image size reduction

[EOS_JD]

I've been playing around with the automate > batch feature of photoshop and if you simply record an action to resize your pics you can run an automated resize on any number of photos. The quality of the resized pics is great compared to the other methods.

I didn't read that far down before responding but that is the way to go

 

[EOS_JD]

My workflow currently is to view my fullsize shots in Adobe Bridge in Review Mode, applying labels to the ones I want to process. My question is: once I have my selected group of photos labelled in Adobe Bridge, what's the easiest way to resize the lot of them into another directory (preserving the original full size files)?

Using Adobe Camera RAW, you can open a number of files at the same time. Look at the text along the bottom of the screen. If you click on the text you can select various resolutions to output to (1024 x768 is not a choice but there is a very slightly larger res). You can use this to down/up scale your images pretty successfully when required.

You can then select all images (see button top left) and then open them.

A far better and easier program for this exact process is Lightroom. Adobe Bridge and Camera Raw are not so useful.

 

[pxl8]

Oh the good old ppi/dpi thread. Quite why something so simple causes so much confusion is one of the great unsolved mysteries

EOS_JD is spot on, ppi/dpi means nothing on it's own, think of it like an exchange rate - it's useless without an amount of currency to convert - in this case the currency is pixels.

So if you've got 800 pixels and you want to translate that to paper inches how many you get depends on the exchange rate. If you're rate is 72 pixels to the inch then you'll get lots of inches, if the rate is 300 then not so many.

The image on your monitor is a fixed resolution, say 1280x1024 pixels. When you view an image at 100% then each pixel in the image is represented by one pixel on the screen. DPI/PPI isn't used to define quality, how big the image is on the screen or anything else - it's just plain ignored. The same is true of a full res image out of the camera - it might have a setting of 300 dpi but if you view it at 100% again then it will be huge. Why? Because your monitor only shows so many pixels to the inch (ppi). The 72dpi setting came from old 14" screens running at a resolution of 640x480. The resolution that gfx cards were capable of increased far quicker than the size of the monitors - to get 72dpi on a 1280x960 display you'd need a 28" monitor. If your monitor was smaller then you were effectively working at a high PPI - more pixels to the inch.

Most LCD panels are now 96 PPI - it's simple enough to check, measure the width of the active area and divide it by the horizontal resolution - that's your screen's PPI.

But DPI/PPI don't really mean anything, the only time it might be an issue is when you insert an image into Word, Publisher, etc as they will read the PPI and use it to work out the default size for the image on the page. Again you can try this easily, create two copies of an image, both 800px wide but one set to 72dpi and the other 300dpi. Now insert both into Word or whatever and they'll end up different sizes - that's DPI/PPI in action.

For most printing needs you don't need to worry about this, if you send a jpg to Photobox, etc. and tell them to print it at 30x20" they will regardless of the pixel resolution of the image. Now you can create images at the right output resolution if you know the precise DPI of the printer, most say 300 but it's often different because images are enlarged slightly for printing.

Forget PPI/DPI for the most part - if you're preparing images for web just worry about the pixel size and jpeg quality, nothing else is going to affect the quality of the result.

 

[EOS_JD]

Oh the good old ppi/dpi thread. Quite why something so simple causes so much confusion is one of the great unsolved mysteries

EOS_JD is spot on, ppi/dpi means nothing on it's own, think of it like an exchange rate - it's useless without an amount of currency to convert - in this case the currency is pixels.

So if you've got 800 pixels and you want to translate that to paper inches how many you get depends on the exchange rate. If you're rate is 72 pixels to the inch then you'll get lots of inches, if the rate is 300 then not so many.

The image on your monitor is a fixed resolution, say 1280x1024 pixels. When you view an image at 100% then each pixel in the image is represented by one pixel on the screen. DPI/PPI isn't used to define quality, how big the image is on the screen or anything else - it's just plain ignored. The same is true of a full res image out of the camera - it might have a setting of 300 dpi but if you view it at 100% again then it will be huge. Why? Because your monitor only shows so many pixels to the inch (ppi). The 72dpi setting came from old 14" screens running at a resolution of 640x480. The resolution that gfx cards were capable of increased far quicker than the size of the monitors - to get 72dpi on a 1280x960 display you'd need a 28" monitor. If your monitor was smaller then you were effectively working at a high PPI - more pixels to the inch.

