[RESOLVED] Bitmap Pixels Per Inch/Centimeter.

[BlindSniper]

Hi all, I have scanned a few img's on 600 DPI. When i load them into a Bitmap class They have Vertical and Horizontal Resolution of 96 PPI To my calculation that is very close to 38 Pixels per centimeter. Now the image that I scanned is 4.5 Centimeter (1.77165354 Inch) In Diameter but When I measure It the 4.5 cm img is close to 1080 pixel in Diameter which is 240 Pixels/Centimeter (609.6 Pixels/inch)

What is the correct way to get the actual Pixels per inch/centimeter from a bitmap or was it set incorrectly by my scanner ?

[Poppa Mintin]

I have no idea, but I look at it this way: -
You say the scanned image is 4.5 Centimetres diameter (1.77165354 Inch)
600 DPI is (say) 236.22 DPcm. So 4.5 x 236.22 would be (say) 1063 Dots.
Dots notice, not pixels. I believe 'Dots' is a measure of printer imaging rather than BitMap imaging.

I reckon that if you expect DPI to be Pixels/Inch and your scanner produces an extra 1.6% that's not too far out surley?

1: It has to 'look' through glass plate from maybe 5 or 6 Millimetres.

2: It has to rely on light reflected from the target, that light has to travel to the upper surface of the glass and back again, so any refraction is going to be exacerbated.

Also I wonder how you're measuring your target? how accurate is whatever you're measuring it with ?

Last thought, any half decent image work tool is going to be able to resize your image to however many pixels wide / high your want to end up with as well as however many PPI you would like.

Poppa.

[BlindSniper]

I had to make the tool myself that measures pixels because apparently no one wants to know big objects are in pixels! but im fairly sure my 1080 and 4.5 cm is correct. But my image Definitely is between 1075 and 1081 Pixels and my img in real life Definitely is 4.5 cm

[Poppa Mintin]

Personally I use Paint Shop Pro (still using V 9) for all my image work and re-work. Once an image has been loaded into it, a simple click on 'Image Information' displays, along with a great deal of other information...

Dimensions Pixel - Pixel
Dimensions Inch - Inch
Pixels Per Inch
Pixel depth / colours
Image size on disc
Image size in RAM

Poppa

[.paul.]

[graphics].dpiX + [graphics].dpiY are pixel values. to convert from dots per inch to dots per cm, divide by 2.54

[BlindSniper]

That is Exactly the same as bitmap.verticalresolution

[Poppa Mintin]

Out of interest I scanned in a 6" steel rule with imperial and metric scales, scanned both horizontally and vertically: -

Scanner:   Epson Stylus PHOTO RX685 Multi purpose printer.
Programs:  Epson Scan.  Photo Shop Pro 9.
Settings:  600 dpi.  24 bit colour.

Measured in Horizontal Plane: 45mm = 1064 Pixels
              Vertical Plane: 45mm = 1065 Pixels

Allowing for the fact that at this scale the machined increments of the rule are about 6 pixels wide (high) and the start edge is not too clearly defined either,
I'd say that the result isn't a million miles from the (rough) calculation in my post 2, which maybe brings us back to your apparent distrust of your scanner.

Poppa.


Oh! PS I used .jpg to save the images, but I don't think that should make any difference, especially since I took the measurements before I saved the scans.

Poppa.

PPS. The hardest part was getting the X and Y axes of the rule exactly horizontal and vertical on the scanner bed.

Poppa.

[BlindSniper]

hmm so it seems my scanner scann's the img correctly but it doesn't correctly set the file attributes. Thanks