Tag Archives: sensitivity metamerism index

Phase One IQ3 100MP Trichromatic vs Standard Back Linear Color, Part II

We have seen in the last post that Phase One apparently performed a couple of main tweaks to the Color Filter Array of its Medium Format IQ3 100MP back when it introduced the Trichromatic:  it made the shapes of color filter sensitivities more symmetric by eliminating residual transmittance away from the peaks; and it boosted the peak sensitivity of the red (and possibly blue) filter.  It did this with the objective of obtaining more accurate, less noisy color out of the hardware, requiring less processing and weaker purple fringing to boot.

Both changes carry the compromises discussed in the last article so the purpose of this one and the one that follows is to attempt to measure – within the limits of my tests, procedures and understanding[1] – the effect of the CFA changes from similar raw captures by the IQ3 100MP Standard Back and Trichromatic, courtesy of David Chew.  We will concentrate on color accuracy, leaving purple fringing for another time.

Figure 1. Phase One IQ3 100MP image rendered linearly via a dedicated color matrix from raw data without any additional processing whatsoever: no color corrections, no tone curve, no sharpening, no nothing. Brightness adjusted to just avoid clipping.  Capture by David Chew.

Continue reading Phase One IQ3 100MP Trichromatic vs Standard Back Linear Color, Part II

Linear Color: Applying the Forward Matrix

Now that we know how to create a 3×3 linear matrix to convert white balanced and demosaiced raw data into XYZ_{D50}  connection space – and where to obtain the 3×3 linear matrix to then convert it to a standard output color space like sRGB – we can take a closer look at the matrices and apply them to a real world capture chosen for its wide range of chromaticities.

Figure 1. Image with color converted using the forward linear matrix discussed in the article.

Continue reading Linear Color: Applying the Forward Matrix

Color: Determining a Forward Matrix for Your Camera

We understand from the previous article that rendering color during raw conversion essentially means mapping raw data represented by RGB triplets into a standard color space via a Profile Connection Space in a two step process

    \[ RGB_{raw} \rightarrow  XYZ_{D50} \rightarrow RGB_{standard} \]

The process I will use first white balances and demosaics the raw data, which at that stage we will refer to as RGB_{rwd}, followed by converting it to XYZ_{D50} Profile Connection Space through linear transformation by an unknown ‘Forward Matrix’ (as DNG calls it) of the form

(1)   \begin{equation*} \left[ \begin{array}{c} X_{D50} \\ Y_{D50} \\ Z_{D50} \end{array} \right] = \begin{bmatrix} a_{11} & a_{12} & a_{13} \\ a_{21} & a_{22} & a_{23} \\ a_{31} & a_{32} & a_{33} \end{bmatrix} \times \left[ \begin{array}{c} R_{rwd} \\ G_{rwd} \\ B_{rwd} \end{array} \right] \end{equation*}

Determining the nine a coefficients of this matrix is the main subject of this article[1]. Continue reading Color: Determining a Forward Matrix for Your Camera