[Qt-interest] Fast displaying of 8bit images

[Pavel Koshevoy]

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
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>> Qt-interest at trolltech.com
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>>   
>>     
>
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