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--- base:techtech_fli [2016-04-21 12:52] – Created this page compyx
+++ base:techtech_fli [2016-04-22 12:41] (current) – added a screenshot compyx
@@ Line 3: / Line 3: @@
 ===== Introduction =====
-Below is an example of a tech-tech effect, achieved using a so-called FLI-routine. This is **not** a tech-tech with a FLI logo, its a simple character set based routine using FLI to allow videoram switching at every rasterline.
+Below is an example of a tech-tech effect, achieved using a so-called FLI-routine. This is **not** a tech-tech with a FLI logo, its a simple FLI based routine to allow videoram switching at every rasterline.
+The routine differs from [[magazines:chacking7#tech-tech_-_more_resolution_to_vertical_shift]|this article]] in that it uses videoram banks to create the tech-tech, not character sets. One could combine these two techniques by cleverly interleaving character sets and videoram to allow for a wider sinus, perhaps even bank switching with $dd00 for even more movement. I'll leave that as an exercise for the reader ;)
+{{:base:tech-tech-fli.png?404|Screenshot of the tech-tech in action}}
 I wrote this code as part of an attempt to regain my old VIC-trickery skills, it's been about 25 years since I did a tech-tech. As such, the code may not be up to today's standards. I just published it here since there was no article on tech-tech's using $d011/FLI.
+Note: I don't explain FLI in detail in this article, there are other articles here which explain FLI properly.
 The code can be assembled by using [[https://sourceforge.net/projects/tass64/|64tass]]:
@@ Line 11: / Line 17: @@
+===== The Theory =====
-====== The Theory =====
 //I'll use the word 'logo' here to simplify things a bit. My code doesn't actually include a logo, just some text
@@ Line 46: / Line 51: @@
 This code we need to do for each (raster)line of the logo. But we have a problem...
-===== The problem =====
+===== The Problem =====
 Manipulating $d016 on each raster line works fine, manipulating $d018 doesn't: the videoram pointer only gets updated on bad lines (every 8th raster line), so we need a way to update the videoram pointer on each raster line. That's were FLI comes in.
-===== The solution =====
+===== The Solution =====
+Using the so-called FLI technique, we can trigger bad lines at every raster line, allowing us to manipulate the videoram pointer at every raster line.
+So our basic tech-tech display routine would look like this: (loop unrolled to keep the FLI bug at three characters while using a single sprite to cover the bug)
+<code 6502tasm>
+; sinus = 0
+lda #$1b
+sta $d011
+lda #$00    ; 0 & 7 == 0
+sta $d016
+lda #$20    ; column 0
+sta $d018
+; sinus = 3
+; next rasterline
+lda #$1c
+sta $d011
+lda #$03
+sta $d016
+lda #$20    ; still column 0
+sta $d018
+; sinus = 6
+lda #$1d
+sta $d011
+lda #$06
+sta $d016
+lda #$20
+sta $d018
+; sinus = 9
+lda #$1e
+sta $d011
+lda #$01    ; 9 & 7 == 1
+sta $d016
+lda #$30
+sta $d018   ; column 1: 9 pixels means we need to select
+            ; the second videoram bank to move the logo
+            ; one column to the right
+; and so on for each line of the logo, wrapping the $d011
+; value around from $1f to $18
+</code>
+Now we can tech-tech the logo by storing the correct values for $d016 and $d018 in the unrolled code, using for example a sinus table. The example code contains a routine which pre-calculates the $d016 and $d018 values from a sinus table for faster performance and another unrolled loop to store these values in the FLI-routine each frame, again for performance.
+===== The Code =====
-Using the so-called FLI technique, we can trigger bad lines at every raster line, allowing us to manipulate the videoram pointer at every raster line.
+Here's the actual source code, in 64tass syntax. The FLI routine needs to be unrolled, the sinus calculation can all be done in a loop, but that eats cycles, so I also wrote some unrolled code for the $d016/$d018 values updating.
+Before assembling, one should probably comment out the references to the music, I would be surprised to see everyone having their HVSC at "/home/compyx/c64/HSVC" ;)
 <code 6502tasm>