Re: Using PNG_IO to create a file.

[Ron Baldwin <no-spam-rebaldwin@adelphia.net>]

When I set thiings up this way, the compiler complains that no visible
subprogram matches the definition for grey_sample because it is
defined to return type "sample" instead of Natural.

On Fri, 03 Oct 2003 02:54:45 GMT, "Jeff C,"
<nolongersafeto@userealemailsniff.com> wrote:

>
>"Ron Baldwin" <re(nospam)baldwin@adelphia.net> wrote in message
>news:bV2fb.1204$qj6.1035959@news1.news.adelphia.net...
>> I need to read .PNG format graphics images from a file, process the image
>as
>> a two-dimensional array, then write the processed image out to a  new
>file.
>>
>> I have used the PNG_IO package to read data from files, but I can't figure
>> out how to get the data from a two-dimensional array to the
>Write_PNG_Type_?
>> output procedures.
>>
>> Does anyone have some examples of such an operation?
>
>
>There is an example in the png_test.adb file that ships with png_io.
>
>
>You just need to instantiate one of the Write_PNG_Type_X procedures with the
>appropriate
>parameters.
>
>Now I suspect you looked at this a little already so I am probably telling
>you things you already know.
>
>Lets look at an example. The spec for one of the procedures is
>
> generic
>    type Image_Handle is limited private;
>    type Sample       is range <>;
>    with function Grey_Sample(I    : Image_Handle;
>                              R, C : Coordinate) return Sample;
>  procedure Write_PNG_Type_0(Filename  : in String;
>                             I         : in Image_Handle;
>                             X, Y      : in Dimension;
>                             Bit_Depth : in Depth   := Eight;
>                             Interlace : in Boolean := False;
>                             Ancillary : in Chunk_List := Null_Chunk_List);
>
>This is for a greyscale image which of course is the simplest case...So you
>need to write a function that takes in
>three parameter types. One should be the type of your 2d array, the other
>two are the index values to the array and finally the function should return
>the value at the row and column passed to it.
>
>So, for example assume you created a 2-d array like
>
>   type My_Pixel_Type is range 0 .. 100;
>
>   type Cool_Picture_Type is array (Coordinate range 0 .. 9, Coordinate
>range 0 .. 19) of My_Pixel_Type;
>
>   and you had a variable
>   Picture : Cool_Picture_Type
>
>   that you had read your data into
>
>   you need to write
>
>   function Get_Picture_Pixel(From_Array : in Cool_Picture_Type;
>                                            Row, Column : in Coordinate)
>return Natural;
>
>   and simply implement it as
>
>
>   function Get_Picture_Pixel(From_Array : in Cool_Picture_Type;
>                                            Row, Column : in Coordinate)
>return Natural is
>
>  begin
>     return From_Array(Row, Column);
>  end Get_Picture_Pixel;
>
>
>
>To create a write procedure that uses your function you would do
>
>   procedure Write_Cool_Picture is new Write_PNG_Type_0
>      (Image_Handle => Cool_Picture_Type;
>        Sample            => My_Pixel_Type,
>        Grey_Sample  => Get_Picture_Pixel);
>
>
>
>  You could then use the new procedure you created to write a file
>containing the data from the Picture by doing
>
>     Write_Cool_Picture(Filename => "adarules.png",
>                                     I             => Picture,
>                                     X            => Picture'length(2),
>                                     Y            => Picture'length(1));
>
>
>And that's it.
>
>
>Now obviously the stuff above is not in the order to feed to a compiler.
>Quite possibly I goofed on a few
>syntax elements and/or swaped rows and columns someplace....
>
>But since this seemed more like a basic programming question that is
>somewhat answered by the code I hoped
>that presenting it in this format would be more useful than yet another
>example like in the test driver.
>