Prima::gp-problems - Problems, questionable or intricate topics in 2-D
One of the most important goals of the Prima project is portability
between different operating systems. Independently to efforts in
keeping Prima internal code that it behaves more or less identically on
different platforms, it is always possible to write non-portable and
platform-dependent code. Here are some guidelines and suggestions for
2-D graphics programming.
Minimal display capabilities
A compliant display is expected to have minimal set of capabilities,
that programmer can rely upon. Following items are guaranteedly
supported by Prima:
Distinct black and white colors
Line widths 0 and 1
One monospaced font
rop::Copy and rop::NoOper
All these properties must be present, however it is not required
for them to be changeable. Even if an underlying platform-specific
code can only support one mode for a property, it have to follow
all obligations for the mode. For example, if platform supports
full functionality for black color but limited functionality for
the other colors, the wrapping code should not allow color property
to be writable then.
Black and white colors on paletted displays
Due the fact that paletted displays employ indexed color
representation, ’black’ and ’white’ indices are not always 0 and
2^n-1, so result of raster image operations may look garbled (X).
Win32 and OS/2 protect themselves from this condition by forcing
white to be the last color in the system palette.
Example: if white color on 8-bit display occupies palette index 15
then desired masking effect wouldn’t work for xoring transparent
areas with cl::White.
Workaround: Use two special color constants cl::Clear and cl::Set,
that represent all zeros and all ones values for bit-sensitive
Black might be not 0, and white not 0xffffff
This inevident issue happens mostly on 15- and 16-bits pixel
displays. Internal color representation for the white color on a
15-color display ( assuming R,G and B are 5-bits fields) is
11111000 11111000 11111000
--R----- --G----- --B-----
that equals to 0xf8f8f8. (All)
Advise: do not check for ’blackness’ and ’whiteness’ merely by
comparing a pixel value.
Pixel value coding
It is not checked how does Prima behave when a pixel value and a
platform integer use different bit and/or byte priority (X).
If a non-solid pattern is selected and a background and/or a
foreground color cannot be drawn as a solid, the correct rendering
requires correspondingly 3 or 4 colors. Some rendering engines
(Win9X) fail to produce correct results.
In complex shapes ( FillPoly, for example) the platform renderer
can fill certain areas two or more times. Whereas the effect is not
noticeable with rop::CopyPut, the other raster operations (like
rop::Xor) produce incorrect picture. (OS/2)
NB - has nothing in common with the fill winding rule.
Workaround: Do not use raster operations with complex filled shapes
For a widget that contains a pattern-filled shape, its picture will
be always garbled after scrolling, because it is impossible to
provide an algorithm for a correct rendering without a prior
knowledge of the widget nature. (All)
Workaround: Do not use patterned backgrounds. Since the same
effect is visible on dithered backgrounds, routine check for pure
color might be applied.
Line caps over patterned styles
It is not clear, whether gaps between dashes should be a multiple
to a line width or not. For example, lp::DotDot looks almost as a
solid line when lineWidth is over 10 if the first (non-multiple)
tactic is chosen. From the other hand it is hardly possible to
predict the plotting strategy from a high-level code. The problem
is related more to Prima design rather than to a platform-specific
Workaround: use predefined patterns (lp::XXX)
Joint areas may be drawn two (or more) times - the problem emerges
if logical ROP (rop::Xor) is chosen.(OS/2)
Dithering might be not used for line plotting. (Win9X)
Font metric inconsistency
A font is loaded by request with one size, but claims another
Impact: system-dependent font description may not match to Prima’s.
Advise: do not try to deduce Prima font metrics from system-
dependent ones and vice versa.
No internal function for drawing transparent bitmaps (like fonts).
Therefore, if a font emulation is desired, special ROPs cannot be
reproduced. (OS/2, Win9X, WinNT)
Impact: font emulation is laborsome, primarily because the glyphs
have to be plotted by consequential anding and xoring a bitmap.
Full spectrum of the raster operations cannot be achieved with this
Prima do not use text kernings, nor encourages underlying platform-
specific code to use it - primarily because of its complexity.
