.5 GB. However, even after I've followed the advice from Rob Aubey's article (http://www.sonycreativesoftware.com/dostudio/authoring_your_first_bly-ray_disc) and kept deleting one media file after another, going down to some 12 GB, the video streams that remained in project two were said to be compressed.

I've never had this kind of problems while making DVDs.In this sense, contrary to some suggestions included in Gary Rebholz' webinar (http://www.sonycreativesoftware.com/vegaspro10webinar02), Rob Aubey is right when he shows how different is the Blu-ray processing from DVD making, and how how much calculating it requires. But even in his article I couldn't find a straightforward answer to my problem. Short of two other pieces of advice: "Add, remove or edit a chapter for any title." "Add, remove or change the subtitle file for a title."
Anyway, thanks to both of you, and I will be grateful for whatever more help I could get.

"which does not say a word about Blu-ray making."

Actually, it does; the last six rows in the chart give BluRay compliant properties. They are the same whether the source is MPEG or AVC. Bitrates, as always, must be calculated externally, and the correct profiles are created by Vegas' DVD Architect AVC templates.

Ninan,
(I'm sure you did but) did you in Make BD/ Prepare/ Select Image/ Review Message List go to Optimize... > Optimize Disc dialog? It should give you 'Reasons for required recompression'. 'Context-sensitive' help to be found under '?' as well?

As a quick test, I prepared a short BluRay with no subtitles, imported subtitles, and text subtitles in DVDA. None of them recompressed the video, so subtitles alone are not the problem. Likewise adding a chapter in Architect does not cause the video to render.

I would check DVDA project properties, and use a bitrate calculator to be sure you are figuring the size on disc correctly (Architect does not).

I guess we assumed you are using DVD Architect Pro (you didn't say)?

@musicvid @vkmast thanks a lot. I had not studied the optimize dialog thoroughly enough. The reasons seem to lie in the bit rates that must be lowered to comply with the file properties vis-a-vis the disc space. It seems I have also discovered why the file I share with the other project must now be compressed too. This time I rendered it in VP 12 together with the project markers. Actually the only file that DVDA does not want ro recompress is the one to which I added chapter markers in DVDA. Would it mean that the latter is a safer (more predictable) way? Yes, @musicvid, for some time now I have been using the Pro software. DVDA 5.2 suits me better than DVDA 6. As for the linked table, you are right, sorry I have not identified the six set-ups as the Blu-ray templates. But if I read them correctly, they all refer to the progressive settings. Mine are interlaced. Which leads me to another question: while working on Blu ray discs, is it better to stick to the source footage specs, or rather to force the progressive set-up?

"while working on Blu ray discs, is it better to stick to the source footage specs, or rather to force the progressive set-up?"

That's a good question, and the consensus is that it is best to honor the source interlace properties in the output. One reason is that Vegas deinterlace methods are pretty unsophisticated by today's standards.

Adding the project markers in Vegas or DVDA should make no difference, as per tests I ran this morning. You must make sure to render markers as I-frames though, otherwise the need to re-encode is quite obvious.

Thanks @musicvid. However, unlike the Blu-ray MPEG-2 whose Customize Template allows to check out Insert I-frames with markers, there is no such an option for the Blu-ray AVC template. Does it mean that it is done automatically?

"However, unlike the Blu-ray MPEG-2 whose Customize Template allows to check out Insert I-frames with markers, there is no such an option for the Blu-ray AVC template. Does it mean that it is done automatically?"

Having recently done some testing on markers for another thread, it seems to me that the markers for Blu-ray do not have to be on an I-frame. Maybe Blu-rays are smarter than DVDs.

BluRays also use B-frames as reference frames, so maybe that is what it is doing. Never noticed the absence of that dialog.

Now that I think about it, AVC is in a transport stream container in a BD and MPEG2 is in a program stream container in a DVD. That may be why the AVC can start playing at any frame.

"Video picture type compression" of Wikipedia reads: "In the latest international standard, known as H.264/MPEG-4 AVC, the granularity of the establishment of prediction types is brought down to a lower level called the slice level of the representation. A slice is a spatially distinct region of a frame that is encoded separately from any other region in the same frame. In that standard, instead of I-frames, P-frames, and B-frames, there are I-slices, P-slices, and B-slices." So, let's assume the "slices" are taken care of by default (?).