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120 lines
5.2 KiB
Plaintext
120 lines
5.2 KiB
Plaintext
sms_ntsc 0.2.3: Sega Master System NTSC Video Filter
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----------------------------------------------------
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Author : Shay Green <gblargg@gmail.com>
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Website : http://www.slack.net/~ant/
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Forum : http://groups.google.com/group/blargg-sound-libs
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License : GNU Lesser General Public License (LGPL)
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Language: C or C++
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Overview
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--------
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To perform NTSC filtering, first allocate memory for a sms_ntsc_t object
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and call sms_ntsc_init(), then call sms_ntsc_blit() to perform
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filtering. You can call sms_ntsc_init() at any time to change image
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parameters.
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By default, sms_ntsc_blit() reads and writes pixels in 16-bit RGB. Edit
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sms_ntsc_config.h to change this.
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RGB Palette Generation
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----------------------
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A 4096-color RGB palette can be generated for use in a normal blitter.
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In your sms_ntsc_setup_t structure, point palette_out to a 12288-byte
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buffer (4096 * 3) to hold the palette, then call sms_ntsc_init(). If you
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only need the palette and aren't going to be using the NTSC blitter,
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pass 0 for the first parameter.
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Image Parameters
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----------------
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Many image parameters can be adjusted and presets are provided for
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composite video, S-video, RGB, and monochrome. Most are floating-point
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values with a general range of -1.0 to 1.0, where 0 is normal. The
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ranges are adjusted so that one parameter at an extreme (-1 or +1) and
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the rest at zero shouldn't result in any internal overflow (garbage
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pixels). Setting multiple parameters to their extreme can produce
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garbage. Put another way, the state space defined by all parameters
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within the range -1 to +1 is not fully usable, but some extreme corners
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are very useful so I don't want to reduce the parameter ranges.
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The sharpness and resolution parameters have similar effects. Resolution
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affects how crisp pixels are. Sharpness merely enhances the edges by
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increasing contrast, which makes things brighter at the edges. Artifacts
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sets how much "junk" is around the edges where colors and brightness
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change in the image, where -1 completely eliminates them. (Color) bleed
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affects how much colors blend together and the artifact colors at the
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edges of pixels surrounded by black. (Color) fringing affects how much
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color fringing occurs around the edges of bright objects, especially
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white text on a black background.
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When using custom settings, initialize your sms_ntsc_setup_t using one
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of the standard setups before customizing it. This will ensure that all
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fields are properly initialized, including any added in future releases
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of the library that your current code can't even know about.
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sms_ntsc_setup_t setup;
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setup = sms_ntsc_composite; /* do this first */
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setup.sharpness = custom_sharpness;
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sms_ntsc_init( ntsc, &setup );
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Image Size
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----------
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For proper aspect ratio, the image generated by the library must be
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doubled vertically.
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Use the SMS_NTSC_OUT_WIDTH() and SMS_NTSC_IN_WIDTH() macros to convert
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between input and output widths that the blitter uses. For example, if
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you are blitting an image 256 pixels wide, use SMS_NTSC_OUT_WIDTH( 256 )
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to find out how many output pixels are written per row. Another example,
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use SMS_NTSC_IN_WIDTH( 640 ) to find how many input pixels will fit
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within 640 output pixels.
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Custom Blitter
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--------------
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You can write your own blitter, allowing customization of how input
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pixels are obtained, the format of output pixels (15, 16, or 32-bit
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RGB), optimizations for your platform, and additional effects like
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efficient scanline doubling during blitting.
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Macros are included in sms_ntsc.h for writing your blitter so that your
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code can be carried over without changes to improved versions of the
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library. The default blitter at the end of sms_ntsc.c shows how to use
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the macros. Contact me for further assistance.
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The SMS_NTSC_BEGIN_ROW macro allows starting up to three pixels. The
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first pixel is cut off; its use is in specifying a background color
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other than black for the sliver on the left edge. The next two pixels
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can be used to handle the extra one or two pixels not handled by the
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main chunks of three pixels. For example if you want to blit 257 input
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pixels on a row (for whatever odd reason), you would start the first two
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with SMS_NTSC_BEGIN_ROW( ... sms_ntsc_black, line_in [0], line_in [1] ),
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then do the remaining 255 in chunks of three (255 is divisible by 3).
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Limitations
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-----------
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The library's horizontal rescaling is too wide by about 3% in order to
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allow a much more optimal implementation. This means that a 248 pixel
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wide input image should appear as 563 output pixels, but with this
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library appears as 581 output pixels. TV aspect ratios probably vary by
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this much anyway. If you really need unscaled output, contact me and
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I'll see about adding it.
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Thanks
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------
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Thanks to NewRisingSun for his original code and explanations of NTSC,
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which was a starting point for me learning about NTSC video and
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decoding. Thanks to the Nesdev forum for feedback and encouragement.
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Thanks to Martin Freij (Nestopia author) and Charles MacDonald (SMS Plus
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author) for significant ongoing testing and feedback as the library has
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improved. Thanks to byuu (bsnes author) and pagefault (ZSNES team) for
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feedback about the SNES version.
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--
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Shay Green <gblargg@gmail.com>
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