Raw Image Analyzer User's Guide

Table of Contents

Exposure display

The exposure display mode is another way to analyze the raw image: selected pixels are displayed with red, green or blue, according to the pixel's position in the color filter array block; however, the displayed intensity is fixed, that does not represent the raw pixels' intensity (i.e. the raw pixel value). In other words: the selection does depend on the raw pixel values, but the pixels appearance does not.

Such a display resembles a "proper" image less than the image display modes, but there is no problem with recognizing a pixel of low value.

The way of pixel selection is the same as that in image display mode, except

An entire color channel (even one of the green channels separated from the other) can be excluded from the display by setting the corresponding white balance coefficient to zero; see the detailed description of the white balance feature in the Image Display section.

The exposure display offers an additional choice: if the BL corrected checkbox is checked, then the black level corrected pixel values are compared against the selection range limits (this is independent on the black point setting); otherwise the original raw pixel values are used in the selection.

The exposure analysis is done best in maximized window: here the small images are presented.

Example 1:

Some cameras offer ISO settings at 1/3 stops as well, but are those "real ISOs"? Let's take an image, shot with ISO 125 and look for a pixel value range, which contains many pixels (the histogram helps finding such ranges). Type in a pixel value in the black point group (don't forget to Enter it) and enter the same value in the white point group as well; after pressing Enter, the control remains in that field. The level 1901 has been selected in both groups. Only red pixels will be displayed at the moment; the color needs to be selected only in the black point group, as that governs the values up to 2047.

All the red pixels we can see now have the value 1901; the statistics shows, that there are 666 such pixels in the entire image, and 1470 greens (and no blues, this is suspicios)

As the control is still in the level input field (no matter if in the black point or white point group), pressing the right cursor increases both limits by one. Now we see the red pixels with value 1902; there are 762 such in the image

Then we do the same with 1903 and 1904

This shows the red pixels with the value 1904; there are 712 such red pixels in the image

And now the surprize with 1905:

There are no red pixels with the value 1905 in the entire image

If we keep on stepping through the values, we find that regularly every fourth/fifths value (alternating) is a "gap" in either the red or blue pixels. This indicates, that the ISO 125 values have been created by multiplying the ISO 100 values. There is no visible gap in the green pixel values, because the gaps are alternating between the two green positions of the color filter array block.

Example 2: the saturation level of a certain camera.

The histogram shows a strange behaviour of the green pixels when clipping: the clipping does not occur at once, it occupies a range of pixel values

Let's find out, how the clipping occurs with this camera. Outside range is checked, only the green pixels in the higher range are shown.

There are no pixels with 4026 and above

but there are some with 4025, so that is that ultimate clipping point here

by lowering the upper limit, the outline of a block appears

one level lower the block starts to fill out

only half of the green pixels appear in the upper part, and a quarter of them in the lower part

until the entire block is filled, but only with one of the two green pixels in each color filter array block

then from 4009 the other green pixels are appearing

and at 4001 the block is almost completely filled

The analysis of the entire image reveals, that lower limit of clipping is 3995. The difference between values in the range from 3995 to 4025 is meaningless, i.e. these pixel values do not represent true proportions of the lightness of the captured scenery. The lower limit has to be seen as saturation level.

The importance of knowing the saturation level lies in the clipping indication in image display mode.

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Modified: 2008-04-29