Doug Kerr
Well-known member
The "linear DNG" format (in one situation) is essentially a transformation of a raw file (from a CFA camera) in which the raw data has been demosaiced but remains in the native camera color space (rather than, for example, now being in sRGB or L*a*b*).
This fact is often obscured by the common description of this format. It is often said that the image in this format "is in RGB form" but in a way that allows us to have the same opportunity to apply color balance, correct for inopportune exposure, and so forth that we have when working with raw data.
But the letters RGB there are misleading.
It follows from the fact that the outputs of the typical Bayer CFA camera sensor are, by custom, usually designated R, G, and B. But they are almost nothing like the R, G, and B coordinates of the RGB color model, or of any specific RGB color space.
For one thing they are linear, rather than "gamma-precompensated" (as are the RGB coordinates of any usual RGB-class color space). For another, although they are indeed the coordinates of the camera's native color space (a notion denied by some), they do not relate to any visible "red-ish", "green-ish", and "blue-ish" primaries (which might justify the labels "R", "G", and "B").
Actually, the camera native color space has "imaginary"primaries, which not only do not correspond to any visible radiation, they do not correspond to any radiation at all - they are fictional constructs, having no physical significance, which however can be mathematically manipulated just like "visible" primaries. (If that seems startling, recall that the very same is true of the primaries of the CIE XYZ color space - they also have no physical significance!)
So to avoid the cultivation of any misunderstandings, I refer to the outputs of the three classes of sensels in a Bayer CFA camera as α, β, and γ (alpha, beta, and gamma). They are the coordinates of the color space of the raw data.
In a "linear DNG" file, the image is represented by actual pixels, not sensel outputs at pixel locations (as in a raw file), but they are described in the α-β-γ color space (the camera native color space).
Now, what is the significance of this form being called "linear DNG"? Beats me.
Of course, the color space there has linear coordinates, but so does the color space in which regular raw data is recorded (including in a regular DNG file).
For a moment, it might seem that "demosaiced DNG" would have been a better name.
But in some cases (such as a "linear DNG" file from a non-CFA camera, such as one with a "Foveon" sensor), the data was never "mosaiced" to begin with (there were initially three sensor outputs, in the camera's native color space, for each pixel).
So perhaps "pixel DNG" would be an even better name.
Of course, "DNG" (based on "digital negative") is itself misleading. It is based on the metaphor that the raw data is pretty comparable to a film negative.
But in fact, considering what we can do with it, the raw file is most comparable to the exposed, but undeveloped, film. (That's why processing raw data into an image file is often said to be "developing" it, do you think?)
So I'm not attracted to carrying the "DNG" designation into new areas, expanding the area of misunderstanding.
In any case, what we have in a "linear DNG" file is "pixel data in the camera's native color space". I won't pretend to suggest a catchy short name for that.
Best regards,
Doug
This fact is often obscured by the common description of this format. It is often said that the image in this format "is in RGB form" but in a way that allows us to have the same opportunity to apply color balance, correct for inopportune exposure, and so forth that we have when working with raw data.
But the letters RGB there are misleading.
It follows from the fact that the outputs of the typical Bayer CFA camera sensor are, by custom, usually designated R, G, and B. But they are almost nothing like the R, G, and B coordinates of the RGB color model, or of any specific RGB color space.
For one thing they are linear, rather than "gamma-precompensated" (as are the RGB coordinates of any usual RGB-class color space). For another, although they are indeed the coordinates of the camera's native color space (a notion denied by some), they do not relate to any visible "red-ish", "green-ish", and "blue-ish" primaries (which might justify the labels "R", "G", and "B").
Actually, the camera native color space has "imaginary"primaries, which not only do not correspond to any visible radiation, they do not correspond to any radiation at all - they are fictional constructs, having no physical significance, which however can be mathematically manipulated just like "visible" primaries. (If that seems startling, recall that the very same is true of the primaries of the CIE XYZ color space - they also have no physical significance!)
So to avoid the cultivation of any misunderstandings, I refer to the outputs of the three classes of sensels in a Bayer CFA camera as α, β, and γ (alpha, beta, and gamma). They are the coordinates of the color space of the raw data.
Yes, it's hard for some people to accept that the outputs of the three kinds of CFA sensels are in fact the coordinates of a color space.
In a "linear DNG" file, the image is represented by actual pixels, not sensel outputs at pixel locations (as in a raw file), but they are described in the α-β-γ color space (the camera native color space).
Now, what is the significance of this form being called "linear DNG"? Beats me.
Of course, the color space there has linear coordinates, but so does the color space in which regular raw data is recorded (including in a regular DNG file).
For a moment, it might seem that "demosaiced DNG" would have been a better name.
But in some cases (such as a "linear DNG" file from a non-CFA camera, such as one with a "Foveon" sensor), the data was never "mosaiced" to begin with (there were initially three sensor outputs, in the camera's native color space, for each pixel).
So perhaps "pixel DNG" would be an even better name.
Of course, "DNG" (based on "digital negative") is itself misleading. It is based on the metaphor that the raw data is pretty comparable to a film negative.
But in fact, considering what we can do with it, the raw file is most comparable to the exposed, but undeveloped, film. (That's why processing raw data into an image file is often said to be "developing" it, do you think?)
So I'm not attracted to carrying the "DNG" designation into new areas, expanding the area of misunderstanding.
In any case, what we have in a "linear DNG" file is "pixel data in the camera's native color space". I won't pretend to suggest a catchy short name for that.
Best regards,
Doug