Igor, Thanks, that works for me. I didn't realize raw actually stayed in the sensor format until conversion. Regards, Bob S.
On 6/16/06, Igor Roshchin <[EMAIL PROTECTED]> wrote: > > > If you were look at the same pixel in a 16bit TIFF file rendering of > > the 12bit RAW data, the relative values of the channel assignments > > would be the same, but you have 16x as many numbers available to > > describe the values in the original RAW file which is then scaled to > > a representation in a discrete numeric space 8x larger (0-4095 in the > > RAW data, 0-32767 in the 16bit RGB channel (the topmost bit is not > > used so it's actually 15 bits of data)). Only 4096 of the values in > > the 16bit space are actual photosite RAW values, they're fitted into > > the larger space because current computing machinery manages 16 bit > > numbers with greater efficiency than 12 bit numbers, in general, > > *and* because as you perform Real or Discrete valued operations on > > these numbers, there are more numbers to represent the results, thus > > greater precision and less likelihood of clipping or round off errors. > > While almost everything has been said on the subject, > I would like to mention one important issue, so that a possible > confusion is avoided. > The text above, while being correct, can cause a confusion. > It might seem that since 16-bit TIFF has more bits to operate, > it is a "reacher" format compared to RAW, so no information is lost > in the conversion RAW->TIFF. > (I've seen it people thinking this way, and was under a similar > impression in the very beginning of understanding RAW vs TIFF > differences). > > One has to remember that information in RAW is stored differently. > The jpeg/tiff you get from DS or after conversion in PS is > 3008x2008. This size corresponds to the 6MP size of the sensor, > i.e. to ~ 3008x2008 sensor sites. > (I leave aside the question of a few "extra" pixels which are not > being used in the conversion in PS, but can be used by some other > software). However, in the JPEG/TIFF, - each pixel has information > about 3 colors (RGB), while in RAW - you have a "mosaic": > out of each 4 sensor sites 2 have information about G(reen), > and 1 - R(ed), and 1 - B(lue): > RGRG > GBGB > RGRG > GBGB > (4x4 part of the sensor ) > > Converter from RAW does what is called demosaicing, by > mixing values from the mosaic to assign values to the pixel in between. > While the description below might not be exact, I think it is presents > a picture that is easy to understand. If you were to put the mosaic > described above on a square-ruled paper (matrix), then we can put > "pixels" P on the nodes (corners of the squares): > > R_G_R_G > _P_P_P_ > G_B_G_B > _P_P_P_ > R_G_R_G > _P_P_P_ > G_B_G_B > > In the demosaicing process, essentiall the value of each pixel "P" > is assigned based on the values of the four surrounding sensor sites. > While in a small image above it looks like you have fewer "P"'s than > R's, G's, and B's altogether, but when you have a large matrix, > the difference in number is negligible (in the scenario described > here it would be ~ 5000 pixels for a ~2000x3000 matrix which is ~0.1%, > but in reality it could be a bit more (no pun intended), depending on > the implementation) > > It is this demosaicing process when the white balance, sharpness, gamma > correction, ISO values, etc are being used in the conversion. > > Since the process of demosaicing is fully mathematically reversible, > the loss of the initial information happens at this point. > That's why you have better control over the image when it is in RAW > format even compared to when it is in 16-bit TIFF. > > I think this rather simple documents explains some of these issues > in more detail: > http://www.adobe.com/digitalimag/pdfs/understanding_digitalrawcapture.pdf > > I hope I didn't cause too much confusion, and my explanation was > helpful to somebody. > > Igor > > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
