Get amount of unique colors from picture

[lulZghost]

For my project I need a function way to get the amount of unique colors from a single picture.
My Idea was to make an array of strings, go pixel by pixel, convert the color to string, check if the string already exists and if not add it to the array.

This is my code for it:

Code: Pascal  [Select][+][-]

1. private
2.   colorlist : array of string;    
3.  
4. ---------------------------------
5.  
6. function TFormMain.checkColors(): integer;
7. var i, x, y, doesexist : integer;
8.     colorstring : string;
9. begin    
10.     for y:=0 to image1.canvas.Height do
11.      begin
12.       for x:=0 to image1.canvas.Width do
13.        begin
14.         colorstring:=ColorToString(FPColorToTColor(image1.Canvas.Colors[x,y]));
15.         for i:=0 to length(colorlist) do
16.          begin
17.           doesexist:=0;
18.           if colorlist[i] = colorstring then doesexist:=1;
19.          end;
20.         if doesexist=0 then
21.          begin
22.           SetLength(colorlist, Length(colorlist)+1);
23.           colorlist[High(colorlist)]:=colorstring;
24.          end;
25.        end;
26.       end;
27.  result:=length(colorlist);
28. end;

SO, apparently this doesn't work, because I get this error

Code: Pascal  [Select][+][-]

1. 000000010000AC15 668b41e8                 mov    -0x18(%rcx),%ax

I don't get any syntax errors, so somewhere I have another error, but I don't see it. Maybe someone else can spot the logic error in there, because I honestly don't know what the error could be.

[marcov]

Code: [Select]

for i:=0 to length(colorlist) do

Iterates over length(colorlist)+1, is this really what you want?

Also,  as a rule, don't convert to string what is not a string.

[Blaazen]

I would allocate array with size 256*256*256/8 = 2MB, it will cover RGB values from (0,0,0),(0,0,1)...(255,255,254),(255,255,255) and each bit represents color: 0..not present, 1..present. In the end you can only do sum of "1" bits.

[LemonParty]

Using strings for such purpouses is ineffective.
Blaazen's idea is really good.
Use https://www.freepascal.org/docs-html/rtl/system/popcnt.html to count the number of setted bits.