Most LCD panels are now 96 PPI - it's simple enough to check, measure the width of the active area and divide it by the horizontal resolution - that's your screen's PPI.

But DPI/PPI don't really mean anything, the only time it might be an issue is when you insert an image into Word, Publisher, etc as they will read the PPI and use it to work out the default size for the image on the page. Again you can try this easily, create two copies of an image, both 800px wide but one set to 72dpi and the other 300dpi. Now insert both into Word or whatever and they'll end up different sizes - that's DPI/PPI in action.

For most printing needs you don't need to worry about this, if you send a jpg to Photobox, etc. and tell them to print it at 30x20" they will regardless of the pixel resolution of the image. Now you can create images at the right output resolution if you know the precise DPI of the printer, most say 300 but it's often different because images are enlarged slightly for printing.

Forget PPI/DPI for the most part - if you're preparing images for web just worry about the pixel size and jpeg quality, nothing else is going to affect the quality of the result.

Other than the fact you use dpi instead of ppi you are quite correct

I like your analogy of currency relating to ppi/print size. i'd not seen that before.

 

[AndyB]

OK I am getting it. Neither PPI nor DPI have any effect on the way an image is displayed on a screen.

Can we just look at printing files then so I can understand that better? This is probably where I am confusing myself. Obviously when we define the size of an image for printing we are going to define the actual measurement in inches or cm? So lets say I have an image that has been sized to 10in by 8in. I have two versions - one sized 10x8 and saved at 72DPI/PPI and one the same size at 300DPI/PPI. Am I correct in saying both will print at 10x8 but the 72 DPI version will be lower resolution because each inch is only made up of 72 dots as oposed to 300 dots so the definition is reduced?

That being the case it is important for people to understand that resizing for web or print is very different and the same image needs to go through a completely different workflow depending on the output.

Jeez I am making myself sound like a complete amatuer here

 

[fracster]

I've been playing around with the automate > batch feature of photoshop and if you simply record an action to resize your pics you can run an automated resize on any number of photos. The quality of the resized pics is great compared to the other methods.

Irfanview is free,easy to use and can batch resize images..............

 

[EOS_JD]

OK I am getting it. Neither PPI nor DPI have any effect on the way an image is displayed on a screen.

Can we just look at printing files then so I can understand that better? This is probably where I am confusing myself. Obviously when we define the size of an image for printing we are going to define the actual measurement in inches or cm? So lets say I have an image that has been sized to 10in by 8in. I have two versions - one sized 10x8 and saved at 72DPI/PPI and one the same size at 300DPI/PPI. Am I correct in saying both will print at 10x8 but the 72 DPI version will be lower resolution because each inch is only made up of 72 dots as oposed to 300 dots so the definition is reduced?

That being the case it is important for people to understand that resizing for web or print is very different and the same image needs to go through a completely different workflow depending on the output.

Jeez I am making myself sound like a complete amatuer here

You are on the right tracks and obviously a web image nereds to go through a different process than a prinetd image.

As noted above, what matters is the number of pixels in your image. Regards your two images above if you have an image 10x8 at 72ppi then its size in pixels is as follows

10 x 72 = 720pixels
8 x 72 = 576pixels

720 x 576 is pretty much a web sized image that is easy for your web site and for email.

If you have a 10x8 at 300ppi

10 x 300 = 3000
8 x 300 = 2400

A 3000 x 2400 pixel image is obviously much bigger and perfect for printing a 10x8 print.

Do you see the differences?

Remember that equation

Ps = Px / R
R = Px/Ps
Px = Ps x R

All the same equation written slightly differently depending on the variable you require.

PPI = Image resolution (for print)
DPI = Printer output resolution

 

[pxl8]

Other than the fact you use dpi instead of ppi you are quite correct

Well, to be fair I only used it where it's being used by s/w (usually incorrectly) but like so many things once it's commonly used then that meaning is generally accepted as correct.

Besides, pixels are dots

 

[EOS_JD]

Well, to be fair I only used it where it's being used by s/w (usually incorrectly) but like so many things once it's commonly used then that meaning is generally accepted as correct.

