ArcsinhStretch

2.1 Stretch Factor

The Stretch factor slider has a logarithmic response which allows the same degree of relative control at each end of the scale, i.e. near 1 and near 1000. Values near 1.0 are useful when applying a second iteration of ArcsinhStretch, for instance.

Note that when stretching the faint signal, the background noise is amplified by the same stretch factor. This amplified background noise will typically be the main constraint on how much stretch you wish to apply. If the maximum stretch level of 1000 is insufficient, then a second iteration of ArcsinhStretch can be applied to the image.

In the example below, the black point has been set and a stretch factor of 50 has been used:

Increasing the stretch factor to 300 makes a big difference to this image:

Useful Hint: For some images it can be advantageous to apply the stretch iteratively. For instance, the example below shows the third iteration using a stretch factor of 6.70. Since the product of 6.7 three times is approximately 300 (6.7 × 6.7 × 6.7 = 300.7), the effect is similar but not identical to the above single stretch of 300. However, the iterative approach has made the nebulosity appear more dramatic.

2.2 Black Point

On the ArcsinhStretch interface it can be seen that the black point adjuster has two sliders. Users of the Convolution process will already be familiar with the concept of having two sliders to control a single parameter, one being a coarse adjustment and the other being a fine adjustment.

The lower slider of the two is for fine adjustment. Think of it as analogous to a fine focusing knob. The slider re-centers itself whenever you move the mouse away from the slider, so you can always finely adjust both up and down from the background level you already have.

The black point has six decimal places of precision, and using the keyboard Up/Down arrows on the fine adjustment slider will nudge the final digit. PgUp/PgDn gives an adjustment 10x as much. The mouse wheel gives an adjustment 3x as much. Failing that, just type the background number in the box—it's easy to overtype the final digit as required.

When stretching by a factor of 100 or more, the very tiniest adjustment of the background value makes a very noticeable difference to the image, because its effects are also magnified by a factor of 100 or more! However, the keyboard keys Up, Down, PgUp, PgDn and the mouse wheel work fine for nudging the value.

Useful Hint: It is easiest to set the black point with the real-time preview window activated and with the highlight clipped values option enabled.

2.3 Protect Highlights

Stretching the image data can cause some data values to saturate (i.e. clip with a value of 1.0). The Protect highlights option will identify such pixels and linearly scale the whole image to avoid any individual pixel values becoming saturated. This will make the final image to appear darker.

For users who want a mathematical example of how such clipping can occur, consider an image pixel with RGB value (0.98, 0.25, 0.25). Since the luminance is less than 1.0, the image pixel will be stretched and this could force the red component to exceed 1.0 and therefore clip.

However, there is a known issue with the Protect highlights option. Under some circumstances the stretched image will end up darker than indicated by its real-time preview. This is because the real-time preview works on a smaller sample of the whole image and may not see some of the brighter pixels that exist in the entire image.

2.4 Use RGB Working Space

By default, the luminance value of the pixel is calculated from equally weighted RGB values (1/3, 1/3, 1/3). Setting the Use RGB working space option will use whatever weights are used by the current RGB Working Space (RGBWS); e.g. (0.22, 0.72, 0.06). These weights can be changed by applying the RGBWorkingSpace process to the data. This can have subtle but interesting results because although individual pixels will still have the correct color balance after stretching, the RGBWS can affect the look of the image as a whole by for instance, emphasizing or de-emphasizing areas of red H-alpha emissions or blue reflection nebulosity.