a representative positive - going edge waveform 21 usually has rounded corners ( fig1 ) or angled sides . nevertheless if its convolution is formed with a test function that is a square - cornered vertical jump 11 , and that is aligned as shown with the jump 21 , the convolution responds with a positive output value . such a response can be used , as will be detailed below , to find edges and thereby to recognize text elements . in the real world , however , a row of scanned - in image data typically has more than one feature 21 , and it is necessary to isolate the effects of the different features 21 from one another . this can be accomplished by using a slightly more sophisticated test function 31 ( fig2 ) that also incorporates a window -- consisting of two other vertical jumps 32 , each smaller than , and oppositely directed to , the central jump . the heights of the two smaller window - end jumps 32 typically add up to the height of the central jump , and the window - end jumps are spaced by some number of pixels c , preferably about three pixels as shown ( but not necessarily symmetrically disposed ), to left and right of the central jump . as suggested above , a positive response occurs only if , as shown , the two waveforms 21 , 31 are aligned . in practice the vertical jump 31 is stepped , pixel by pixel , along a row of input - image pixels 121 ( fig3 )-- and as the convolution approaches 122 ( and leaves 124 ) a pixel t1 where there is alignment with an edge 123 , the convolution ideally responds only with zero output 128 . thus ideally the point of alignment is identified as a positive - going square - cornered peak 127 , one pixel wide , in the convolution output signal 127 - 129 as a function of position along the pixel row . in practice , however , because the pixel structures of the scanner and original image ( if indeed the latter was created by a pixel - based system ) are in general not perfectly aligned , the peak 127 will extend over more than one scanner pixel width -- and will not be square - cornered . similarly a peak 129 will occur if the vertical jump 31 is convolved with an edge waveform 125 that is negative - going ( and again only at the moment , or position t2 along the pixel row , at which the two waveforms 125 , 31 are aligned )-- but in this case the peak 129 in the convolution 127 - 129 , too , will be negative - going . the actual numerical processes required to perform the convolutions are extremely simple and well documented , and can be implemented in firmware by a competent programmer without further explanation . they are also quite fast : if the window interval c ( fig2 ) is set to three at both sides of the central jump , all three convolutions can be done with only twelve additions and subtractions per pixel -- four per chromatic primary ( red , green and blue ). only a single row of buffering need be used . for such purposes each plane of an image that has been scanned in may be taken as f ( x ), where x is pixel position along each row , and with values typically in the range from zero through two hundred fifty - five ( i . e ., assuming eight - bit processing ). the vertical jump is taken as h ( x )=- 1 for x & lt ; 0 , and h ( x )=+ 1 for x & gt ; 0 , given that | x |& lt ; c , and otherwise h ( x )= 0 -- all just as shown in fig2 . the convolution then is given as a sum -- ## equ1 ## which will be found to entail only addition and subtraction , no nontrivial arithmetic multiplication since | h | is everywhere either unity or zero . the convolution also will be found to require a number of such additions and subtractions equal to only 2c - 2 for each plane -- because no calculation at all is done where the jump function h is undefined , at 0 , - c and + c . this number of calculations , if c is set to three , is 2c - 2 = 4 per plane , or for example twelve total , as mentioned earlier . the spacing c is subject to optimization for the types and sizes of text ( and other features ) that are anticipated , and therefore can be controlled indirectly in response to general operator settings if desired . if preferred for greater speed of actually performing the convolutions , or to partially release a central system processor for other tasks , the procedures just described can be implemented in custom hardware -- such as , for example , an application - specific integrated circuit . as will be appreciated by those skilled in the art , such implementation should be within the capability of a competent senior electronics designer . the waveforms 121 , 127 - 129 at upper right in fig3 may be taken as the response of a blue - sensitive photodetector 72b ( fig5 ) to the black numeral &# 34 ; 1 &# 34 ; which is represented by dense shading 1k at upper left in fig3 . it is to be understood that fig3 is somewhat schematic or conceptual . in actuality a text numeral in most practical fonts will be occupy at least perhaps five to ten times more pixel columns than suggested in the drawing -- since modern pixel grids are typically very fine , namely some 1 / 12 or 1 / 24 millimeter ( 1 / 300 or 1 / 600 inch ) per step . the vertical scale is somewhat more realistically labeled to suggest that each row of squares in the drawing represents ten pixel rows rather than one . in any event the drawing suffices for purposes of describing the invention . now when a scanner 72r / 72g / 72b ( fig5 ), after passing the blank pixel rows 490 through 510 ( fig3 ), reads 41 ( fig4 ) image rows 520 , and when these latter rows are convoluted 43r / 43g / 43b ( fig4 ) within the searching means 73 ( fig5 ) with the windowed vertical jump 31 ( fig2 ), the response at pixel columns t1 through t2 ( fig3 )-- and just before and after those columns -- is very much as indicated at right in fig3 for rows 570 . the same holds true at rows 550 . for the intermediate rows 530 and 540 , the same statement still holds , except that the leading edge is farther to the left than at 123 , so that