Cropping is a process where you remove undesirable regions of an image. Typically, cropping removes regions at the border of an image, or simply makes the whole image smaller from a larger image.

PixInsight’s DynamicCrop process makes it easy not only  to crop a single image, but also to crop multiple images with the same settings thereby allowing you to crop individual images early in the processing workflow.

DynamicCrop is a dynamic process, which means that you can see the results of the crop as you make changes to the image so you can easily target specific areas before you commit to the change by applying the process.

In this article, I’ll explain how to use DynamicCrop and then show you how to translate the settings so that you can use it on another image. This is useful in cases where you want to crop your luminance image and want to crop your RGB image in the same way early in the processing workflow.

How To Use DynamicCrop

There are a couple of ways to use DynamicCrop and I show you both here.

You need to have at least one image open on your PixInsight desktop to use the process.

Start the process by selecting from the menu Process – Geometry – DynamicCrop (or select it from the <All Processes> section of the Process menu option).

When initially opened, the process window’s controls will all be disabled (greyed-out) and this is expected behavior. DynamicCrop is waiting for your to make a selection.

To crop your image, simply drag and drop your mouse over the image to highlight the area to be cropped, as shown:

When you release your mouse, PixInsight highlights the selected area and changes your mouse pointer as shown in the bottom-right corner of the following screenshot:

When you move your mouse over the border of the selected area, it changes to the pointer shown here:

You can drag and drop using this pointer to adjust the cropped area; you can adjust any of the four sides in this way and you can adjust from the corners too.

You can even rotate the cropped region – simply move your mouse pointer away from the edge of the image until it changes to the pointer shown here:

If you wish to have better control over the size of the area to be cropped, you can enter pixel values in the Width and Height boxes (do not press enter because that triggers the crop process to execute; simply move the cursor out of the box by clicking in another box or press the tab key). Once you have selected a size, you can move the selected area around by hovering your mouse over the selected area and dragging and dropping as necessary.

Another way to use DynamicCrop is to click somewhere on your image and then, clicking the Reset button at the lower-right side of the DynamicCrop window first. Doing this makes the selected region the whole image where you can then adjust the borders as necessary. Using this approach can be easier when you just want to remove your image’s borders.

Once you are satisfied with your selection, click the green checkmark at the lower-left corner of the DynamicCrop window to execute the crop. Your image will change to reveal only the cropped area.

So what if you just cropped your luminance image and wish to crop your RGB image in the exact same region? I discuss how to do that next.

Reapplying DynamicCrop To Another Image

Early in your processing workflow, you’re likely to have both a luminance and RGB image on your PixInsight desktop. You can apply the same crop to your other images by taking advantage of PixInsight’s HistoryExplorer. The History Explorer keeps track of all of the changes you have made to an image and allows you to extract the settings used with any process. You can use this feature to translate DynamicCrop from one image to another.

Assuming you just cropped your luminance image, right-click the name of your image on the left side of your image, and select Load History Explorer as shown:

Locate the line where it says DynamicCrop and click and drag the word ‘DynamicCrop’ to somewhere on your PixInsight desktop, as shown:

Next, make the image you want to crop active by clicking on its title bar and then double-click the icon you just dropped on your desktop. The region will now be highlighted show you the exact same settings you used earlier:

You can click the green arrow in the DynamicCrop window that pops-up at this point to crop the region in your image and you can close the DynamicCrop window.

Conclusion

In this article, you learned about the DynamicCrop process and learned how to use it to crop an image. You also learned how to apply the crop settings to another image, as is often the case early in the processing workflow that you have more than one image to crop as part of the same image set.

More Articles In This Series

This article is part of a whole series of articles about processing images using PixInsight: