Update calibration.md

doc/calibration.md

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 # Multiple Camera Calibration
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 This is a manual to guide you through the automatic spatial alignment of a multi-sensor setup comprising of 4 Intel Real Sense cameras. The proposed method is reliant on an easily-assembled calibration structure which lifts the requirement of placing/gluing markers onto it. 
 In essence, it includes two independent steps in order to achieve high quality estimations. Firstly, given as an input a quadraple of depth images (180x320 px) it utilizes machine learning techniques to produce initial estimations. Secondly, towards further refining the initial results, a graph-based optimization scheme is used to maximize the overlap of point-clouds of neighboring viewpoints.
 
-## Prerequisites
+## Prerequisites 
+
 The goal is to deliver an easily-assembled reference structure for the calibration procedure to the public. Thus, the proposed structure requires 4 low-cost and commercially available packaging boxes from the IKEA, and specifically the [JÄTTENE boxes](https://www.ikea.com/ie/en/products/small-storage-organisers/storage-boxes-baskets/j%C3%A4ttene-packaging-box-brown-art-60047151/). 
 In practice, the calibration structure could be assembled using any 4 boxes, sized 56 x 33 x 41 cm each.
 
-## Assembling the Calibration Structure
+
+## Assembling the Calibration Structure 
+
 Since the proposed calibration structure does not require markers, users should only pay attention in placing the boxes, one on top of each other following a 90^o^ rotational pattern, as depicted in the following pictures. Snapping is performed using each respective bottom boxes sides.
 The first image shows the assembling procedure from a diagonal perspective. For more descriptive design we use color-coded representation. The first (bottom-most) box is colored in red, the second in green, the third in blue while the last (top-most) is depicted in yellow. 
 ![Calibration Assembling (Diagonal-View](https://github.com/VCL3D/VolumetricCapture/raw/master/doc/StructureGuideTextureColored.png)
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+
+---
 The succeeding image displays the identical positioning of the boxes as viewed from the front-view.
 ![Calibration Assembling (Front-View)](https://github.com/VCL3D/VolumetricCapture/raw/master/doc/StructureGuideFront.png)
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+
+---
 
 And this last image depicts the corresponding calibration procedure from the top-view.
 ![Calibration Assembling (Top-View)](https://github.com/VCL3D/VolumetricCapture/raw/master/doc/StructureGuideTop.png)
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-## Sensors Placement
+---
+
+## Sensors Placement 
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 The formerly placed calibration structure acts as the global coordinate system anchor. 
 - The setup requires all 4 Intel RealSense sensors to be placed __perimetrically around the calibration structure and looking inwards (i.e., towards the structure)__. 
 - In order to capture the desired object in full-360^o^, sensors should be placed approximately at __90^o^ intervals__,