the width of the plateau is greater than at 124 . the resulting positive - going peak in the convolution is further to the left than 127 , and correspondingly the central region of the convolution somewhat longer than the region between 127 and 128 . in all these cases , down through rows 560 inclusive , precisely the same responses are to be expected for a green - sensitive 72g ( fig5 ) or a red - sensitive detector 72r . hence in these simple cases the first forty pixel rows of the black numeral &# 34 ; 1 &# 34 ; 1k are very easily recognized as black - on - white text by the coincidences detected 44 in the analyzing means 77 ( fig5 ) in the convolution - based edge - detection system described above . the conditions that dark pixels be found between peak ( e . g ., 127 ) and trough ( e . g ., 129 )-- and light pixels between a trough and the next following peak ( not shown in fig3 )-- are likewise met . inspection for satisfaction of the peak / trough , trough / peak conditions is readily performed 46 , 47 ( fig4 ) during the process of reading in the image and doing the convolutions ; and the required coincidence alternations too can be tested 45 in close association with the coincidence detection 44 itself . when the peak / trough , trough / peak tests 46 , 47 and coincidence alternation test 45 are all found 51 to be satisfied , the system in effect declares 52 that a candidate text segment is being examined . this decision of course cannot be made until a trough or negative - going peak 129 is encountered -- but is then made retroactive to the most recent positive - going peak . the uppermost portion of the numeral 1k is also surrounded , or to be more precise bordered on three sides , by great numbers of white pixels -- and so easily satisfies the pixel - mix condition too . preferably , however , for reasonable efficiency in extending recognition of a relatively long run of text , this latter condition is not tested 55 until after the system has recognized 53 the end of a pixel row or the encountering 61 of a pictorial region ( discussed later ). thus after a candidate text segment is started 52 the system cycles back 54 to read more data , continuing to read and evaluate 42 - 51 those data until the end of a row is encountered or a pictorial region is encountered . actually in cycling through additional data regions after a candidate text segment has been started , as will be appreciated the test at 51 works somewhat differently : now , encountering a coincidence failure does not invalidate the entire candidate text segment up to that point . rather the system first concludes processing of the candidate text by temporarily skipping 52 &# 39 ; to the &# 34 ; end of row &# 34 ; test 53 ( which here instead merely recognizes the end of the candidate text segment ), then testing 55 for mix of pixels etc . as before , and printing 62 either with or without enhancement 56 . at this point , if the mix of pixels were found inadequate for recognition of text , the candidate text segment previously declared open 52 would be disqualified . for the top of the numeral 1k , however , the pixel - mix test 55 yields acceptable results . therefore the text identifying means 78 order the desired enhancement 56 to be performed by the text enhancing means 79 . next the enhanced image portion is directed to the printer 81 ( fig5 ) for printing 62 . after concluding that work with the previously open text segment , the system returns to the negative output line n from the both - conditions - met test 51 , which line leads to printing 62 of the newly read region -- without enhancement . the searching means 73 , coincidence analyzing means 77 , text identifying means 78 , and text enhancing means 79 may all be implemented as a part of a unitary integrated - circuit digital microprocessor 82 . as mentioned earlier , however , the searching and coincidence analyzing means 73 , 77 are alternatively implemented in custom hardware . the same large numbers of white pixels , in the rows 520 - 540 under discussion , also meet the condition that the system is not examining a pictorial element . that fact is recognized 42 early - on by presence of more than just a very few light pixels . thus without qualification the system or procedures of the present invention recognize rows 520 through 550 as textual , and based on this recognition can readily proceed to enhance those rows by the enhancement protocol mentioned earlier . if the black numeral 1k were the only feature in the representative image of fig3 then the waveforms 121 , 127 - 129 could be regarded as the response of a photodetector with any color sensitivity . in fig3 however , additional green leafy shapes 1g and analogous red shapes 1r are assumed to be closely associated with the central and lower portions of the numeral 1k , as at pixel rows 560 through 620 . thus a waveform showing response of a green - sensitive detector at pixel rows 560 would have a considerably broader plateau , incorporating the detector response to one of the &# 34 ; leaves &# 34 ; 1g which touches the left side of the numeral 1k . such a waveform would also include another , separated plateau representing response to another one of the leaves 1g which is drawn spaced away from the right side of the numeral 1k . these responses , however , would be very low or zero in the regions of the red shapes 1r ; therefore the green - sensitive detector would register solid color to only one side , not both , of the numeral 1k . similarly a red - sensitive detector would produce a waveform with an additional , separate plateau to the right representing the upper - right red shape 1r , and a conjoined broader plateau toward lower - left representing the lower - left red shape 1r . as drawn , these shapes thus preclude coincidence of detected edges at rows 560 and 600 on the left side of the numeral 1k , and at rows 600 and 610 at both sides . therefore the coincidence detection 44 ( fig4 ) fails ( and with it necessarily the peak - to - trough and trough - to - peak tests 46 , 47 ). accordingly from the negative n output line of the coincidence test 44 a path is followed directly to the print function 62 . in other words the system cannot enhance these particular portions of the text element 1k , and the printer 81 ( fig5 ) prints them without enhancement . generally speaking , as fig4 shows , a negative result at any one of the major tests 44 - 47 , 51 , 55 results in a similar program flow to the printing function 62 -- bypassing text enhancement 56 . at all or part of rows 570 through 590 , however , white spaces 1w included very near the black numeral 1k enable the system to find the necessary edge coincidences -- and also to pass the tests 45 , 46 - 47 , 42 for ( 1 ) black - and - white pixel mix , ( 2 ) dark pixels between peak and trough , and ( 3 ) absence of a pictorial element . with respect to at least the first of these latter three tests -- and one or more of the others as well , if desired -- as can be seen the number of light pixels is relatively very small . the system surmounts this challenge , however , by virtue of applying the historically controlled sliding - scale threshold mentioned previously . because text was recognized 55 previously ( though not immediately preceding ) at rows 520 through 550 , the system here uses -- as the number of light pixels required for satisfaction of the test or tests -- a relatively very low threshold . to accomplish this , the same mix - of - pixels test 55 in addition to commanding enhancement 56 also sends a status feedback signal 57 to a small buffer memory 58 . this first - in / first - out buffer , in turn , directs a delayed threshold - control status feedback signal 59 to a variable threshold control block 64 . this block in turn feeds back different threshold signals 67 to the mix - of - pixels test 55 -- and if desired even further back up to the peak / trough , trough / peak tests 46 , 47 too , as shown at 66 , 65 . if desired , variable thresholding may also be applied to the few - light - pixels test 42 shown near the top of fig4 . ( another variant within the scope of my invention is to modify the definitions of pixel darkness / lightness used for the various thresholds -- instead of , or in addition to , modifying the pixel counts .) the number of light pixels found at rows 570 - 590 , while small , is well above zero and exceeds the low test threshold imposed by the feedback loop 57 - 59 , 64 - 67 . in these regions the system therefore can correctly identify the textual element 1k , and can proceed to enhance it . if there were only just a very thin white border at both left and right of the character 1k , the variable threshold technique would enable the invention to recognize and enhance the numeral 1k in rows 560 / 600 - 610 too . as a practical matter , the visual system of a person viewing the finished drawing tends to integrate the enhancement effects over such relatively very small gaps where the character is printed in at least gray but not actively enhanced . nevertheless , it is possible to implement broad aspects of my invention in such a way as to minimize occurrrences of text nonrecognition in situations such as rows 560 , and rows 600 through 610 . for example , my invention can be practiced to define and find &# 34 ; text &# 34 ; as regions of coincidence for any two of the three color planes , rather than requiring all three . yet another variant is to enable such finding of &# 34 ; text &# 34 ; for coincident edges in any two planes only in cases of reduced threshold , just as described above for pixel darkness or pixel count . thus for instance the system may be programmed to recognize &# 34 ; text &# 34 ; upon finding of coincident edges in any two planes , if text was found in recent ( even though not immediately preceding ) pixel rows ; and even to recognize &# 34 ; text &# 34 ; upon finding of coincident edges in just one plane , e . g . where aligned text was found immediately preceding . still other variants include specifying particular color planes for use in these various circumstances . for example , a decision might be made to rely upon edges found only in signals from a green - sensitive detector . contrariwise , there is some basis for a converse rule : edges found only in signals from a red - sensitive detector may be deemed to represent an image region where black text without enhancement is particularly difficult to visually distinguish in appearance from adjoining color materials . thus black may be readily visible next to blue or green , since the eye is relatively sensitive to those colors . under this philosophy there may be minimal need for enhancement in such regions . edges found only in signals from a red - sensitive detector , however , may be deemed to represent a region that is particularly needful of enhancement -- for here only indistinct borders otherwise appear to the human eye , which is relatively insensitive to red . in any event , as can now be appreciated a number of varying implementations of the invention are practical and readily accomplished . these are considered within the scope of certain of the appended claims . another black character , an alphabetic letter &# 34 ; d &# 34 ;, is seen in dense shading dk at rows 660 through 750 , spaced downward from the numeral 1k in the drawing . at right are the resulting waveforms d21 , d27 - d29 , with features identified by callouts having the prefix &# 34 ; d &# 34 ; instead of &# 34 ; 1 &# 34 ;; the remainder of each callout is the same as for the corresponding callout in the waveforms above for the character 1k . the waveform features here include an image signal waveform d21 , with a broader plateau d24 arising from the broader top stroke of the character dk , and corresponding convolution waveform d27 - d29 with its broader intermediate zero segment d28 . some of the red shapes 1r -- associated with the