Besides, pixels are dots

Yes it's common use even in magazines....

Have we not had this argument before though?

Pixels are not dots

Pixels exist ONLY in the digital world

Dots are physical matter

 

[pxl8]

Straw poll 100 people, show them a single white pixel on black screen and ask them what it is. How many will call it a dot?

 

[EOS_JD]

Straw poll 100 people, show them a single white pixel on black screen and ask them what it is. How many will call it a dot?

I would agree with that. A monitor shows dots hence monitor res is in dpi

The image is still made up of digital numbers (making up colours, brightness etc)

 

[jaffamuffin]

IF your OS/application is aware of your SCREENs ppi, then choosing to display the image 'at print size' or 'actual size' (NOT 100%) in your image editor, you should get a picture that will be the same physical size as the print out.

e.g. If you open a 800x600 300ppi image in photoshop, and then change the the ppi to 150, (without resampling i.e. the image is STILL 800x600) then got to View-> Print size, the image will be shown at ~%50 zoom. (on my 1600x1200 20.1 inch screen- If you have a smaller screen but same res, the zoom will be less this is my point...) If you print the picture out and hold it to the screen it will match up. The image is still the same size.

 

[jaffamuffin]

Yes it's common use even in magazines....

Have we not had this argument before though?

Pixels are not dots

Pixels exist ONLY in the digital world

Dots are physical matter

Indeed, and a printer often requires multiple dots in order to reproduce a single pixel of an image.

 

[pxl8]

I would agree with that. A monitor shows dots hence monitor res is in dpi

The image is still made up of digital numbers (making up colours, brightness etc)

Ok, I'll bite - what's a pixel then?

 

[turbotoes]

Ok, I'll bite - what's a pixel then?

I'd have thought you would be well placed to answer that, I thought you had eight of them

thanks to you and eos_jd for the explainations though, I'm starting to understand what you're both on about :bonk:[

 

[Paul MR]

A pixel short for picture element.
A digital color image pixel is just a RGB data value (Red, Green, Blue).
8bit 1 byte per RGB (3 bytes per pixel) max 256 colours
16bit two bytes per RGB (6 bytes per pixel) = 65536 colours
24bit three bytes per RGB (9 bytes per pixel) = 16.7 million colours.
MOst common used pixels have an aspect ratio of square, but there are quite a few variations.
If you have Photoshop have a play with: Image - Pixel Aspect Ratio

 

[EOS_JD]

Indeed, and a printer often requires multiple dots in order to reproduce a single pixel of an image.

Yes.

 

[EOS_JD]

IF your OS/application is aware of your SCREENs ppi, then choosing to display the image 'at print size' or 'actual size' (NOT 100%) in your image editor, you should get a picture that will be the same physical size as the print out.

e.g. If you open a 800x600 300ppi image in photoshop, and then change the the ppi to 150, (without resampling i.e. the image is STILL 800x600) then got to View-> Print size, the image will be shown at ~%50 zoom. (on my 1600x1200 20.1 inch screen- If you have a smaller screen but same res, the zoom will be less this is my point...) If you print the picture out and hold it to the screen it will match up. The image is still the same size.

Not sure if this is correct. When printing an 800x600 image it depends on the resolution you set. If I print at 300ppi ir will print pretty small yet at 150ppi it will print at double the 300ppi size..... Not sure were the screen size correlation is?

Pixels map to the screen resolution - not the print size.

 

[EOS_JD]

Ok, I'll bite - what's a pixel then?

I know it's not a physical dot. Surely it's a digital set of data that allows a screen to show the data as a colour/brightness dot on the screen (the screen has the dots not the digital image). Without a monitor the pixel still exists. A screen only allows the pixel to be displayed.

 

[jaffamuffin]

Not sure if this is correct. When printing an 800x600 image it depends on the resolution you set. If I print at 300ppi ir will print pretty small yet at 150ppi it will print at double the 300ppi size..... Not sure were the screen size correlation is?

Pixels map to the screen resolution - not the print size.

Decent apps will have a option for viewing at 'print size' or 'actual size'

Say if your screen was 300 dpi and your printer was 300dpi, 'actual size', and '100%' would be the same physical size - i.e. measure the print, and measure the picture on the screen - they would be the same.

