IMAGE INTERFERENCE IN DIGITAL SYSTEMS

Although image interference can be performed with traditional photographic films, digital techniques speed up the procedure considerably and allow for controlled experimentation and exact repeatability. The options available in the blending modes of Adobe Photoshop, although they do not allow total control of the settings, are eminently useful for this purpose.

The blending modes can be used with different drawing tools, but for the present purpose they only need to be used from the “Layers” palette or from the “Calculations” option on the “Edit” menu. The options that are most useful are “Difference” and “Subtract.” They work in similar ways, by subtracting the pixels value of one layer from that of the other. See “Basic Concepts” for an explanation of some terminology of digital images as used in Adobe Photoshop.

The blending modes in Adobe Photoshop act by calculating a new value for each pixel based on the values of the corresponding pixels in the two original images (which must have a one-to-one pixel correspondence). “Difference” and “Subtract” both calculate the numerical difference between the two values, but “Difference” returns negative numbers as positive, whereas “Subtract” returns them as 0.

The results of the two blending modes is shown diagrammatically in Figure 10, which represents a corresponding row of pixels in two versions (A–B) of a simplified grey-scale image. The blue diagrams (C–J) show three applications of “Subtract” and one of “Difference,” with levels not adjusted (left column) and adjusted (right column). “Subtract” renders negative resulting values (light blue in C) as 0, thus losing part of the information, but this may be countered by using an “Offset” setting that will bring all values above 0 (E, G). Switching the order of subtraction (from A – B to B – A) creates images that are each other’s negatives (E–F vs. G–H), provided that no subtraction values are negative.

Because “Difference” makes no difference between positive and negative results (I–J), it does not matter which image is subtracted from which; this method may therefore create artifacts, such as pits in the surface being rendered light (cf. F and J). “Difference” is, however, more flexible to use in Adobe Photoshop than “Subtract”, because it can be simultaneously applied to multiple channels (e.g., colour pictures).  Further, because it can be applied from the “Layers” palette, it allows one to view continuously the effects of adjustments of highlights, shadows, and midtones of the two original images. One may therefore use the slide controls in the levels adjustment to obtain maximum contrast visually.

Figure 11 shows the effect of the two blending modes on a chart having different shades of grey.