Does PI reduce image resolution to 72 ppi regardless of input files resolution?

[Rudy Pohl]

Hi there:

I am new to PI and part way through my 45-day trial period. I have a couple of questions about resolution and printing final images.

1. I notice that no matter what resolution my images are that I input into PI for the Integration process I always end up with a file that is 72 ppi. Is this normal?

2. I always print my images files using a commercial printing service and send them Photoshop files at 300 ppi. Should I resample my final Pixinsight images of 72 ppi up to 300 ppi?

Thanks,
Rudy

[Juan Conejero]

1. I notice that no matter what resolution my images are that I input into PI for the Integration process I always end up with a file that is 72 ppi. Is this normal?

Yes, this is the expected behavior. The preprocessing task (calibration, demosaicing, registration, integration) works with data, not with pictures. Properties like printing resolution and color profiles, among others, do not make any sense for linear raw data. After image integration, you get a newly created image with default settings. Once the image is in a nonlinear state, in the postprocessing stage, you have to assign the appropriate printing resolution parameters and color profile in order to obtain correct results on specific output devices and media.

To assign printing resolution parameters, you can use several processes of the Geometry category with the standard force resolution parameter enabled. For example, you can use Crop with the four margins set to zero (no pixels altered) and the required horizontal and vertical resolution, resolution units, and force resolution = on. Or, if you are going to resize your image, Do the same with the Resample process to perform both operations at the same time.

2. I always print my images files using a commercial printing service and send them Photoshop files at 300 ppi. Should I resample my final Pixinsight images of 72 ppi up to 300 ppi?

Basic concept: Don't confuse printing resolution with pixel dimensions. Resolution parameters are just metadata; they have nothing to do with pixel contents. For example, you can print a 1x1 image on a square meter at one pixel per meter resolution. Or you can print a 1200x1200 image on a square inch at 1200 dpi, but also the same image on a square of 1.2x1.2 millimeters at 10000 pixels per centimeter.

[Rudy Pohl]

Hi Juan:

Thanks so much for your reply.

Unfortunately, I'm still not clear on my question, Could I rephrase and have you answer me again?

When I input my images to PI for debayering, star aligning and image integration, these images are at a resolution of 240 ppi which is the native resolution of my camera. This means that there are 240 pixels packed into every inch of image. When the stacked is completed and I save it as a 16-bit image and load it into Photoshop CS5 it tells me that the resolution of the image is 72 ppi.

So here's my question: Does the stacked image now have 72 pixels in every inch of image or does it have 240 pixels in every inch.

Thanks,
Rudy

[NGC7789]

I think the best way to understand this is that your image doesn't have inches. It has pixels. Until you output it, be it to a screen or a paper or other physical medium, it does not have inches or other similar units. So, pixels per inch does not have any meaning to the image itself. As Juan said it's just metadata. If your camera has a "native resolution" of 240 ppi it's just that the camera sets the metadata to this value. It probably does this because many sources state 240 ppi as the recommended resolution for photo quality output.

Changing dpi or ppi never requires resampling. Changing the number of pixels requires resampling.

[tsaban]

Juan explained everything.

Btw. the 72ppi can be changed in Preferences/Default Image Resolution