However, screens are typically 90-150dpi, so if you have a small, high res laptop screen for example with a dpi of 150. If you choose to view at 'actual size' the application (e.g. photoshop) will DISPLAY (note it doesn't change the image data) the image on the screen at ~50% size, so that the printed image, still matches the dimensions of the image on the screen.


Think about a word processor - if it is aware of yoru screens dpi, regardless of screen resolution or size, a 72pt font should always measure 1 inch (72pt) high on the screen. and shold always print at 1inch high on a bit of paper, regardless of the number of pixels it takes to actualy display that image on the screen.

 

[EOS_JD]

Decent apps will have a option for viewing at 'print size' or 'actual size'

Say if your screen was 300 dpi and your printer was 300dpi, 'actual size', and '100%' would be the same physical size - i.e. measure the print, and measure the picture on the screen - they would be the same.

However, screens are typically 90-150dpi, so if you have a small, high res laptop screen for example with a dpi of 150. If you choose to view at 'actual size' the application (e.g. photoshop) will DISPLAY (note it doesn't change the image data) the image on the screen at ~50% size, so that the printed image, still matches the dimensions of the image on the screen.

The only way the print size will equal actual print size is to resize to the monitors resolution at the proposed print size. In my case 96ppi to get 50% I need twice the resolution..... and so on.

Screens do not have a res of 1500ppi......

Think about a word processor - if it is aware of yoru screens dpi, regardless of screen resolution or size, a 72pt font should always measure 1 inch (72pt) high on the screen. and shold always print at 1inch high on a bit of paper, regardless of the number of pixels it takes to actualy display that image on the screen.

I see what you say in this regard and Ps displays images the same way - the difference is the size dends on the resolution of the image. It's rarely set to the same as the monitor resolution (or multiples of it) that are scaleable easily.

 

[jaffamuffin]

The only way the print size will equal actual print size is to resize to the monitors resolution at the proposed print size. In my case 96ppi to get 50% I need twice the resolution..... and so on.

I don't quite understand but I think I get you. You are agreeing that the display on the screen is resized (to view) which it is, because the display will be lower res than a printing device.

Screens do not have a res of 1500ppi......

not any that I have seen. My screen has 100ppi, a 17" 1920x1200 laptop will have about 133ppi, so close(ish) to 150ppi. Some of the new ebook readers have 150ppi+ screens, but that's a different thing again.

I see what you say in this regard and Ps displays images the same way - the difference is the size dends on the resolution of the image. It's rarely set to the same as the monitor resolution (or multiples of it) that are scaleable easily.

The resolution of the image shouldn't/doesn't affect the size (dimensions) of the image. This is what you were saying a few posts back. What the resolution of the image does is affect the size of the display of the images, be it on a VDU or a printer.

PS. I just remembered about an old IBM screen I think, that had suc a high resolution you could actually use a loupe to see more detail on what ever it was displaying. this was a few eyars ago, trying to find a link. Was pretty impressive at the time. heres the press release. http://domino.research.ibm.com/comm/pr.nsf/pages/news.20001110_display.html

 

[EOS_JD]

I don't quite understand but I think I get you. You are agreeing that the display on the screen is resized (to view) which it is, because the display will be lower res than a printing device.

Not sure I get you either! Printing device has nothing to do with the display of the image.

If you monitor displays an image at 96dpi and you are looking at an image on your screen at 96ppi res, then that image will show at the same size as it would print at that resolution.

Change the res and you cannot view an image at 100% to show the size of the print - rarely does anyone print at 96ppi!

not any that I have seen. My screen has 100ppi, a 17" 1920x1200 laptop will have about 133ppi, so close(ish) to 150ppi. Some of the new ebook readers have 150ppi+ screens, but that's a different thing again.

Yes but who uses a 17" monitor at that resolution?... I don't see the point of this discussion though - why do you want to view your image on screen at the size you print?

The resolution of the image shouldn't/doesn't affect the size (dimensions) of the image. This is what you were saying a few posts back. What the resolution of the image does is affect the size of the display of the images, be it on a VDU or a printer.

