Originally Posted by hwjohn
Can you explain how an upconverting player "twists" the space? I was under the impression that Rec 709 simply moves the primaries in relation to Rec 601.
It ought to be a relatively straight-forward matrix multiplication (3x3 matrix). If they do anything differently, then they either know way more, or way less, than I do. Somewhere in Charles Poynton's website, he shows what that matrix is (I could calculate it quickly, but formatting the table is more effort than it is worth).
Lets say the upconverting DVD player encounters a frame with pure red Rec 601 primary. Shouldn't it "remap" that color to the red primary as defined in Rec 709?
That is essentially what happens if it doesn't do the "twist". What you want it to do is to desaturate the "pure" (HD) red so that the color red that is produced is the correct shade of red (SD) that was intended in the encode.
In other words, the Rec709 red primary is located at xy( 0.64, 0.33). For SMPTE-C (effectively Rec 601), red is located at xy( 0.63, 0.34). This means that the HD red is more saturated than the SD red in both x and y. With SD content, though, you want that "pure" SD red, and all shades of red in-between, to be based upon the SD red primary, not the HD primary. If you don't, then ALL reds would be somewhat incorrect not just a pure red.
If that is the case, then wouldn't alluringreality's propsed check work correctly (assuming we are using an upconverting player to calibrate for HD material)?
Please correct me if possible (I may be too far off for correction as a result of me being an idiot
Not an idiot. This stuff is just a little complex and almost completely undocumented in the enthusiast space. The interesting thing is that once enough users have calibration equipment and decent meters, we ought to be able to tell as a community who is doing it right and who is not. Then manufacturers will have to pay more attention so that they don't miss a fairly trivial step in the process.
Hopefully this little bit helps.