Computing question

Turtles · 16 Apr 2009 at 12:26

Some guy wants to make some copies of photographs. He intends to use a scanner to capture the photographs and an ink jet printer to print the images.

A scanner i set to a resolution of 1200dpi using 24 bit colour depth and the photographs are 6 inches by 8 inches. Calculate the uncompressed size of the file. Express your answer in appropriate units.

The answer is supposed to be 197.8 MB but I'm struggling to work out how you'd get there.

I was thinking it was something like 1200 * (6*8) * 24

then dividing by 8 and then 1024 and 1024 again to get it in MBs, but I don't get the 197.8 MBs that's in the answers.

I know I'm missing something simple out so any help much appreciated :s

Burnsy2023 · 16 Apr 2009 at 12:32

I am also confused by this. I would have done what you put. ((1200x24)x48) and then convert from bits to bytes (divide by 8). I get about 172KB

p4radox · 16 Apr 2009 at 12:39

8 inches * 1200dpi * 6 inches * 1200dpi as the dpi scale is linear. It's not dots per square inch.

Then multiply by 24.

Then divide by 8 to get the answer in bytes.

then divide by 1024^2 to get the answer in Mb.

this gives you 197.8Mb.

However, this is a badly worded question. It should be stated that it's only a monochrome image. In real life each dot would be stored as 3 colour intensities (RGB) so the uncompressed file size would be 3 times larger.

edit: so to get the answer it's:
8*1200*6*1200*24/(8*1024*1024) = 197.8Mb

domthecondom · 16 Apr 2009 at 12:42

tell your teacher he is WRONG

VeNT · 16 Apr 2009 at 12:47

1200 pixels an inch
48 inches on image
57,600 pixels total
24 bits a pixel
1,382,400 bits total
172,800 bytes
172KB

doesn't sound right.
do we times 3 for RGB?

so the pixels gets trippled

edit, beaten like a ginger stepson! that will teach me to make a cuppa while posting!

SoapSurgeon · 16 Apr 2009 at 12:48

But 24 bit colour depth is 3 bytes... so isn't that RGB?

9600x7200 (resolution) x 3 bytes = same answer.

Could be completely wrong of cours

Blackvault · 16 Apr 2009 at 12:49

p4radox said:
8 inches * 1200dpi * 6 inches * 1200dpi as the dpi scale is linear. It's not dots per square inch.

Then multiply by 24.

Then divide by 8 to get the answer in bytes.

then divide by 1024^2 to get the answer in Mb.

this gives you 197.8Mb.

However, this is a badly worded question. It should be stated that it's only a monochrome image. In real life each dot would be stored as 3 colour intensities (RGB) so the uncompressed file size would be 3 times larger.

edit: so to get the answer it's:
8*1200*6*1200*24/(8*1024*1024) = 197.8Mb

Yes is correct, or at least sounds it. I work in a photolab and taking the size of the image, dpi etc into consideration it sounds roughly right.

Blackvault

SoapSurgeon · 16 Apr 2009 at 12:50

VeNT said:
1200 pixels an inch
48 inches on image
57,600 pixels total
24 bits a pixel
1,382,400 bits total
172,800 bytes
172KB

doesn't sound right.
do we times 3 for RGB?

it is dots per inch not dots per square inch. Therefore for every inch wide there are 1200 pixels and for every inch down there are 1200 pixels - so i worked it out at 9600x7200 resolution. Then multiply this by 24 bits for colour and then divide by whatever that other guy said to convert to MB

Burnsy2023 · 16 Apr 2009 at 12:51

I see. That makes more sense.

kaiowas · 16 Apr 2009 at 12:52

6x8 image at 1200 dpi gives an image size of 6x1200 by 8x1200 (7200 by 9600 pixels)

7200x9600 = 6912000pixels = 65.918megapixels
Each pixel is stored as 24 bits (ie 3 Bytes) so filesize is 65.918 x 3 MegaBytes
65.918 x 3 = 197.754MB

VeNT · 16 Apr 2009 at 12:54

SoapSurgeon said:
it is dots per inch not dots per square inch. Therefore for every inch wide there are 1200 pixels and for every inch down there are 1200 pixels - so i worked it out at 9600x7200 resolution. Then multiply this by 24 bits for colour and then divide by whatever that other guy said to convert to MB

hmmm interesting if true.
strage how I've never seen a scanner listed at 1200x1200 as 1200DPI.

Helium_Junkie · 16 Apr 2009 at 12:55

You were incredibly close

Its just that 1200dpi means 1200 each side, so an actual dp sq i of 1440000.
Which makes quite the difference

Turtles · 16 Apr 2009 at 13:02

Thanks guys

yer_averagejoe · 16 Apr 2009 at 13:06

yup, 24 bit colour is RGB8 - one byte for each component.

Helium_Junkie · 16 Apr 2009 at 14:11

VeNT said:
hmmm interesting if true.
strage how I've never seen a scanner listed at 1200x1200 as 1200DPI.

Well, many photo prnts are 300dpi, try to picture how that would look if it was merely 300 per square inch...
Would be like a mini rasterbation

matt_fsr · 16 Apr 2009 at 14:28

Helium_Junkie said:
Would be like a mini rasterbation

It's a good job you don't have Dyslexia

tripitaka · 16 Apr 2009 at 15:06

SoapSurgeon said:
But 24 bit colour depth is 3 bytes... so isn't that RGB?

9600x7200 (resolution) x 3 bytes = same answer.

Could be completely wrong of cours

Exactly, 8 bits each for the R, G, and B components.

Energize · 16 Apr 2009 at 16:00

p4radox said:
However, this is a badly worded question. It should be stated that it's only a monochrome image. In real life each dot would be stored as 3 colour intensities (RGB) so the uncompressed file size would be 3 times larger.

The image is not monochrome, an rgb24 format means 24-bits are used to represent an rgb colour value, 2^24 = 16.7 million colours. If it was monochrome you would multiply by 2 instead of 24.

FrostedNipple · 16 Apr 2009 at 16:07

Hmm, i was always under the impression that images of the same size could be different file sizes, shows how much i know

, although i would be able to do the copies because i have an 8Gb mem stick that can store ANYTHING!

Energize · 16 Apr 2009 at 16:10

FrostedNipple said:
Hmm, i was always under the impression that images of the same size could be different file sizes, shows how much i know , although i would be able to do the copies because i have an 8Gb mem stick that can store ANYTHING!

Uncompressed images of a given resolution will always be the same size if the format is the same.