pyopengl-devel

[PyOpenGL-Devel] glReadPixels with null pointer

[Tim Sheerman-C. <ord...@sh...>]

Hi all,

I had problems when using glReadPixels when the data buffer pointer 
passed to the function is zero:

glReadPixels(0, 0, capSize[0], capSize[1], GL_BGRA, GL_UNSIGNED_BYTE, 0)

I was doing this while reading from a pixel buffer object. This caused a 
segmentation fault in OpenGL/GL/images.py when simple.glReadPixels is 
called. I found a way to fix this by setting the variable to ensure it 
is a "None" pointer.

         if array is None:
             array = images.SetupPixelRead( format, (width,height), type )
         else:
             if array != 0:
                 array = arrayType.asArray( array )
             else:
                 array = arrayType.asArray( None )
         imageData = arrayType.voidDataPointer( array )
         simple.glReadPixels(
             x,y,
             width, height,
             format,type,
             imageData

This branch fixes the segfault problem: 
https://code.launchpad.net/~thetourist/pyopengl/tim

Regards,

Tim Sheerman-Chase

[PyOpenGL-Devel] glReadPixels with null pointer

[Tim Sheerman-C. <ord...@sh...>]

On 04/09/13 22:05, pyo...@li... wrote:
> On 13-09-02 01:59 PM, Tim Sheerman-Chase wrote:
>> >Hi all,
>> >
>> >I had problems when using glReadPixels when the data buffer pointer
>> >passed to the function is zero:
>> >
>> >glReadPixels(0, 0, capSize[0], capSize[1], GL_BGRA, GL_UNSIGNED_BYTE, 0)
> Hi Tim,
>
> I've merged your patch into bzr head, though with some modifications, as
> "if array != 0:" will trigger "cannot determine the scalar value of an
> array" errors when array is a numpy array (it creates a boolean array of
> size of array then attempts to determine whether array is True/False and
> blows up).
>
That seems sensible to me. However, I am still having problems with the 
current trunk version #623 when using glReadPixels in the previously 
described situation. I did a quick hack to try to fix it but I am unsure 
if this does not break something else. 
https://code.launchpad.net/~thetourist/pyopengl/tim2

My code breaks if the ctypes pointer created by ctypes.c_void_p is 
passed through arrayType.voidDataPointer. I have skipped this step. I am 
a little unclear as to what what would be returned in each of the cases 
and it might be worth checking.

Thanks,

Tim

Re: [PyOpenGL-Devel] glReadPixels with null pointer

[Mike C. F. <mcf...@vr...>]

On 13-09-04 08:05 PM, Tim Sheerman-Chase wrote:
...
>
> That seems sensible to me. However, I am still having problems with the
> current trunk version #623 when using glReadPixels in the previously
> described situation. I did a quick hack to try to fix it but I am unsure
> if this does not break something else.
> https://code.launchpad.net/~thetourist/pyopengl/tim2
>
> My code breaks if the ctypes pointer created by ctypes.c_void_p is
> passed through arrayType.voidDataPointer. I have skipped this step. I am
> a little unclear as to what what would be returned in each of the cases
> and it might be worth checking.

Ah, the problems of fixing code without a test case to run :D .  You are 
correct, the voidDataPointer call should only be happening in cases 
where the data is *not* a c_void_p, the whole point of that being to 
create a c_void_p out of an array data type.

Merged, thanks,
Mike

-- 
________________________________________________
   Mike C. Fletcher
   Designer, VR Plumber, Coder
   http://www.vrplumber.com
   http://blog.vrplumber.com

Re: [PyOpenGL-Devel] glReadPixels with null pointer

[Mike C. F. <mcf...@vr...>]

On 13-09-02 01:59 PM, Tim Sheerman-Chase wrote:
> Hi all,
>
> I had problems when using glReadPixels when the data buffer pointer
> passed to the function is zero:
>
> glReadPixels(0, 0, capSize[0], capSize[1], GL_BGRA, GL_UNSIGNED_BYTE, 0)

Hi Tim,

I've merged your patch into bzr head, though with some modifications, as 
"if array != 0:" will trigger "cannot determine the scalar value of an 
array" errors when array is a numpy array (it creates a boolean array of 
size of array then attempts to determine whether array is True/False and 
blows up).

With the modified code *any* integer/long passed in will be converted to 
a ctypes.c_void_p( value ).  I believe that should be fine for all 
normal use cases of the function.

HTH,
Mike

-- 
________________________________________________
   Mike C. Fletcher
   Designer, VR Plumber, Coder
   http://www.vrplumber.com
   http://blog.vrplumber.com