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POPIL's Image.frombuffer expected data length when using ctypes array
 

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  1. POPIL's Image.frombuffer expected data length when using ctypes array
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    <p>I am using Python, PIL and ctypes to do image manipulation. As I hacked stuff together, I used PIL's <code>fromstring</code> function to get the pixel buffer from ctypes into a PIL object. I simply iterated over the array, building the python string. </p> <p><strong>This works</strong></p> <pre><code>tx = foo.tx tx.restype = POINTER(c_ubyte) result = tx(...args...) #TODO there must also be a better way to do this pystr = "" for i in xrange(w*h*4): pystr += result[i] i = Image.fromstring("RGBA", (w, h), pystr) i.save("out.png") </code></pre> <p>It wasn't pretty, but it worked. Commented with a TODO and moved on. After getting the initial plumbing in place, profiling showed significant performance issues with this code block. Not surprising, I guess.</p> <p><strong>This does not work</strong></p> <p>Similar to this question: <a href="https://stackoverflow.com/questions/6368645/pixel-manipulation-with-pil-image-and-ctypes">Pixel manipulation with PIL.Image and ctypes</a>, I am trying to use <code>frombuffer()</code> to get the pixel data into the PIL object more efficiently: </p> <pre><code>tx = foo.tx tx.restype = POINTER(c_ubyte) result = tx(...args...) i = Image.frombuffer('RGBA', (w, h), result, 'raw', 'RGBA', 0, 1) i.save("out.png") </code></pre> <p>Despite <code>fromstring</code> working, using <code>frombuffer</code> results in the following exception:</p> <pre><code>Traceback (most recent call last): File "test.py", line 53, in &lt;module&gt; i = Image.frombuffer('RGBA', (w, h), res, 'raw', 'RGBA', 0, 1) File "/Library/Python/2.7/site-packages/PIL/Image.py", line 1853, in frombuffer core.map_buffer(data, size, decoder_name, None, 0, args) ValueError: buffer is not large enough </code></pre> <p><strong>Environment</strong></p> <p>The buffer is <code>malloc</code>'ed in C as:</p> <pre><code>unsigned char *pixels_color = (unsigned char*)malloc((WIDTH*HEIGHT*4)*sizeof(unsigned char*)); </code></pre> <ul> <li>The buffer has 4 bytes per pixel for each of the bands RGBA.</li> <li>Mac OS X 10.7, python2.7.1, PIL 1.1.7</li> </ul> <p><strong>Edit</strong></p> <p>Based on <a href="https://stackoverflow.com/users/205580/eryksun">eryksun</a>'s comment and answer below, I moved the buffer allocation from the <code>malloc</code> in the C library into Python and passed the pointer into C:</p> <pre><code>tx = foo.tx tx.restype = POINTER(c_ubyte) pix_clr = (c_ubyte*(w*h*4))() tx(...args..., pix_clr) i = Image.frombuffer('RGBA', (w, h), pix_clr, 'raw', 'RGBA', 0, 1) i.save("out.png") </code></pre> <p>This works as expected. It still doesn't explain <em>why</em> PIL was unhappy with the C-allocated buffer, but this is the better structure for memory management anyway. Thanks to both ErykSun and HYRY!</p>
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    1. COWhy is `tx.restype` set to `POINTER(...)` ? Are you returning the pointer, or any other pointer?
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    2. CO@Unapiedra, yes - the tx function is C and is returning a unsigned char * pointer to the buffer of pixel data.
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    3. COOkay. Now, I am confused. why are you passing `pix_clr` into the function as a parameter? And why is the `char * pointer` (or its data) not destroyed after `tx()` has executed?
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