Hi
I'm hoping someone here can tell me what I'm doing wrong as I've spent
the best part of three days reading docs and trying code.
I want to port a windows DLL that controls a USB camera to Python
using C types. I've happily converted a lot of the functions but am
stuck with one vital function that requires a structure to be passed
to the DLL for filling with data.
I have the C declarations
CAM_API BOOL LUCAM_EXPORT LucamGetFormat(HANDLE hCamera,
LUCAM_FRAME_FORMAT *pFormat, FLOAT *pFrameRate);
where the LUCAM_FRAME_FORMAT is defined as
typedef struct {
ULONG xOffset; // x coordinate on imager of top left corner of
subwindow in pixels
ULONG yOffset; // y coordinate on imager of top left corner of
subwindow in pixels
ULONG width; // width in pixels of subwindow
ULONG height; // height in pixels of subwindow
ULONG pixelFormat; // pixel format for data
union
{
USHORT subSampleX; // sub-sample ratio in x direction in pixels
(x:1)
USHORT binningX; // binning ratio in x direction in pixels (x:1)
};
USHORT flagsX; // LUCAM_FRAME_FORMAT_FLAGS_*
union
{
USHORT subSampleY; // sub-sample ratio in y direction in pixels
(y:1)
USHORT binningY; // binning ratio in y direction in pixels (y:1)
};
USHORT flagsY; // LUCAM_FRAME_FORMAT_FLAGS_*
} LUCAM_FRAME_FORMAT;
In my Python code I have
#------------- Frame format --------------------
class FRAME_FORMAT_UNION(Union):
__fields__ = [("subSample", c_ushort), # sub-sample ratio in x
direction in pixels (x:1)
("binning", c_ushort )] # binning ratio in x
direction in pixels (x:1)
class LUCAM_FRAME_FORMAT(Structure):
__fields__ = [( "xOffset", c_ulong), # x coordinate on imager of
top left corner of subwindow in pixels
( "yOffset", c_ulong), # y coordinate on imager of
top left corner of subwindow in pixels
( "width", c_ulong), # width in pixels of
subwindow
( "height", c_ulong), # height in pixels of
subwindow
( "pixelFormat", c_ulong), #pixel format for data
( "XUnion", FRAME_FORMAT_UNION),
( "flagsX", c_ushort), # LUCAM_FRAME_FORMAT_FLAGS_*
( "YUnion", FRAME_FORMAT_UNION),
( "flagsY", c_ushort)]
LP_FRAME_FORMAT = POINTER(LUCAM_FRAME_FORMAT)
and make the call like this
FrameRate = c_float(0)
FrameFormat = LUCAM_FRAME_FORMAT()
FrameFormat.xOffset = 0
FrameFormat.yOffset = 0
FrameFormat.width = 0
FrameFormat.height = 0
FrameFormat.pixelFormat = 0
FrameFormat.XUnion = 0
FrameFormat.flagsX = 0
FrameFormat.YUnion = 0
FrameFormat.flagsY = 0
lucam = windll.lucamapi
error = bool()
GetFormat = lucam.LucamGetFormat
GetFormat.argtypes = ( HANDLE, LP_FRAME_FORMAT, POINTER(c_float) )
GetFormat.restype = BOOL
error = GetFormat (hCamera, FrameFormat, FrameRate )
On return the FrameRate parameter is correct but the FrameFormat
structure no longer has the any field attributes. Further more it
often generates an access violation, apparently in python26.dll. I
guess the call is writing to unallocated memory but I have no idea
why.
Can anyone help?
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