# Characterization of Prints Based on Microscale Image Analysis of Dot Patterns
## 3 Measurement, Modelling and Analysis at the Microscale of Printed Dots
### Separation of ink Colors (Step 1)  
The separation of ink colors is not an easy task as there is often some mis-registration between the printed color channels. Figure 4 shows examples of printed cyan, magenta and yellow dots shifted in position. Figure 2: (**a**) Example of selected samples from the IT8.7/3 Color Chart: sample C10—Cyan with 20% ink coverage; sample D10—Magenta with 20% ink coverage, sample E10—Yellow with 20% ink coverage, (**b**) cropped image of a rotogravure printed cyan sample magnified \(\times\)5. The problem of halftone misregistration was studied in [1]. Figure 4a shows an example of a halftoning scheme with different screen angles for cyan, magenta, yellow and black channels. According to [PERSON] [1], black, the color with the strongest  
Figure 4: (a) Example of halftone misregistration (of offset along X and Y axis) for a CMY image with 20% of ink coverage for each channel. (b) Even if the magenta dots are perceptible in the cyan image, there is enough contrast between the cyan dots and the background to segment cyan dots. (c) Even if the cyan dots are perceptible in the magenta image, there is enough contrast between the magenta dots and the background to segment magenta dots. (d) Magenta dots cannot be dissociated from the yellow dots in the yellow image. (e) Magenta dots segmented in the magenta image are removed from the yellow image in order to properly segment yellow dots. Figure 3: Framework of the proposed image processing pipeline. contrast, is halftoned and placed at 45\({}^{\circ}\), due to the lower sensitivity of eye at this angle; meanwhile, yellow, the weakest color, is halftoned at 0\({}^{\circ}\), where the human eye is most sensitive. According to [PERSON] [1] using a different angle for each channel reduces the effect of misregistration but, on the other hand, introduces a type of pattern, named a rosette pattern, which is quite visible at microscale. The problem of halftone misregistration was studied in [1]. Figure 5a shows an example of the halftoning scheme with different screen angles for cyan, magenta, yellow and black channels. Black, the color with the strongest contrast, is halftoned and placed at 45\({}^{\circ}\) due to the lower sensitivity of eye at this angle, meanwhile yellow, the weakest color, is halftoned at 0\({}^{\circ}\), where the human eye is most sensitive [1]. Using a different angle for each channel reduces the effect of misregistration, on the other hand it introduces rosette patterns, which are quite visible at microscale [1]. [PERSON] proposed, in [1], using different wavelengths to separate colors, but the proposed approach for separating ink colors in color prints was limited to two colors. Moreover, this solution is limited to the available wavelengths, and sometimes it is hard to find good wavelengths to separate more than two color inks. The main issue with the separation of ink colors in color prints when the dots are printed on top of each other (dot-on-dot) is that the effect of one ink cannot be completely removed when sending the light at the absorbing wavelength band of the other ink, as a consequence a shadow effect appears. The term shadow effect of one ink means the contribution of that ink to the reflectance spectra of the other ink [1]. Considering that, in our study case, even if cyan dots were printed on the top of the magenta layer, there is no problem of dot detection in both the Cyan and Magenta channels. Due to the misregistration between these two channels, we process these two channels independently from the others. On the other hand, considering that the yellow dots were printed on the top of the cyan layer and that there is a problem of dot detection on the Yellow channel due to the misregistration between these two channels, we decide to mask the dots detected on the cyan image in the yellow image in order to be able, in the next step, to process the Yellow channel independent from the others. Images corresponding to samples number 7, 8, 9, 10, 14, 15, 16 and 18 in Tables 1-3 show a similar misregistration problem.