[Qt-interest] Fast displaying of 8bit images

[Pavel Koshevoy]

Never mind, I was confused by the colortable variable in your code.
Still, most of what I've said so far applies -- if you want to display 
images fast, you need hardware acceleration.  That mean OpenGL, or 
DirectX if you don't care about cross-platform compatibility.

    Pavel.

Pavel Koshevoy wrote:
> Ugh, I did't fully understand your message.  You want to display 
> color-mapped 8-bit images?  You can still use OpenGL, although 
> GL_LUMINANCE is probably the wrong texture type in that case.  Not all 
> OpenGL hardware supports what you want to do directly, but you can also 
> accomplish what you want using GL_LUMINANCE textures and a small 
> fragment shader program running on the GPU to do the color mapping. For 
> fragment shader you could use OpenGL 2 GLSL, or NVIDIA Cg.
>
>     Pavel.
>
>
> Pavel Koshevoy wrote:
>   
>> Use OpenGL.  Treat your image as GL_LUMINANCE texture. If your image 
>> size is larger than max texture size supported by OpenGL you'll need to 
>> do some creative tiling of GL_QUADs (or tiangles, or triangle strip, 
>> etc...).  On some platforms (OSX) there exist OpenGL extensions that 
>> avoid copying texture data from main memory to the graphics card memory, 
>> you may want to use that.
>>
>>     Pavel.
>>
>> Jiri Horky wrote:
>>   
>>     
>>> Hi all,
>>>
>>> I have a big uchar array, where my image (2448x2048) is stored as 1 byte
>>> (gray degree) per 1 pixel.  Pixels start from left corner, going
>>> line-by-line to right bottom corner of image ( it is PGM image, but
>>> without header). What I need to do is simple - just display the image,
>>> but as fast as possible. From what I've found so far, I tried to convert
>>> my image into QImage class and then using fromImage function loading it
>>> into QPixmap, as in following code:
>>> (please excuse using fprintf in C++, but time measuring was copied from
>>> another project :-)
>>>
>>>     QVector<QRgb> colortable;
>>>     QRgb rgb;
>>>     for ( int i = 0; i < 256; i++ ) {
>>>         rgb = qRgb(i,i,i); // grey
>>>         colortable.append(rgb);
>>>     }
>>>
>>>     g_timer_start(timer);  // start of measuring
>>>
>>>     QImage * image = new QImage (img, 2448, 2048, QImage::Format_Indexed8);
>>>     image->setColorTable(colortable);
>>>     *pixmap = pixmap->fromImage(*image, Qt::ThresholdDither);
>>>
>>>     g_timer_stop(timer);
>>>      fprintf(stderr, "Via QImage it takes %lfs\n",
>>> g_timer_elapsed(timer, NULL));
>>>
>>> My measurement shows that it takes about 0.135 ms to convert from uchar
>>> array to Pixmap.
>>>
>>> I am wondering if there isn't any faster method, because manual of
>>> fromImage function says :
>>>
>>> In case of monochrome and 8-bit images, the image is first converted to
>>> a 32-bit pixmap and then filled with the colors in the color table. If
>>> this is too expensive an operation, you can use QBitmap::fromImage
>>> <qbitmap.html#fromImage>() instead.
>>>
>>> But as manual also says, QBitmap is suitable only for 1-bit images....
>>>
>>>
>>>
>>> My next question is: would be possible to prepare QPixmap in another
>>> thread, if I allocate it in GUI thread and make sure it wont be
>>> manipulated from another thread when displayed?
>>>
>>>
>>> I am testing it on Linux, in case it matters.
>>>
>>> Thank you for any ideas!
>>>
>>> Jiri Horky
>>> _______________________________________________
>>> Qt-interest mailing list
>>> Qt-interest at trolltech.com
>>> http://lists.trolltech.com/mailman/listinfo/qt-interest
>>>   
>>>     
>>>       
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