Standard home television screens are best suited for low resolution displays. Low resolution mode provides 320 pixels for each horizontal line. High resolution monochrome and RGB monitors can produce displays in high resolution mode, which provides 640 pixels for each horizontal line. If you define an object in low resolution mode and then display it in high resolution mode, the object will be only half as wide. To set horizontal resolution mode, you write to bit 15, HIRES, in register BPLCON0 : High resolution mode -- write 1 to bit 15. Low resolution mode -- write 0 to bit 15. Note that in high resolution mode, you can have up to four bitplanes in the playfield and, therefore, up to 16 colors. Interlaced mode allows twice as much data to be displayed in the same vertical area as in non-interlaced mode. This is accomplished by doubling the number of lines appearing on the video screen. The following table shows the number of lines required to fill a normal, non-overscan screen. NTSC PAL ---- --- Non-interlaced 200 256 Interlaced 400 512 Table 3-6: Lines in a Normal Playfield In interlaced mode, the scanning circuitry vertically offsets the start of every other field by half a scan line. _____________________ line 1 _|_____________________|_ |_____________________| \ | _________ | \ | _________ | \ | | \ | Odd Field | \ _____________________ | _________ | \_|_____________________|_ Line 1 | _________ | _|_|___________________|_ Line 2 |_____________________| / | | |_____________________| / | | | |_____________________| / | Video Display | / | | (400 lines NTSC | _____________________ / | 512 lines PAL) | line 2 _|_____________________|_/ | | | |_____________________| | | | _________ | |\|/ | | _________ | |_____________________| | | | Even Field | Same physical space as used | _________ | by a 200-line (256) PAL), | _________ | noninterlaced display. |_____________________| |_____________________| |_____________________| Figure 3-5: Interlacing Even though interlaced mode requires a modest amount of extra work in setting registers (as you will see later on in this section), it provides fine tuning that is needed for certain graphics effects. Consider the diagonal line in Figure 3-6 as it appears in non-interlaced and interlaced modes. Interlacing eliminates much of the jaggedness or "staircasing" in the edges of the line. ___________________ ___________________ |#|#| | | | | | | | | |#|#|_|_|_|_|_|_|_|_| |#|#|_|_|_|_|_|_|_|_| |_|#|#|_|_|_|_|_|_|_| | | |#|#| | | | | | | |_|_|#|#|_|_|_|_|_|_| |_|_|#|#|_|_|_|_|_|_| |_|_|_|#|#|_|_|_|_|_| | | | | |#|#| | | | | |_|_|_|_|#|#|_|_|_|_| |_|_|_|_|#|#|_|_|_|_| |_|_|_|_|_|#|#|_|_|_| | | | | | | |#|#| | | |_|_|_|_|_|_|#|#|_|_| |_|_|_|_|_|_|#|#|_|_| |_|_|_|_|_|_|_|#|#|_| | | | | | | | | |#|#| |_|_|_|_|_|_|_|_|#|#| |_|_|_|_|_|_|_|_|#|#| |_|_|_|_|_|_|_|_|_|#| non-interlaced interlaced Figure 3-6: Effect of Interlaced Mode on Edges of Object When you use the special blitter DMA channel to draw lines or polygons onto an interlaced playfield, the playfield is treated as one display, rather than as odd and even fields. Therefore, you still get the smoother edges provided by interlacing. To set interlaced or non-interlaced mode, you write to bit 2, LACE, in register BPLCON0 : Interlaced mode -- write 1 to bit 2. Non-interlaced mode -- write 0 to bit 2. As explained above in Setting the Number of Bitplanes , bits in BPLCON0 are not independently settable. The following example shows how to specify high resolution and interlaced modes. MOVE.W #$A204,BPLCON0+CUSTOM ; Write to it The example above also sets the following parameters that are also controlled through register BPLCON0 : * High resolution mode is enabled. * Two bitplanes are used. * Hold-and-modify mode is disabled. * Single-playfield mode is enabled. * Composite video color is enabled. * Genlock audio is disabled. * Light pen is disabled. * Interlaced mode is enabled. * External resynchronization is disabled. The amount of memory you need to allocate for each bitplane depends upon the resolution modes you have selected, because high resolution or interlaced playfields contain more data and require larger bitplanes.
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