Consistent terminology

[chrisn]

Originally raised by @fintelia as part of #346:

A bunch of sections use different terminology, likely because they were written at different times by different people:

Section 4.2: Talks about a "source image" getting transformed into a "reference image", with the former explicitly left out of scope of the spec and the later consisting of dimensionless RGB or RGBA channels with $2^{sampledepth}$ bits.

cHRM chunk: Talks about display primaries "used" in "the image", without indicating whether that's the source image, reference image, or some other image.

sRGB chunk: Now the samples "conform" to a colour space and have a "rendering intent"

cICP and mDCv chunks: TV terminology as mentioned

Section 12.1 This section leans heavily into talking about samples being "light intensities" in an abstract 0-1 range. The concept of > "original scene intensity" is mentioned but glossed over.

transfer function definition: "image luminance", "image samples"

[ProgramMax]

Assigning to CS as discussed last meeting.

[ProgramMax]

Also adding Simon.

[chrisn]

Another one I noticed: in Table 2, for mDCV there's "eventual display" but elsewhere in the spec the same concept is "display device".

[chrisn]

I created a draft PR #449 to fix some of these, but it's not clear to me how to address them all. @fintelia, do you have suggestions on changes to be made?

[fintelia]

I don't think there's any quick fixes that will make everything consistent and to be honest I'm not sure it is worth trying to align everything. If you wanted to, it would be a matter of picking one set of terminology and then rewriting each section to use that same terminology. So the gAMA chunk would say that it is a transfer function, sRGB and cICP would use the same terminology, etc.

[svgeesus]

I agree this is actually quite a lot of work.

It might be possible to do this incrementally.

[svgeesus]

There have already been some useful improvements resulting from this issue. Doing all of this correctly would substantially rewrite large sections of the spec.

In the interests of spec stability for PNG 3rd edition, in my opinion the milestone should be changed to Future edition considerations

[ProgramMax]

Agreed