Resolution of an image affects the PRINT SIZE - NOTHING ELSE. The number of pixels in the image (and ratio of those pixels) affects the size and how it is displayed on your LCD.

PS. I just remembered about an old IBM screen I think, that had such a high resolution you could actually use a loupe to see more detail on what ever it was displaying. this was a few eyars ago, trying to find a link. Was pretty impressive at the time. heres the press release. http://domino.research.ibm.com/comm/pr.nsf/pages/news.20001110_display.html

Interesting!! Must cost a few quid! (dollars)

 

[jaffamuffin]

Not sure I get you either! Printing device has nothing to do with the display of the image.

If you monitor displays an image at 96dpi and you are looking at an image on your screen at 96ppi res, then that image will show at the same size as it would print at that resolution.

Indeed. But what if your screen is 96ppi, and your printer prints at 300ppi, and you want to see the image at the same size it would print?

Yes but who uses a 17" monitor at that resolution?... I don't see the point of this discussion though - why do you want to view your image on screen at the size you print?

Many people. The new macbook 17" have this res for example.

Ever heard of WYSIWYG? That is the point of having the image on screen the same size as the print.

Resolution of an image affects the PRINT SIZE - NOTHING ELSE. The number of pixels in the image (and ratio of those pixels) affects the size and how it is displayed on your LCD.

UNLESS you view it at 'print size' 'real size' 'actual size', which is what I was saying. The size of the diplayed image when viewed as such will depend on your screen ppi value and your Operating sytem being aware of it. The image doesn't change size. Just the display of the image...


Say I'm doing a A4 page layout and I want to get an idea of what the document will look like before I print it - I choose view print size, and if my display subsystem is correctly configured, I could hold up a page of A4 to the screen and the display on the screen would measure 210x297 mm. If I have a higher res screen, I will get a more detailed render of the page, but it will be the same size in mm. 'Real view' 'actual size' 'print size' are all references to the Real world, the Actual size of the Printed document.

 

[EOS_JD]

Indeed. But what if your screen is 96ppi, and your printer prints at 300ppi, and you want to see the image at the same size it would print?

Why would you want to? You can't! It's impossible if the image res is higher (or lower)

Many people. The new macbook 17" have this res for example.

Ever heard of WYSIWYG? That is the point of having the image on screen the same size as the print.

But you are not printing at 96ppi (the wysiwig resizes images to a lower res to do that)

UNLESS you view it at 'print size' 'real size' 'actual size', which is what I was saying. The size of the diplayed image when viewed as such will depend on your screen ppi value and your Operating sytem being aware of it. The image doesn't change size. Just the display of the image...

My 7x5 at 300ppi is close to the size of a 6x4 on my screen....

Actual size shows the ppi mapped to dpi on the monitor and my 7x5 at 300ppi more than fills my 21" monitor.

I see what you say though.

Say I'm doing a A4 page layout and I want to get an idea of what the document will look like before I print it - I choose view print size and if my display subsystem is correctly configured, I could hold up a page of A4 to the screen and the display on the screen would measure 210x297 mm. If I have a higher res screen, I will get a more detailed render of the page, but it will be the same size in mm. 'Real view' 'actual size' 'print size' are all references to the Real world, the Actual size of the Printed document.

Maybe mine isn't configured correctly hmmmm..... I do now see more of what you say though.

 

[jaffamuffin]

But you are not printing at 96ppi (the wysiwig resizes images to a lower res to do that)

It doesn't resize the image.. It resizes the Display of the image. That is the whole point..

My 7x5 at 300ppi is close to the size of a 6x4 on my screen....

(presumably you are viewing at print size? - a 7x5 at 300ppi would be 2100 pixels wide...) - You see ! So when you look at the image on your screen and it looks and actually measure 6x4 on the screen, it actually prints at 7x5. Does that seem normal/correct to you?

Actual size shows the ppi mapped to dpi on the monitor and my 7x5 at 300ppi more than fills my 21" monitor.

confused.. (edit - yes actually I see now. It will - Actual size being in this case 100% - are you using Photoshop? 7x5 will be 2100 px wide, and your screen will only have probably 1600 wide, so yes it will fill over the edge. Some apps use actual size to mean print size, and some use print size to mean print size, and some use actual size to mean 100% etc etc.

