First it is worth reminding yourself what the histogram is and what it is not. This explains it:
http://www.blackwaterskies.co.uk/2013/12/how-to-interpret-image-histogram.htmlSecondly, it is also important to understand that the histogram transformation process and the curves process are functionally equivalent. All they do is apply a transfer function to the image. Basically you have a curve on a graph, with the input values on one axis and the output values on the other axis. Both axes would have a range of values representing the full range of 8, 14, 16 bit values for the image (or whatever range is appropriate for the image format).
To perform the transfer, you just read up from the X axis from each possible pixel value to intercept the transfer curve, and then across to the Y axis to find the new value. Every pixel that had a value of (say) 1,000 is converted to a new value of (say) 1,500, and from 1,001 to 1,503, etc. As you transform values, the histogram of the new image will change shape to reflect the newly transformed set of pixels.
The histogram transformation process applies a simple mid-tone transfer function, whereby you change the shape of the curve by moving the ends (the black and white points) plus a third point which is the mid-tone slider. The curves tool allows you to create an arbitrary number of control points to build a more complex curve shape. It is perfectly possible to create a mid-tones transfer function using the curves tool, but you can't create a more complex curve in the HT tool as you only have three fixed control points on the curve.
When dealing with an RGB image, you can create three MTF curves (using the HT tool) or and ditto you can create three different curves for RGB in the curves tool, one for each of the colour channels. Doing so will shift the colour balance in the image. (Or you can create one MTF or curve to apply to all three channels equally, preserving the current colour balance).
Sticking with the HT tool for now:
- If you want to make the histogram narrower, you can drag the black and white points in towards the centre. At some point you may start clipping the darkest / brightest pixels - essentially the start or end of the transfer curve is flat and every pixel below the black point is mapped to zero, and everything above the white point is mapped to the highest available value. This is usually a bad thing because you have now lost information in the image.
- The only other option is to move the mid-tones slider. As you move it you will tend to spread out or make narrower the histogram peak, and as a result you will make the peak higher or lower.
All that said, your problem is that you want to colour balance the image, but basically you are concentrating on completely the wrong thing. The height of the colour peaks is not relevant to a good colour balance in an astro image. Your aim is to align the three RGB peaks as best you can and you should then have a good colour balance.
There is a really easy way to do this:
- Open the ScreenTransferFunction process from the process menu (don't use the icons on the toolbar).
- Now click the 'radioactive' icon in the process window to apply a basic STF. (Click the 'chain' icon just above in the process window to unlink the channels if you don't have good colour balance, or leave them linked if you do).
- You should now have your image showing with a suitable STF.
- Next open the HistogramTransformation process and start the real-time preview.
- Drag the blue triangle from the ScreenTransferFunction process window to the bottom bar of the HistogramTransformation process. This will transfer the STF parameters to the HT process and give you a reasonable starting point for doing the stretch.
- A this point the preview window will white out or be massively over-stretched, because you have double-applied the stretch (once from the active STF and once from the HT preview).
- You now just need to de-activate the STF - close the STF process and if necessary click the icon next to the radiation icon on the toolbar to clear the active STF.
- You should find the preview now shows a similar result to what you had when you first applied the STF, i.e. a good starting stretch.
- Now tweak the HT process as you wish to get a better stretch - you should only need to tweak the K curve (applied to all three channels) and leave the individual RGB curves alone to retain the new colour balance.
- Finally drag the blue triangle from the HT process to the image window to apply the stretch.
- Save the image and when you re-open it later, you should find it has the stretch applied as you'd expect.
