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Twynn
Account closed
Registered: Nov 2005
Posts: 9 |
Half pixel shifting
First of all: It's good to see that the C64-scene is still alive.
I kinda left the scene about ten years ago but after accidently crossing this site I got interested a bit again. Downloaded some demo's to see what happened since I left and I was quite amazed. Not those ugly 8x8 modes anymore, but good looking demos.
Anyway, when I was watching krestage I saw this intro for each demopart which used this high-resolution text which is said to use half pixel shifting? I'm still familiar with the normal vic-tricks, but this one made me curious.
Now I wonder how this is done. Is it just a visual trick or has the vic really some possibility to do half pixel movements or something? |
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Oswald
Registered: Apr 2002
Posts: 5094 |
just a visual trick, tho I dunno how is it done, and if it can be at any level considered as a half pixel shift. |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Just move the gfx one pixel each 2nd frame. E.g. if moved vertically the eye will be between two lines on the 2nd frame, since the eye will move smoothly rather than stopping and starting 25 times per second. If you then display the equal lines of the gfx on one frame and the odd lines on the other it will almost look like there's twice as many lines.
Same concept as interlace on a normal TV signal, except here the lines are auto-displaced half a line each frame, so you don't have to move your eyes.
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HCL
Registered: Feb 2003
Posts: 728 |
No, it's just a visual trick.
Theoretically you may think that the graphics is twice as wide as the screen, so when you show it you can only display every second pixel. But since the scroller moves one pixel each frame, you will display the other second half of the gfx the next frame. and so on..
Since the scroller moves with a constant speed, it also flickers between the two graphic screens at a constant rate. This makes it look good. |
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Oswald
Registered: Apr 2002
Posts: 5094 |
I understand then. In short this is the same as IFLI interlacing, or as in the demo 400, which doubles the vertical resolution. But doing this with a diagonal scroll you may *think* that you doubled both resolution. As the displayed graphics only alternates between 2 of a kind, its not even virtually 640x400 imho, as to really display the amount of data needed you would need to display 4 kinds of gfx. |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
IFLI is not the same since in IFLI you actually shift half a multicolor pixel, thus you don't have to scroll the image to get the impression of higher resolution, you actually get true interlacing in IFLI. |
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Twynn
Account closed
Registered: Nov 2005
Posts: 9 |
Ok, thanx all.
Just a little interlace trick.. Well, still nicely done.
JackAsser: isn't IFLI actually more ment for producing more colors instead of a higher resolution? |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Twynn: If you shift half a pixel you get higher resolution, if you don't shift you get color blending, that's basically it when it comes to interlacing. However, the artist may use interlacing as a means of color blending if the pixels are put correct. |
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algorithm
Registered: May 2002
Posts: 705 |
Doesn't really matter if it is a genuine VIC trick or illusion. As long as it gives the effect, that is all that matters.
Some other examples are alternative interlace methods which seem to reduce flicker (line interlacing or cross hatch)
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Oswald
Registered: Apr 2002
Posts: 5094 |
anyone knows of a ifli pic where each line is shifted to the opposite direction ? I wonder if that method is ufeful. |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
oswald: i tried this a (long) time ago...it actually improves the quality of the interlace quite a bit (depending on the picture ofcourse). its a simelar thing with evening out the luminance of both half frames. the first will improve horizontal color blending (since the picture will be displayed in a "checkerboard" style all the time) and the second will reduce flickering. |
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