Aligning
Alignment is the stage where the alignpoints of the images get "registered" compared to a referenceframe. In RegiStax V4 the referenceframe was allways the frame the user had located in the sequence of images to position the alignpoints on. In RegiStax V5 we allow the user to load a referenceframe which serves two purposes, first off all it allows the user to find the best alignpoints better (for instance using the estimate-alignpoints function) and secondly it can be a far better reference for the topology of the image.
Lets assume we have selected a referenceframe and have also set our alignpoints. The next step is to align (register) the alignpoints. RegiStax V5 has several new ways to align images. We will describe a few:
- Simple Aligning
This is more or less the same as the previous RegiStax versions. The main difference is that multi-point alignment will be done in one single processing run. In V4 every alignpoint would be processed in seperate runs during alignment, so for 50 alignpoints that would mean all images had to be loaded 50 times. Loading images is time-consuming and slows down processing. In V5 we have therefore changed this to a simultaneous alignment of all alignpoints. That speeds up alignment of multi-points considerably, using the new maximum amount of alignpoints (500) is however still timeconsuming ! During alignment you will see the alignpoints at every image in the sequence, when an alignpoint drifts off compared to the "master alignpoint #1" it will blink in red during alignment. RegiStax will limit the drift to the value set by the user (see Setting Alignpoints). During alignment you will see in the statusbar on the lower-right how much time RegiStax still thinks Alignment will be needed. - Aligning using Gravity-Centre
Sometimes we are dealing with images that are difficult to align, think about a solar image that nearly fills the whole image but hardly has any clear features (eg. solar spots) visible. In Registax V4 aligning these images is hardly possible. In V5 we have developed "gravity-centre" alignment, this type of alignment calculates the "centre of gravity" in each image based. You can use this form of alignment as a single option, the gravity-centre will then also be the alignpoint, but you can also use this form of alignment with multiple alignpoints. The alignpoints can be set as a user can always do but the "gravity-centre" will serve as the master alignpoint. Again, alignpoints will be limited in drifting away from the position of the gravity-centre point. Although this method was mainly developed for aligning large solar/full moon images it can be usefull in several other occasions. For instance if you have a large series of an object (planet/moon/ISS) that is jumping around over the image nearly erratic. The gravity centre can easily find the object as long as there are no alike objects in the scene. A bright object on a dark background is what can be used the best. - Aligning using filters
Some images are difficult to align due to noise, little contrast or other image-artefacts. RegiStax V5 has a set of 4 different filters that can be used to pre-process images before they are aligned.
The filters are :
- Blur :uses a gaussian blur with a radius set by the user to blur the image. This is often usefull for images that have "static" noise. This type of noise is less random and tricks the alignment to stay locked on a pattern. After stacking and using wavelets the user often will see diagonal lines. Aligning with this filter can prevent this from happening.
- Gamma: sometimes the features a user wants to align on are very dim. Setting the gamma value of the image different can help in improving the contrast.
- Stretch: this allows the user to clip (stretch) the intensity values of the image. Everything below the low-value will be 0(black) and everything above the high-value will be 255 (white). Especially raising the low-value can be helpfull to erase background noise.
- Negative this simple creates a negative image, some users use this to make dim/dark areas better visible to position alignpoints.