*2.3. "Washed out" IMDC Textural Identification*

In the majority of cases, the IMDC identification methods described above work quite well. However, in certain cokes, some IMDC areas are quite dark in appearance, even darker than epoxy, and cannot be thresholded as a part of the coke matrix. In these cases an extra identification method is needed (Figure 5). Several explanations can be offered for such appearance of IMDC. One possibility is that such IMDC may have very fine porosity which is not impregnated by epoxy during block preparation. Alternatively, some of these areas may have a very weak structure and so are destroyed, plucked out and/or "washed out" (the general term used to call these areas) during the polishing of the block surface. The rest of the coke matrix is brighter than epoxy, so these dark areas are not included when coke matrix is segmented.

**Figure 5.** Photomicrograph of coke with the significant presence of dark/"washed out" IMDC.

For segmentation of "washed out" IMDC, thresholding of areas with reflectivity higher than that of porosity but lower than epoxy is performed (Figure 6a,b). Next, moderate dilation is performed (Figure 6c) to preserve the washed out IMDC areas during following strong erosion (Figure 6d). After dilation, compensating for that strong erosion, filling holes and scrapping of undersized objects (Figure 6e), the areas of washed out IMDC are determined. Masking those areas with the combination of the map in Figure 6a and the coke matrix gives the actual map of washed out IMDC (see Figures 2b and 6f).

There exists a possibility that the described method may also identify dark minerals with reflectivity between that of porosity and epoxy. If such minerals are known to be sufficiently present in the sample as relatively large grains, they should be identified texturally prior to IMDC identification in the workflow. If that cannot be achieved, a decision should be made about whether the "washed out" identification method should be included in the whole IMDC identification procedure. For the coke shown in Figure 5, it was in fact critically important to include such identification due to the significant presence of "washed out" areas.

**Figure 6.** Segmentation of "washed out" IMDC: (**a**) thresholding of areas darker than epoxy but lighter than porosity; (**b**) reflectivity histogram with thresholds corresponding to (**a**); (**c**) dilation of binary map obtained from thresholding; (**d**) strong erosion leaving denser IMCD-like areas; (**e**) areas of washed out IMDC; (**f**) final map of washed out IMDC.