If you haven't try this - resample your image to say 50 or 75 ppi - the pixel dimensions will change as you are resampling, then try viewing at actual and print size again. Print size should stay the same, but actual size will change)

Maybe mine isn't configured correctly hmmmm..... I do now see more of what you say though.

Huzzah!

 

[pxl8]

Gawd, you two...

Displaying an image or document at "print" size means show a scaled version so that if it was measured it would be the same size as the print.

The ability to do this has been around for a long time and can be extremely useful, esp. for design work to get a "feel" for how a layout work, how readable fonts will be, etc.

Going back to the subject of pixels, if you've got 90mins have a watch of this:

http://video.google.com/videoplay?docid=-5655850487750051532

A summary for those that don't want to sit through it:

Pixels are something you count, resolution is something you measure (counting pixels is one way). It traces the history of the word and its origins from Picture Element, one of the first uses was an early form of fax. Pixels really have nothing to do with RGB values or digital data, the are the smallest discreet element that makes up an image - or to put it another way, dots whose size depends on the system in which they are applied

 

[Paul MR]

Don't agree with your statement. If you are talking pixels in camera before the picture is taken IE: the sensor then YES it does not have anything to do with data. Once the picture has been taken and stored on to media it then becomes data.

 

[pxl8]

What if the media isn't media at all? What if it's an analog signal?

 

[Paul MR]

You are now changing the subject. The subject was digital pictures not cameras.

 

[pxl8]

I know it's not a physical dot. Surely it's a digital set of data that allows a screen to show the data as a colour/brightness dot on the screen (the screen has the dots not the digital image). Without a monitor the pixel still exists. A screen only allows the pixel to be displayed.

You are now changing the subject. The subject was digital pictures not cameras.

The subject is pixels and I was responding to the definition offered above. Pixels were around long before digital cameras had even been thought of.

 

[Paul MR]

You have taken the whole thing out of context.
But it does bring a saying to mind...
Don’t argue with idiots they will drag you down to their level and beat you with their experience.

 

[jerry12953]

Aaaaarrrgggggghhhhh........!!!!!

 

[pxl8]

.

 

[EOS_JD]

Gawd, you two...

Displaying an image or document at "print" size means show a scaled version so that if it was measured it would be the same size as the print.

The ability to do this has been around for a long time and can be extremely useful, esp. for design work to get a "feel" for how a layout work, how readable fonts will be, etc.

Going back to the subject of pixels, if you've got 90mins have a watch of this:

http://video.google.com/videoplay?docid=-5655850487750051532

A summary for those that don't want to sit through it:

Pixels are something you count, resolution is something you measure (counting pixels is one way). It traces the history of the word and its origins from Picture Element, one of the first uses was an early form of fax. Pixels really have nothing to do with RGB values or digital data, the are the smallest discreet element that makes up an image - or to put it another way, dots whose size depends on the system in which they are applied

i see what you say but don't agree they are dots.
Surely a dot must have an RGB value?

 

[EOS_JD]

What if the media isn't media at all? What if it's an analog signal?

then dots would be accurate. As soon as you scan the dots, the dots become pixels.

 

[EOS_JD]

The subject is pixels and I was responding to the definition offered above. Pixels were around long before digital cameras had even been thought of.

you don't require a camera to make a pixel.

 

[pxl8]

then dots would be accurate. As soon as you scan the dots, the dots become pixels.

And so we come to the point in hand. Early uses of the word pixels included large (I mean inches in size) "dots" created from the transmission of analogue signals. It wasn't describing the data in transit or stored somewhere, it was describe the elements that made up the printed image output at the end of the process.

Pixels aren't exclusive to digital images, the word has been around a long time.

Surely a dot must have an RGB value?

Only if you are talking about a bitmapped image. The pixels on your camera's sensor don't have RGB values.

 

[jaffamuffin]

a pixel is just a sample point. put many of them together and you get a picture. Yes it is that ambiguous. The use of the word is highly context sensitive.....

 

[EOS_JD]

Only if you are talking about a bitmapped image. The pixels on your camera's sensor don't have RGB values.

But a camera and bitmap images are use pixels not dots