On Jun 25, 7:26 pm, Terry Reedy <[EMAIL PROTECTED]> wrote: > idiolect wrote: > > Hi all - Sorry to plague you with another newbie question from a > > lurker. Hopefully, this will be simple. > > > I have a list full of RGB pixel values read from an image. I want to > > test each RGB band value per pixel, and set it to something else if it > > meets or falls below a certain threshold - i.e., a Red value of 0 > > would be changed to 50. > > > I've built my list by using a Python Image Library statement akin to > > the following: > > > data = list(image.getdata()) > > > Which produces a very long list that looks like [(0,150,175), > > (50,175,225),...]. I'm trying to figure out a fast and pythonic way > > to perform my operation. The closest I've come so far to a succinct > > statement is a list comprehension along the syntax of: > > Why are you trying to do this with a list comprehension? Learn the > basics first. Perhaps you have read too many of the recent threads > presenting diverting challenges for bored experienced programmers. Some > of these were definitely not Pythonic code for real use. > > First question: do you really want to create a new 'very long list' or > modify list 'data' in place. Let's assume the latter. > > for i,tup in enumerate(data): > data[i] = replace(tup) > > where replace(tup) is an expression or function that produces a tuple > meeting your criteria. Simplest is > (max(tup[0],Rthresh), max(tup[1],Gthresh), max(tup[2],Bthresh)). > > If nearly all your pixels are ok, add the following before the > assignment so you only make replacements when actually needed: > if tup[0] < Rthresh or tup[1] < Gthresh or tup[2] < Bthresh: > > Terry Jan Reedy
A giant thank-you to all who've posted in response to my query - these are all much better approaches to my problem. I think I got hooked on using a list comprehension as it seemed the most concise approach vs. other techniques after a bunch of Google searches, but all of you have pointed out more efficient methods. I appreciate your willingness to indulge a n00b who hasn't thought his problem through, apparently. I'll try all of these approaches out over the next day and see what works best, although I suspect you've all posted sufficient solutions. Can't wait to try these suggestions out - cheers, idiolect -- http://mail.python.org/mailman/listinfo/python-list
