In addition to all of the functions described above, the blitter can draw patterned lines. The line draw mode is selected by setting bit 0 (LINEMODE) of BLTCON1 , which changes the meaning of some other bits in BLTCON0 and BLTCON1 . In line draw mode, the blitter can draw lines up to 1024 pixels long, it can draw them in a variety of modes, with a variety of textures, and can even draw them in a special way for simple area fill . Many of the blitter registers serve other purposes in line-drawing mode. Consult Appendix A for more detailed descriptions of the use of these registers and control bits. You should also see Appendix C for the new limits on line-drawing mode in the Enhanced Chip Set (ECS). In line mode, the blitter draws a line from one point to another, which can be viewed as a vector. The direction of the vector can lie in any of the following eight octants. (In the following diagram, the standard Amiga convention is used, with x increasing towards the right and y increasing down.) The number in parenthesis is the octant numbering; the other number represents the value that should be placed in bits 4 through 2 of BLTCON1 . | | \ (2) | (1) / \ | / \ 3 | 1 / \ | / \ | / (3) \ | / (0) \ | / 7 \ | / 6 \|/ -------------*------------- /|\ 5 / | \ 4 / | \ (4) / | \ (7) / | \ / | \ / 2 | 0 \ / | \ / (5) | (6) \ | | Figure 6-8: Octants for Line Drawing Line drawing based on octants is a simplification that takes advantage of symmetries between x and -x, y and -y. The following Table lists the octant number and corresponding values: Table 6-3: BLTCON1 Code Bits for Octant Line Drawing BLTCON1 Code Bits Octant # ------------------- -------- 4 3 2 - - - 1 1 0 0 0 0 1 1 0 1 1 2 1 1 1 3 1 0 1 4 0 1 0 5 0 0 0 6 1 0 0 7 We initialize BLTCON1 bits 4 through 2 according to the above Table. Now, we introduce the variables dx and dy and set them to the absolute values of the difference between the x coordinates and the y coordinates of the endpoints of the line, respectively. dx = abs(x2 - x1) ; dy = abs(y2 - y1) ; Now, we rearrange them if necessary so dx is greater than dy. if (dx < dy) { temp = dx ; dx = dy ; dy = temp ; } Alternately, set dx and dy as follows: dx = max(abs(x2 - x1), abs(y2 - y1)) ; dy = min(abs(x2 - x1), abs(y2 - y1)) ; These calculations have the effect of "normalizing" our line into octant 0; since we have already informed the blitter of the real octant to use, it has no difficulty drawing the line. We initialize the A pointer register to 4 * dy - 2 * dx. If this value is negative, we set the sign bit (SIGNFLAG in BLTCON1 ), otherwise we clear it. We set the A modulo register to 4 * (dy - dx) and the B modulo register to 4 * dy. The A data register should be preloaded with $8000. Both word masks should be set to $FFFF. The A shift value should be set to the x coordinate of the first point (x1) modulo 15. The B data register should be initialized with the line texture pattern, if any, or $FFFF for a solid line. The B shift value should be set to the bit number at which to start the line texture (zero means the last significant bit.) The C and D pointer registers should be initialized to the word containing the first pixel of the line; the C and D modulo registers should be set to the width of the bitplane in bytes. The SRCA, SRCC, and DEST bits of BLTCON0 should be set to one, and the SRCB flag should be set to zero. The OVFLAG should be cleared. If only a single bit per horizontal row is desired, the ONEDOT bit of BLTCON1 should be set; otherwise it should be cleared. The logic function remains. The C DMA channel represents the original source, the A channel the bit to set in the line, and the B channel the pattern to draw. Thus, to draw a line, the function _ AB + A is the most common. To draw the line using exclusive-or mode, so it can be easily erased by drawing it again, the function _ _ ABC + AC can be used. We set the blit height to the length of the line, which is dx + 1. The width must be set to two for all line drawing. (Of course, the BLTSIZE register should not be written until the very end, when all other registers have been filled.) Register Summary for Line Mode

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