resize images without losing accuracy ?

[Omer_Media]

I understand that you wish to print for a book, If this is the case we have to think in dpi.

The reason why I set the image to 300dpi first is a pc, will calculate its algoratham from a higher dpi to give better data for the print.

Starting on a lower algoritham may look the same but the image will print with dots slightly further apart reducing quality.

Ask any other printer and they will tell you the same.

You will also need to consider what type of printing technology and print stock you will be using as this also effects quality and bleed when colours are over laid onto each other.

 

[Omer_Media]

In photoshop open 2 files.

one at 300 dpi and one at 72dpi.

Draw a circle filled in black both the same size.

now change the one with 72dpi to 300dpi.

zoom into both of these.

one which started as a 300dpi has the better quality and no fuzz round the sides.

This is because the algoritham works diffrently on lower dpi.

Now imagine these dots printed onto paper, no fuzz better quality at they are packed together.

Remeber how newspapers use to look years ago all with lots of dots and white space ? In todays age they have better quality due to printing process able to use more dots tightly packed.

 

[PatriK-b]

Not sure what you are saying here but if I'm right it's the second one (resampling) that is the one you should avoid in your scenario. Resizing does not affect the original image (other than cropping some away).

First, thank for you previous reply which clarify the ppi, because resolution often confuses people. ppi is useful only to calculate physical printing area of a picture, if you give number of pixels and resolution instead of forcing print area. Changing this value for photo print has no meaning, has you constraint the print size.

I just opened ps and understand why my words are confusing you.

PS do not use 'resample' term as other softwares (and I) do.

With resample checkbox unchecked you only change the ppi information field (and again, yes, it's only a field, it does not affect image data)
With resample checkbox checked, the number of pixels is changed, a filter is applied to get the new picture.

I try to sum up here my minds (and what can be found in a non-ps world )
- resize just remove (when reducing) or clone (when enlarging) pixels to get the new image. This is very fast, generally used for preview only and this does not exist in ps.
- resample create new pixels, calculated by applying a filter to the original image pixels, this is in ps.

I hope I clarified. :???: not easy to do it in english.

 

[EOS_JD]

I understand that you wish to print for a book, If this is the case we have to think in dpi.

The reason why I set the image to 300dpi first is a pc, will calculate its algoratham from a higher dpi to give better data for the print.

Starting on a lower algoritham may look the same but the image will print with dots slightly further apart reducing quality.

Ask any other printer and they will tell you the same.

You will also need to consider what type of printing technology and print stock you will be using as this also effects quality and bleed when colours are over laid onto each other.

Not sure where you are getting your info from but you are 100% wrong.... Fine if you switch off resampling as you are then not changing the document. It's down to ONLY THE NUMBER OF PXELS IN YOUR OIGINAL. You may only require to resize - not resample.

 

[EOS_JD]

In photoshop open 2 files.

one at 300 dpi and one at 72dpi.

Draw a circle filled in black both the same size.

now change the one with 72dpi to 300dpi..

THis is not what you shopuld be doing. What you don't seem to understand is that you can change the image to 300ppi without resampling (adding pixels).....

You are actually changing the image by adding these pixels. You can change to 300ppi without adding pixels (honest you can).

Here try this - What is different about these two image sizes

1. 10"x8" @ 72ppi
2. 2.4" x 1.92" @ 300ppi

Both images are the same..... 720px x 576px

one which started as a 300dpi has the better quality and no fuzz round the sides.

Because you changed the number of pixels in the image (resampled). You can resize without resampling.

This is because the algoritham works diffrently on lower dpi.

No it's because you resampled. I'm saying don't resample because it will reduce the quality of your image (as you are indicating).

Now imagine these dots printed onto paper, no fuzz better quality at they are packed together.

You pack them closer together by reducing only the print size. Not by adding more pixels though.

 

[EOS_JD]

First, thank for you previous reply which clarify the ppi, because resolution often confuses people. ppi is useful only to calculate physical printing area of a picture, if you give number of pixels and resolution instead of forcing print area. Changing this value for photo print has no meaning, has you constraint the print size.

You don't need to constrain the print size though.

I just opened ps and understand why my words are confusing you.

PS do not use 'resample' term as other softwares (and I) do.

Ps uses the correct terms

With resample checkbox unchecked you only change the ppi information field (and again, yes, it's only a field, it does not affect image data)

You can change print size and ppi (as you amend one the other changes automatically.

With resample checkbox checked, the number of pixels is changed, a filter is applied to get the new picture.

Yes that's correct. It's also called interpolation.

I try to sum up here my minds (and what can be found in a non-ps world )
- resize just remove (when reducing) or clone (when enlarging) pixels to get the new image. This is very fast, generally used for preview only and this does not exist in ps.

Not sure that is correct? You can certainly do anything in Ps regards adding canvas or deleting pixels (cropping) without resampling. But I'm not 100% sure what you mean.

- resample create new pixels, calculated by applying a filter to the original image pixels, this is in ps.

You can do this in Ps yes.

I hope I clarified. :???: not easy to do it in english.

I'm struggling to understand your thoughts exactly. You can increase the print size by reducing the ppi of an image and conversely you reduce the print size by increasing the ppi.

By adding or subtracting pixels by you are changing the number of pixels in the image therefore in my mind you are resampling. Although I do see that if you crop off pixels or add pixels by increasing the canvas size, you'll not resample the image - but I am not talking about this above (or below)

Use the equation I provided

Pixels (P)= Resolution (PPI) / Print Size (PS)
So lets try a couple of things.

I'll use my 20D image files here which are 3504 pixels x 2336 pixels.

When the come into Photoshop, generally they are displayer as 48.667" x 32.444" @ 72ppi. This is exactly the same as 35.04 x 23.36" @100ppi or 11.68" x 7.7867 @300ppi.

Multiply the PS x PPI and you'll get the number of pixels. So from this you can edit to almost any size without changing the number of pixels in your image. Obviously if you want to print around say 24 x 16 (which prints at 146ppi) or larger, then you may want to consider resampling a bit but even at 146ppi you'll get a quality image that will look great on a wall.

Does this help?