Get Photos Right Print Size - And All The Same!

[abrogard]

I am trying to make passport photos for three different people.

And I think to finish up with an image suited for your typical print shop where they print on 4" x 6" paper and usually get about eight passport pics on the one card.

So I've been making the passport pics from different photos all supposedly crafted to be of the required size (48mm x 33mm in this case ) and I have finished up with the 'passport pics'  with these details according to GIMP:

1.   737 x 1072 pixels.   4" x 5.9"             180 x 180 ppi
2.  689 x 1003 pixels   3.82" x 5.57"        180 x 180 ppi
3.  630 x 918 pixels     3.5"  x 5.1"            180 x 180 ppi

So it hasn't worked out.  They're all different.

So am I using the wrong method?

Or can this all be fixed at this stage?

Right now I can't even cut and paste them successfully onto a new canvas of 4" x 6".   Can paste them on but they're too big and I can't move them around to tuck them into the corner.  Hence I never even get past the first one.

The method is to calculate the pixels per mm by measuring the head height in pixels right there in GIMP and dividing that number by the required head height in mm as specified.  (30mm)

I then know the pixel dimensions of the rectangle I need to cut out because I just multiply those pixels/mm by the height and width (48 x 33).

Sounds alright.   But it's not working too good.

Unless it can all be fixed now.

Or is there an entirely different way to go about it?

[rich2005]

First 180 ppi is not wonderful for printing photographs, but it might be ok.

Conversion from inches to pixels, which is not difficult because generally pixels-per-inch (ppi) are used.

But why not use Gimp to do the arithmetic for you.

For the 6" x 4" print size @ 180 ppi File -> New and open the Advanced Options Set resolutions to 180. Set the units to inches (in) Set the Width and Height to 6 and 4.

Under that is the size in pixels: 1080 x 720 <<< the size for the print shop.

   

The same with the individual passeport photos, except this time set the units to mm underneath is the size in pixels 234 x 340 <<< the size for each 'headshot'

   

Quote:ight now I can't even cut and paste them successfully onto a new canvas of 4" x 6". Can paste them on but they're too big and I can't move them around to tuck them into the corner. Hence I never even get past the first one.

All different sizes. You need to scale them down to 234 x 340 pix. but you will lose quality.

Those fit together like : You can change ruler dimension bottom of the image window if it helps but the image is Gimp it is in pixels and the size will show top of the window.

   

[rich2005]

Can I suggest for those 3 photographs a modification to your procedure. The 3 are correct width-to-height ratios but different sizes in pixels.

A good common size might be 585 x 830 pixels but @ 450 ppi. Scale each Image -> Scale Image set the resolutions to 450 set the size to 33x48 mm

   


A suitable canvas for this, as before but with the resolutions set to 450 ppi matching the scaled photos.

   

[abrogard]

Thank you muchly for the responses. I will have to sit down and read them carefully, do what's suggested and absorb it all, learn what it means.

Meanwhile I had a thought in the night...

Isn't the crux of my problem my insistence upon getting the mix of photos onto one printout?

Together with letting GIMP decide the pixel size of the printout canvas?

GIMP has decided 180 ppi I suppose because that's the default on my setup when I ask for a new canvas.

Each photo actually has a different ppi because though all taken with the same camera at the same resolution my cutting a part out to use for the passport means I am making a different ppi for each one based on the number of pixels I've had to cut out from that photo - requiring each cutout to shoehorn into 48mm x 33mm. Is that right?

So that 737 x 1072 actually is meant to be a 33x48mm pic with a dots per mm of 22.3 which at 25mm per inch would be something like 558dpi.

So I, for that pic, should tell GIMP I want a new canvas measuring (4 x 558) x (6 x 558) in inches. And so on...

I think I've got that right have I? Learned something there?

I will try that and as I say I will try your suggestions and really learn something, hopefully.

One needs understanding isn't it? Not just blindly doing things. My current understanding is so miniscule. I don't know the difference between a mask and a layer. I don't know what to do, if anything, about 'floating layers', don't know about these alpha channels and whatnot.... don't know nuffin....

later: ah. not so difficult. both of you are simply talking about scaling. and it seems to be the answer. thanks. that way I don't need to make different printouts for each.

GIMP is that clever ? It actually changes the pixels per inch? What's happening there? It must be throwing pixels away on some edits, creating new pixels on others?

[rich2005]

Going back to the subject: "Get Photos Right Print Size - And All The Same!"

1. All The Same: Gimp works in pixels.
If you want the individual photos all the same size then you need to scale each photo individually to the same dimensions in pixels. It sounds like you are doing that in place on the canvas using the scale tool. I would pre-process each before starting using Image -> Scale Image Both get there in the end, but Scale Image is that little bit more predictable.

2. Right Print Size Printing is where pixel-per-inch (ppi) comes in. You can use any value for this, eye-of-the-beholder comes into play. Viewing the print at a distance (a poster) and a small value is ok, a 6"x4" photo and a larger ppi is needed. The convention for photographs is use 300 ppi. Larger is ok, some scanned images might use 600 - 1200 ppi but generally not much benefit using large values.

Quote:GIMP is that clever ? It actually changes the pixels per inch? What's happening there? It must be throwing pixels away on some edits, creating new pixels on others?

Gimp is not that clever, a pixel is a pixel, scale down and pixels are thrown away, scale up and pixels are guessed (interpolated) and added. Both actions degrade the image although you can get away with scaling down a little.

Worst case for your photos 737x1072 pix > 234x340 pix scaled down 3 times. That is why I recommend ditching your 180 ppi and go for a higher print resolution. The print shop will still print a 6"X 4" whatever the print resolution, it is only a setting in the image file.

What does happen: Copy & Paste, Import as Layers, drag-n-drop and the properties of the destination image are used. Your 6"x4" canvas @ 180 ppi (1080x720 pix) then that is used. Use 6"x4" @ 450 ppi (2700x1800 pix) that is used. Your individual photos do not change, a pixel is a pixel

Quote:GIMP has decided 180 ppi I suppose because that's the default on my setup when I ask for a new canvas.

180 ppi is unusual for a default. Used to be 72 ppi which is/was the pixel resolution for a computer monitor matching the requirements for web-pages. Look at the drop down list of standard templates. Lots @ 300 ppi for printing but the Web Banners still 72 ppi, while the phone templates high for those displays.
Using 72 ppi is one of the main reasons for disappointed beginners when their work is printed (think logo on tee-shirt)
Gimp 2.10 and the default canvas is now 300 ppi (1920x1080 pix)

Quote:. I don't know the difference between a mask and a layer.

You can see layers, a stack of images one on top of another. Some will hide anything underneath. Think of a mask as making a layer into a window, parts of the window are opaque, parts are transparent. You can see through the transparent bits to whatever is underneath.

Quote:..I don't know what to do, if anything, about 'floating layers'..

Typ. (Floating Selection)Pasted layer It is temporary. Treat as another layer and move, scale, rotate if required. When finished make it permanent, either anchor it, which merges into the active layer or make it into a new layer. Look in the Layers menu.

Keep trying, you get there in the end.

[abrogard]

Thanks for all that.  I will, I'll get back to it....