[Lazarus] Strange issue with TImage and lazarus 2.2.2

[Ondrej Pokorny]

On 30.06.2022 18:20, Werner Pamler via lazarus wrote:
> Am 30.06.2022 um 17:03 schrieb Ondrej Pokorny via lazarus:
>> On 30.06.2022 16:54, Werner Pamler via lazarus wrote:
>>> Am 30.06.2022 um 15:11 schrieb Mattias Gaertner via lazarus:
>>>> Maybe instead of the GraphicClass.ClassName the first extension can be
>>>> written.
>>>> That means, that an old IDE cannot read the graphic.
>>> This will break Delphi compatibility, i.e. forms with an image in 
>>> which the Picture has been loaded by Delphi will not be readable by 
>>> Lazarus any more, and vice versa. Just checked again the signature 
>>> that Delphi writes to the begin of the Picture.Data: it's the class 
>>> name, not the extension.
>>
>> And what about writing the extension to the data end?
>>
>>     Picture.Data = {
>>       0B 54 49 6D 61 67 69 6E 67 50 4E 47 89 50 4E 47 0D 0A 1A 0A 00 
>> 00 00 0D 49 48 44 52 00 00 01 C6
>>       11 T  I  m  a  g  i  n  g  P  N  G                // 11 is the 
>> length byte
>>       03 p  n  g
>>
>> That should be both Delphi and legacy Lazarus compatible if the 
>> reader reads only the defined count of bytes for the classname?
>>
> But isn't this the same problem? When Vampyre is not installed, and 
> the streamer sees the TImageingPNG classname there is no way how to 
> find the end of the Picture.Data block.
>
Oh yes, you are right. The ClassName is written in the beginning of the 
stream. From the sample I assumed it is written in the end but that 
doesn't make sense. My bad.

---

Well there is an easier solution. I just remembered that I added an 
image handler resolver TFPCustomImage.FindHandlerFromStream() to FPC 
three years ago: https://gitlab.com/freepascal.org/fpc/source/-/issues/33955

So if the ClassName is not found, the FP image handler can be found from 
the actual image stream. Every image format can be detected directly 
from the stream, so the ClassName is actually redundant. When we have 
the TIHData for the stream, then we have everything - the extension and 
the FP reader/writer. From this information we can find the LCL image 
handler from the registered list - of course only if at least /some/ 
handler is registered for the extension.

Of course, we are limited to the FPC version - I don't know if the 
feature is already in a released version. But we can add the support for 
the FPC versions with this feature.

Ondrej

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