BeRoPNG - A very tiny but complete PNG loader

[BeRo]

function Swap32(x:word):word;
begin
 result:=(Swap16(x and $ffff) shl 16) or Swap16((x and $ffff0000) shr 16);
end;

BeRoPNG should be portable and small (even at source code level) at the same time, so that it should containing no or only few inline assembler parts, but a good optimizing compiler with inline-support and good code peephole replace templates should optimize this with "bswap eax" on x86-32 and x86-64 (what Delphi&FPC does it not yet, but some C/C++ compilers. My big hope for this is the LLVM backend for FPC). The "and $ffff0000" is just for that it's more readable, the compiler will optimizing it out, but that is something you should know (as you probably already do).

[BeRo]

Any ideas?

cheers,
Paul

Just remove DataSize:longword; and the DataEnd checks from BeRoPNG (change "while (pansichar(DataPtr)+11)<pansichar(DataEnd) do begin
" to "while true do begin" and so on, but without this it's a security leak then. :-)

[paul_nicholls]

Just remove DataSize:longword; and the DataEnd checks from BeRoPNG (change "while (pansichar(DataPtr)+11)<pansichar(DataEnd) do begin
" to "while true do begin" and so on, but without this it's a security leak then. :-)

Thanks I think I might just leave it then haha

cheers,
Paul

[deathshadow]

Still, you're swap32 function should be longword, not word, no?

word and $FFFF0000 === 0 -- ALWAYS
word shl 16 === 0 -- ALWAYS.

So in effect:

result:=(Swap16(x and $ffff) shl 16) or Swap16((x and $ffff0000) shr 16);

is the same thing as saying

result:=0;

Which means on big-endian systems when your swap function is called, you'll get nothing but zero.

Also, you're sending zero to your swap16 function...

function Swap32(x:longword):longword;
begin
 result:=(Swap16(x and $ffff) shl 16) or Swap16(x shr 16);
end;

Is what you should have there. What you have right now won't work... Thankfully it's only called on big endian systems -- and you've probably only tested little endian.

It may also need typecasting forced, since swap16 returns word, not longword meaning that shl 16 could STILL result in zero.

[BeRo]

word and $FFFF0000 === 0 -- ALWAYS
word shl 16 === 0 -- ALWAYS.

Sorry, you're wrong. See my code again:

function Swap32(x:word):word;
begin
 result:=(Swap16(x and $ffff) shl 16) or Swap16((x and $ffff0000) shr 16);
end;

The first swap16 call gets "x and $ffff", and the result of it will shifted 16 bits to left, which's correct.
The second swap16 call gets "(x and $ffff0000) shr 16" or short "x shr 16", and the result of it will not shifted because it was already shifted 16 bits to right, which's also correct.

I guess, you had simply overlook the correct nested levels of the (...) blocks in my code.

[deathshadow]

Forgot about this thread... but:

function Swap32(x:word):word;

word is 16 bit unsigned... can only contain 0..$FFFF -- therein:

x and $ffff returns X

x and $ffff0000 returns 0, always...

since $FFFF is the highest number WORD can contain, $0000FFFF and $FFFF0000 == 0, ALWAYS. X in your function CANNOT hold a value that would return anything but zero on that second AND.... EVER.

See what I'm saying? for that function to work X would have to be DWORD and/or Longint... 32 bit... not 16 bit which is what WORD is.

[BeRo]

See what I'm saying? for that function to work X would have to be DWORD and/or Longint... 32 bit... not 16 bit which is what WORD is.

Yeah, I know (see the rest of the code), but this is unimportant, since this is 1. just a typo error in the typedef in the functiondef paramdef header (which's quick fixable) and 2. unused on little-endian systems

[BeRo]

I've uploaded a new pure pascal version under the same url http://rootserver.rosseaux.net/stuff/BeRoPNG.pas , which's a bit faster and also compatible to Delphi XE3 & XE4 now.

And the other http://rootserver.rosseaux.net/stuff...oidSpecial.pas variant fallbacks to cpng (a wrapper unit to the on ARM Android faster c-based libpng implementation, which's a bit for Android's own libpng.so modified png.pas from FPC, so you do need http://rootserver.rosseaux.net/stuff/cpng.pas ), if you're building for the Android target, otherwise the pure pascal code part will be used, if you're building for other targets than Android.