Newbie- please tell me what this DPI change behavior means

So, I’m trying to enlarge my images for print (my capture device is only an iPhone 12 Pro) and I don’t understand what the Gigapixel is doing. Justin Hedrick from support emailed me that I should process in 2 steps: first set the resolution to 300dpi, then resize the width to larger. However, it looks to me like it has locked the image size in step 1. When I set the resolution to 300 dpi, he said that should be a 4x increase from 72 dpi, (does he mean file size or output dimensions?) but I’m seeing the width decrease in output size because the file size stays the same. Is there a setting I’m missing? I expected the file size to increase if I ask Gigapixel to increase the dpi.

I meant to add- it actually seems to be indicating that the file size is decreasing when I first only change the DPI. I made a screen capture, and it shows JPG 11.1MB => 3.8 MB which is not what I expected.

9 posts were split to a new topic: DPI vs PPI and General Didcussion

Thanks for your response, so just begs more questions

  1. why did support say to adjust DPI in the first pass? Is that wrong? and
  2. did you see what I wrote about the file size that is shown when processing? It actually says it’s getting smaller. I’ll try to upload the screenshot.

    These are the instructions I received from support. Maybe I am not understanding them correctly:

Blockquote

To do this, you will want to use the height or width option. When either is set to inches, a new field will open for PPI. Here, you can set the resolution to 300. From 72 dpi, this would be an increase of about x4. You won’t want to scale more than 6 times the original in one process. More will cause some loss in quality. However, you can run multiple processes on the same image for more enlargement

Because of this, you may want to increase the resolution first but leave the dimensions the same to start, and then do a second process to scale up the size to the desired end result. 300 PPI is generally recommended for print quality, then you can change the size to the specific dimensions you want, like 24x30, for example.

The DPI settings are used to upscale the image and does not refer to file size. You can only increase the dimensions by adding the number of inches you want in upscale box ( choose IN instead of PX), either width or height, then choose the DPI you want.

For example, you can enter 10 inches @ 300 DPI which will give a pixel count of 3000.

File size is determined in the output settings by the quality you want.

That is correct, you just forgot to enter the size in inches that you wanted.

The application is using the settings to UPSIZE and DPI is a relevant setting for printing images.

thank you, yes, I am just posting what the application is displaying. I think it makes sense to set both the resolution and width at the same time. I think support is worried, however, that it might be too much of an upscale in one step which would degrade the output quality. However, probably not an issue in my case since GP AI is only indicating less than 2x scaling. Sorry, do not understand the trolling comment.

Please post a screenshot of the settings you are applying in the upscale option. If the upscale settings are less than 6x AI will be used to upscale otherwise bi-cubic will be used which will not give the quality you want. Note you can upscale multiple times.

screenshots:

That looks good as it is less than 2x so there is no problem with the AI upscale … I assume you are using as a standalone so when you choose Save Image make sure you choose an output format other than Preserve Input Format. Then choose the format and the quality you want, if a JPEG the maximum is 95% which is more than enough.

File size is often going down on processed images even if they are of higher resolution because TPAI denoises the image.
Noise is the one thing that kills jpg (but also other) compressions.
A smooth, homogeneous area can be compressed much better than a grainy mess.

So, yes, the processed image can have higher quality / be better looking and still have a lower file size than the noisy original.

yes, thanks for pointing out the export Format. It defaults to Preserve Input Format, which was ok since it was JPEG, but I wasn’t able to select the quality with that setting.

That’s good to know. I realized later that Graphic Converter, which I use only to as my finder for images, has been sometimes displaying 1:1 compression and other times displaying files as 3:4 compression. This is not related to this topic but it’s why I actually saw the file size go down- the compression went up as you point out.

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