[fpc-pascal] Why is FPImage dog slow in reading jpeg's?

[michael.vancanneyt at wisa.be]

On Mon, 30 May 2011, Tomas Hajny wrote:

> On Mon, May 30, 2011 16:16, michael.vancanneyt at wisa.be wrote:
>>
>> Try the following code:
>>
>> Img:=TFPMemoryImage.Create(0,0);
>> Img.UsePalette:=False;
>> Img.loadFromFile(FileList[i]);
>>
>> That should work much faster. The reason is probably that the default
>> image
>> created by the reader uses a palette, which is of course dog slow when
>> reading
>> large images.
>
> Interesting. What is supposed to be a "large image"? I ask because the
> original post mentioned 35 kB and 65 kB (and indirectly also dimensions
> around 600x400 if I understand the program output correctly) which I
> certainly wouldn't consider as "large" (and the mentioned time necessary
> for reading it surely not appropriate regardless of using palette or not).
> I guess that some optimization may be reasonable because such pictures
> could be loaded and displayed (including conversion from true colour to
> 256 colours palette) faster even on a 486 machine...

It depends on the colors used. The default palette algorithm simply 
allocates a slot per found color in the jpg and returns the index of the
slow. For a picture, this will create a huge palette, in which the search 
for a color is in essence linear, so n^2 lookups.

So, dog slow.

You could obviously speed this up by creating a palette yourself, and 
limiting the number of colors.

Or, best for pictures is  not to use a palette, which is what my solution
entails.

Michael.