[Haskell-cafe] Right way to implement setPixel function

[CK Kashyap]

thank you very much Job.
Regards,
Kashyap

On Thu Aug 20th, 2009 1:13 PM EDT Job Vranish wrote:

>Opps:
>setPixel = State setPixel'
>
>should be:
>setPixel x y rgb = State $ setPixel' x y rgb
>
>- Job
>
>On Thu, Aug 20, 2009 at 1:05 PM, Job Vranish <jvranish at gmail.com> wrote:
>
>> Your setPixel function is almost ready to work in a State monad
>> If you modify your setPixel function slightly like so:
>>
>> setPixel' :: Int -> Int -> Color -> B.ByteString -> ((), B.ByteString)
>> setPixel'  x y (r,g,b) image = ((), B.concat [beforePixel, pixel,
>> afterPixel])
>>
>> and then wrap it in the State monad constructor:
>>
>> setPixel = State setPixel'
>>
>> then you can do
>>
>> drawPixels = do
>>   setPixel 5 10 (200, 0, 0)
>>   setPixel 20 1 (0, 200, 0)
>>   setPixel 90 2 (0, 0, 200)
>>
>> modifiedImage = execState drawPixels originalImage
>>
>> See! you were already using a monad and didn't even know it! :D
>>
>> Performance wise,  B.concat is O(n), which is very not good for your
>> purpose. It copies the whole string and the optimizer won't be able to
>> magically make it go away. For something that works in O(1), you will have
>> to use something like STArrays instead of bytestrings.
>>
>> - Job
>>
>>
>>
>> On Thu, Aug 20, 2009 at 2:32 AM, CK Kashyap <ck_kashyap at yahoo.com> wrote:
>>
>>> Hi,
>>> I had posted a note on line drawing algo with Haskell some time back. Now,
>>> I am trying to write a PNM image.
>>>
>>> import qualified Data.ByteString as B
>>>
>>> width = 256
>>> height = 256
>>> bytesInImage = width * height * 3
>>> blankImage =  B.pack $ take bytesInImage (repeat 0)
>>>
>>> type Color = (Int,Int,Int)
>>> setPixel :: B.ByteString -> Int -> Int -> Color -> B.ByteString
>>> setPixel image x y (r,g,b) = B.concat [beforePixel, pixel, afterPixel]
>>>         where
>>>                 beforePixel = B.take before image
>>>                 afterPixel = B.drop (before+3) image
>>>                 pixel=B.pack [(fromIntegral r),(fromIntegral
>>> g),(fromIntegral b)]
>>>                 -- number of bytes before the 3 bytes of
>>>                 -- the pixel at x y
>>>                 before = (y * width * 3) + (x * 3) - 3
>>>
>>> main = do
>>>         putStrLn "P6"
>>>         putStrLn ( (show width) ++ " " ++ (show height) )
>>>         putStrLn "255"
>>>         -- Set a red pixel at 100 100
>>>         B.putStr (setPixel blankImage 100 100 (255,0,0))
>>>
>>>
>>> Can I please have some review comments on the code above? Would recreating
>>> the entire ByteString for each setPixel be an overhead?
>>> Also, I am barely beginning to grasp the Monad concept....I was wondering
>>> if there could be a monadic style of implementation of this - that could
>>> potentially have a series of setPixels inside a do block?
>>>
>>> Regards,
>>> Kashyap
>>>
>>>
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>>