Odd glitch with tiling flats - how to fix?

[durian]

So, I've been experimenting with making an image to use as a flat, but since the spot where I'd want it to be placed doesn't lie neatly on the grid, I've made four flats which tile to make the image. However, when I tested with both pr-boom and ZDoom, I get an ugly black line between the different flats.

See here (overlook the quality of the flat itself - this is just a test).





Have I done something wrong, or is this unavoidable? Any thoughts greatly appreciated :)

[scifista42]

Software renderer don't display flats ABSOLUTELY in correct places. If you look from different angles, it may happen that the flat is "shifted" a little, and then it starts to tile, as usual. So in your case, a little tiled strip of one of those flats is displayed between them. And since the flat is probably black on the opposite side to the white light, you see a black line.
This is a normal behaviour. I don't know if OpenGL rendered is more accurate when displaying flats. You can, hypothetically, improve it by adding some white pixels on the other side of the flat, where it would normally tile. At least one or two rows. You see:

          THE FLAT   .....................
                     .XXXXXXXXXXXXXXXXXXX.
                     .XXXXXXXXXXXXXXXXXX<--- X = black
                     .XXXXXXXXXXXXXXXXXXX.
                     .  XXXXXXXXXXXXX    .
                     .  XXXXXXXXXXX      .
       Two rows ------> XXXXXXXXXX       .
       of white      .  XXXXXXXXXX      <--- Your white
       on the left   .  XXXXXXXXXX       .      light
                     .  XXXXXXXXXXX      .   (one half)
                     .  XXXXXXXXXXXXX    .
                     .XXXXXXXXXXXXXXXXXXX.
                     .XXXXXXXXXXXXXXXXXXX.
                     .XXXXXXXXXXXXXXXXXXX.
                     .....................

Also - the shot looks neat :)

[durian]

That is tremendously helpful!
So, I tested it with the white lines, and while it removed the black line, it was replaced with a white line. However, in view of your explanation, I tried pasting the opposing image on the other side of the flat, and now it tiles perfectly:



Thanks alot - I owe you a coke! :)