24 bits reading

[gelatine1]

unfortunately im reading a file (bmp image file) which has its data packed in 3 bytes. So my question is how should I read this ? I have the file mapped into my memory so I have a pointer to it but I can't use

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dword ptr [esi] or

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word ptr [esi] or anything similar right ?

[dedndave]

you can, if ESI is set to the pointer   :P

[gelatine1]

I don't really get what you mean ?  :icon_confused:

[dedndave]

I have the file mapped into my memory so I have a pointer to it but I can't use...

you say you have the pointer - let's say it's lpView

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    mov     esi,lpView
now, you can use ESI   

truthfully, though.....
BMP images are never so large that file mapping is needed
you can probably fit the entire file in memory at once

you could use HeapAlloc to allocate a large block of memory, then read it in
or, you could use LoadImage and set the LR_CREATEDIBSECTION flag

a DIB section is a bitmap with a pointer and direct access to the image data bits

after using LoadImage to create a DIB section,
use GetObject to fill a DIBSECTION structure to get the data pointer

http://msdn.microsoft.com/en-us/library/windows/desktop/ms648045%28v=vs.85%29.aspx

http://msdn.microsoft.com/en-us/library/dd144904%28v=vs.85%29.aspx

http://msdn.microsoft.com/en-us/library/dd183567%28v=vs.85%29.aspx

[gelatine1]

Well wait I think I asked my question in a wrong way. So I guess I'll rephrase my question. I meant to ask if there is some kind of way to subtract 3 bytes from memory and move it to a 4 byte register. Something that would look like

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movzx eax,word ptr [esi] to subtract 2 bytes from memory and get it in a 4 bytes register.

I guess I'll take a look a look into the functions you gave me there. thank you

[KeepingRealBusy]

You just set a pointer to the lowest byte, then move a dword to a register and mask out the most significant byte and advance the pointer by the 3 bytes you already have

Dave.

[dedndave]

right - as Dave mentioned...

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    mov     eax,[esi]
    and     eax,0FFFFFFh

the address in ESI is calculated, based on the location of the desired pixel
image base address + ( line number * bytes per line) + ( 3 * column number)
notice that most BMP images are upside-down (top image line is last in file)
the sign of the height value in the header tells you if it is right-side up   :P

bytes per line is always some multiple of 4
so, if you have a 24-bit image that is 3 pixels wide, the bytes per line will be 12

[Tedd]

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@@:
mov eax,[esi]       ;get 4 bytes
and eax,00ffffffh   ;mask out the top one, leaving the 3 bytes you want
...                 ;do stuff
add esi,3           ;move to the next 3 bytes
jmp @B              ;repeat..