Post by thread:[OT]: Megapixels rerquired for document storage

I wondered what digital camera resolution I required to archive documents of various sizes. This is simply stating the numerically fairly obvious in a way which may be useful to others. (Many could derive this themselves in 2 shakes of a lambs tail (if NZers :-) ) - this saves the shaking.) SUMMARY This assumes the camera image is in approximately "A" format (long side is 1.414 x short side) 1. Megapixels required to store an image of width W and "A" format aspect ratio (A4, A5 etc) MP = 1.4 x W^2R^2/1000000 ... [1] W = width in units of your choice R = resolution in pixels per unit of width MP = megapixels Rearranging 2. Resolution required in pixels per unit width for an A format document of width W R = sqrt [ (MP x 1000000) / (1.4 x W^2) ] ... [2] or A4 sheet, 1 MP ==> 105 dpi A4 sheet, 3 MP ==> 300 dpi Conclude that 3 MP is fairly desirable if a camera is to be used for this purpose. ___________________________________ Discussion: An A4 sheet (or any sheet in the A series) has a height which is sqrt(2) = 1.414 greater than its width. If short side has length L then area is L x L x 1.4 For a resolution of R dots per unit length (lets start with dots per inch = dpi as it's commonly used) then total dots = (L x R) x (L x R) x 1.414 = 1.414 x L^2R^2 = number of pixels If this page is digitised this becomes the number of pixels resolution needed for a given L and R or, as more commonly expressed, if we divide by 1 million, the megapixel rating required. Megapixels = MP = 1.4 x L^2R^2/1000000 ... [1] Rearranging R = sqrt [ (MP x 1000000) / (1.4 x L^2) ] ... [2] (1,000,000/1,414 = 707,000 near enough Equations 1 & 2 are useful for determining either how good a "camera" we need to archive a given document or how good a job a given "camera" will do. A standard A4 sheet is 8" wide. How many DPI will I get in a 1 MP image? R = sqrt (707000/64) = 105 dpi This is about poor fax quality What about with a 3 MP camera A 3 appears inside the sqrt so the figure increases by root(3) = 1.732 to give about 300 dpi This is early laser quality but without the edge rounding and line smoothing used in all modern laser printers (added first by HP AFAIK.) ie a 3 megapixel camera will provide a reasonable copy of an A4 document but the quality will still be less than ideal. A 3 MP image saved at 1 byte per pixel (8 bit grey scale) requires 3 MByte storage per image with no compression. JPEG will reduce this to 100k-300k for a typical document with not too much quality loss. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body