numeral 1k -- extend downward from that numeral and into the leftward portion of the vertical space between the bottom of the numeral 1k and the top of the character dk . ( these are not manifested in the waveforms d21 , d27 - d29 from the blue - sensitive detector .) these nearby red shapes 1r tend against recognition of the top of the letter dk at row 660 . furthermore row 660 is now relatively remote from the last incidence of text that was actually recognized , namely at row 590 ; therefore the variable threshold , too , will tend against recognition of the top of the letter . nevertheless the black &# 34 ; d &# 34 ; letter dk should be readily recognized by virtue of the ample number of nearby white pixels and the cleanly isolated edges in all three planes . therefore this character too is identified as text and accordingly printed with enhancement . an example of an opposite kind of image material appears at rows 770 and following . here the floral shapes pg , pr , pb in green , red and blue clearly constitute -- for the human eye -- pictorial regions . the present invention , too , will recognize them as such , based upon the absence of light - colored pixels starting at row 780 and continuing through rows 800 . the relatively small incursion of white pixels pw in row 810 , being considerably smaller than a textual feature ( as seen in characters 1k , dk above ), would not likely result in recognition of a black feature as &# 34 ; text &# 34 ; even if a black detail were present in the region . preferably the invention performs this simple , quick very - few - light - pixels test 42 before starting into the several more - elaborate steps 43 - 61 discussed above . this sequence has an important advantage . a positive result of this test 42 means a positive result as to presence of pictorial elements , and therefore in effect a negative finding as to the presence of text . this finding is passed along the positive output line y from the few - light - pixels test 42 to a decision 61 that there should be no text enhancement . from this decision block 61 are issued two commands , as shown : one straight down to the print function 62 , and the other line 63 directly back up to the read - image function 41 . in this way the option of text enhancement is preserved without wasting much processing time in clearly adverse situations . in other words , pictorial - region determination overrides text enhancement , and leads to omission of all the testing associated with text enhancement . classification of a region as pictorial is passed down to corresponding pixels of subsequent rows -- until there are more light pixels . pictorial - region classification tends to limit the overall processing to enhancing black text on a white region ; but this limitation need not be rigid , and as suggested earlier the invention is amenable to variation and optimization of conditions for different types of images or different desired results . in particular the invention straightforwardly manages black text on different types of printing media , because the parameters used in the algorithms may be made variable rather than constant . these variables are functions of the lightness / darkness of the printing medium . preferably in practice of this invention an additional step is performed to tie together adjoining textual regions -- that is , pixel sequences that meet all the conditions through the pixel - mix test 55 . if the regions between textual regions again have the right mix of pixels , then the two textual regions are made into one . this step obviates the need to detect top and bottom edges -- time consuming due to required buffering . for instance , the top of a letter &# 34 ; t &# 34 ; often consists entirely of pixels that are intermediate between dark and light -- denying the system well - defined edges at either side . such a top edge does not belong to a textual region , as determined by previously described forms of the invention . by tying adjoining textual components together , the top edge of the &# 34 ; t &# 34 ; and other like textual components are made parts of textual regions and so can be enhanced . discussion of the invention in much of this document refers to colors generally -- resolution of a scanned - in image into three colors , coincidence of all three colors in a three - color system , or of plural colors or a single color , etc . most commercial scanner systems , however , now operate on the basis of separate subsystems for detecting red , green , and blue ( rgb ) light from an image to be scanned . the invention is very straightforwardly practiced in this form . therefore , solely for the sake of definiteness and relative simplicity of the disclosure , the foregoing &# 34 ; detailed description &# 34 ; passages explicitly refer to resolution , coincidence etc . in rgb terms . the invention , however , is not in the slightest limited to rgb implementations . to the contrary , the invention is entirely amenable to operation in other plural - chromatic - color coordinate systems , whether or not the color system used for initial scanning operations is rgb . practice of the invention is also particularly straightforward upon conversion of scanned - in color inputs into color - coordinate systems in which the nonchromatic component is a separate dimension , separately managed . such systems include for example the hue - plus - gray ( hpg ) or hnk system , and the better - known lab systems . with color specified in any of these ways , it is necessary to test for left and right edges in only the nonchromatic constituent -- namely , k ( in hpg ) or l ( in lab or the like ) alone . for certain forms of the invention as discussed above this mode of implementation may be substantially more convenient or efficient . the above disclosure is intended as merely exemplary , and not to limit the scope of the invention -- which , rather , is to be determined by reference to the appended claims .