RP9 files may contain one or more
application preview images, for display by
playback software and in other contexts
(preview thumbnails, directory listings,
etc.)
Preview image files must be in PNG
format, with square pixels, i.e. with a
ratio of 1:1. Raw PAL pixel grabs are
acceptable, but NTSC grabs are not, due to
their "tall" pixel ratio (10:11).
The name of the default preview image
inside the RP9 container is
"rp9-preview.png". For games and demoscene
productions, this image should be a
meaningful and memorable "in-game" shot (not
a title screen or box shot).
It is also possible to include a title
screen image, named "rp9-title.png". For
games and demoscene productions, this image
should show the title of the application as
displayed on screen, or whatever comes
closest to it. "rp9-title.png", if present,
should ideally be in the same size as
"rp9-preview.png" (which must be present).
The color depth of the image should
either reflect the original (e.g.
palette-based 256 colors or less), or be
true color. If the image is resized, it is
highly recommended to convert it to true
color before resizing it, and then save it
as true color.
Supported bitmap sizes are:
- 640x480 (use only if this is the
original "1X" format, not if upscaled)
- 320x240 (recommended, unless
upscaled)
- 160x120
Rendered with square pixels (as is common
on digital processing and displays), these sizes
mirror the shape of the universal 4:3
standard-definition TV format while
minimizing the need for subpixel scaling
along one or both axes in consideration of
common retrogaming resolutions. Both PAL and
NTSC televisions had a "4x3 format",
independently of the resolution and pixel
shape. Implementations
like RetroPlatform Player should and will
automatically convert between 640x480, 320x240 and
160x120 as necessary.
When processing existing images, it may
be necessary to take into account ratio
distortions and bars at the top and bottom.
For example, some popular C64 game
screenshot archives use 320x200 images,
which is usually based on NTSC data, but
can't be rendered properly on 1:1 displays,
so it often results in distorted images
(circles are not circles any more).
Conversely, some popular Amiga archives use
320x256 images, expecting PAL content, and
adding black bars at the top or bottom if
the source was NTSC.
An automated import procedure that takes into
account existing variations could be
something like the following:
- If the source is a "double barrel"
image (simple check for a frequent
scenario: exactly 640x256; longer check:
width/height >= 2.5 unless hires
non-interlace): the right half is
processed into "rp9-preview.png" (unless
entirely black), and the left half is
processed into "rp9-title.png" (unless
entirely black or identical to the right
half)
- If the source image is 640x480, 320x240 or
160x120 don't do anything, just keep it
as is (unless it has obvious distortions
or overscan areas)
- In all other cases, convert it to
true color before resizing it (unless a
perfect crop without resize can be
achieved); there is always a loss of quality in
resizing, so resize only if necessary,
and be sure to pick a good resize
algorithm (e.g. bicubic)
- If the source is exactly 320x200
(frequent scenario) or 640x400:
resize it to 320x240 or 160x120, without
preserving the original ratio (just
stretch it to the target size)
- If the source is exactly 320x256
(frequent scenario): if
there are same-color bars at the top and
bottom (320x8 each), cut the central
320x240 part; after this optional step,
resize (with stretch, as before)
whatever is left (i.e. 320x256 or
320x240) to 320x240 or 160x120 as
necessary
- If the image is still not 640x480,
320x240 or 160x120, crop external
same-color (presumed overscan) areas,
resize down (maintaining ratio) to
640x480, 320x240 or 160x120, and apply
black bars to make it exactly 640x480,
320x240 or 160x120
- Exceptions: sources known to be NTSC
("tall pixels") and/or hires
non-interlace modes ("double width")
need to be resized to the square-pixel
destination format taking into account
their original ratio
The image import feature of RetroPlatform
Player (as used in Amiga Forever and C64
Forever 2010.1 and higher) actually employs
a similar algorithm when the image is not
already in a supported format. The format
conversion can also be invoked from the
command line:
PlayerName.exe -SetPreview <source.png|bmp|gif|jpg> <destination.png>
In many occasions common sense has to be
used, for example because the screenshot
contains what should be a circle (which is a
good indication of the target ratio), or
because stretching can be avoided by just
cutting a few scanlines.
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