Thx for all the suggestions on my problem.

It is really about the aspect problem. For now, my findings is about the 
texture mapping rather than the image resolution.

I've try to use the the suggestion about resize the image with the width 
of 352 pixel rather than the 384. But it turns out that the marker 
pattern is still distorted (taller and thinner). Therefore, I use the 
original 384x288 resolution and compare again. I've use the ruler to 
measure all objects in the povray generated image and in the camera 
captured image (there is a pixel ruler under linux, really convenient~). 
I've found that those objects (the A4 paper) I defined is actually 
having the same projected dimension with the real one. But for the 
marker's patter, it become taller and thinner.

Therefore, is my texture mapping code actually wrong?

Here is my texture mapping source code:

#this is simulating a marker printed on an A4 size paper
    box
    {
       <0, 0, 0>, <21, 29.7, 0.01>
       pigment
       {
          image_map{png "marker1.png" once interpolate 2 map_type 0}
          scale <21, 29.7, 0.01>
       }
    }



regards
Colin

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