Bitmap/Histogram: Difference between revisions
Image histogram |
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Revision as of 18:36, 7 December 2008
You are encouraged to solve this task according to the task description, using any language you may know.
Extend the data storage type defined on this page to support image histograms. The image histogram contains for each luminance the count of image pixels having this luminance. Choosing the histogram data format take care about the data type used for the counts. It must be at least NxM, where N is the image width and M is the image height.
Histogram is useful for many image processing operations. As an example, use it to convert an image into black and white art. The method works as follows:
- Convert image to grayscale;
- Compute the histogram
- Find the median of. The median is defined as the luminance such that the image has an approximately equal number of pixels with lesser and greater luminance.
- Replace each pixel of luminance lesser than the median to black, and others to white.
Use read/write ppm file, and grayscale image solutions.
Ada
Histogram of an image: <ada> type Pixel_Count is mod 2**64; type Histogram is array (Luminance) of Pixel_Count;
function Get_Histogram (Picture : Grayscale_Image) return Histogram is
Result : Histogram := (others => 0);
begin
for I in Picture'Range (1) loop for J in Picture'Range (2) loop declare Count : Pixel_Count renames Result (Picture (I, J)); begin Count := Count + 1; end; end loop; end loop; return Result;
end Get_Histogram; </ada> Median of a histogram: <ada> function Median (H : Histogram) return Luminance is
From : Luminance := Luminance'First; To : Luminance := Luminance'Last; Left : Pixel_Count := H (From); Right : Pixel_Count := H (To);
begin
while From /= To loop if Left < Right then From := From + 1; Left := Left + H (From); else To := To - 1; Right := Right + H (To); end if; end loop; return From;
end Median; </ada> Conversion of an image to black and white art: <ada>
F1, F2 : File_Type;
begin
Open (F1, In_File, "city.ppm"); declare X : Image := Get_PPM (F1); Y : Grayscale_Image := Grayscale (X); T : Luminance := Median (Get_Histogram (Y)); begin Close (F1); Create (F2, Out_File, "city_art.ppm"); for I in Y'Range (1) loop for J in Y'Range (2) loop if Y (I, J) < T then X (I, J) := Black; else X (I, J) := White; end if; end loop; end loop; Put_PPM (F2, X); end; Close (F2);
</ada>