From the other hand, sometimes glyph position cannot be determined
correctly if no information for the text kerning is provided.
Fractional text position
If the font glyphs have fractional widths, it might be observed
that letters may change their position in a string.
Example: A set of glyphs has width of 8.6 pixels for each symbol.
If the string "abcd" is drawn at position 0, then black part of "d"
starts at 25th pixel, but if "cd" is drawn at 17th, as it supposed
to be if the integer arithmetics is used, it starts at 24th pixel.
Solution: Do not rely to Drawable::get_text_width information,
because it always returns integer value, but to
Drawable::get_font_abc, which returns real values.
If a text is drawn with non-CopyPut raster operation, text
background is not expected to be mixed with symbols - however this
is hardly reachable, so results differs for different platforms.
Text background may be only drawn with pure ( non-dithered ) color
(Win9X,WinNT) - but this is (arguably) a more correct behavior.
Advise: Do not use ::rop2 and text background for special effects
Internal platform features
Font change notification is not provided. (X, OS/2)
Raster fonts cannot be synthesized (OS/2, partly X)
Raster operations ( ROPs)
Background raster operations are not supported (X,Win9X,WinNT) and
foreground ROPs have limited number of modes (OS/2,X). Not all ROPs can
be emulated for certain primitives, like fonts, complex shapes, and
It is yet unclear which primitives have to support ROPs, - like
FloodFill and SetPixel. Behavior of the current implementation is that
they do not.
Platforms tend to produce different results for angles outside 0 and
2pi. Although Prima assures that correct plotting would be performed
for any angle, minor inconsistencies may be noticed. If emulating,
note that 2 and 4-pi arcs are not the same - for example, they look
differently with rop::Xor.
Some displays are unable to change their hardware palette, so
detecting 8- or 4- bits display doesn’t automatically mean that
palette is writable.(X)
Widget::palette property is used for explicit declaration of extra
color needs for a widget. The request might be satisfacted in
different ways, or might not at all. It is advisable not to rely
onto platform behavior for the palette operations.
Dynamic palette change
It is possible (usually on 8-bits displays) for a display to change
asynchronously its hardware palette in order to process different
color requests. All platforms behave differently.
Win9X/WinNT - only one top-level window at a time and its direct
children ( not ::clipOwner(0)) can benefit from using
Widget::palette. System palette is switched every time as different
windows moved to the front.
OS/2 - not implemented, but in principle the same as under win32.
X - Any application can easily ruin system color table. Since this
behavior is such by design, no workaround can be applied here.
Scaling is invalid (Win9X) or not supported (X). Common mistake is
to not take into an account the fractional pixels that appear when
the scaling factor is more than 1. This mistake can be observed in
Large scale factors
Request for drawing a bitmap might fail if large scaling factor is
selected. (OS/2,Win9X,WinNT). This effect is obviously due that
fact that these platforms scale the bitmap into a memory before the
plotting takes place.
Some ROPs are ambiguous - SRCTRANSPARENT, for example. Some times they
work, some times they don’t. The particular behavior depends on a video
Circles cannot be drawn using an even diameter.
Fast GDI operations on HWND_DESKTOP may be delayed, thus GetPixel may
return invalid pixel values.
Amount of GDI objects can not exceed some unknown threshold -
experiments show that 128 objects is safe enough.
Color cursor creation routine is broken.
Filled shapes are broken.
No bitmap scaling
No font rotation
No GetPixel, FloodFill ( along with some other primitives)
White is not 2^n-1 on n-bit paletted displays (tested on XFree86).
Filled shapes are broken.
Color bitmaps cannot be drawn onto mono bitmaps.
Palettes are not implemented
Plotting speed of DeviceBitmaps is somewhat less on 8-bit displays than
Images and Icons. It is because DeviceBitmaps are bound to their
original palette, so putting a DeviceBitmap onto different palette
drawable employs inefficient algorithms in order to provide correct
Image that was first drawn on a paletted Drawable always seen in 8
colors if drawn afterwards on a Drawable with the different palette.
That is because the image has special cache in display pixel format,
but cache refresh on every PutImage call is absolutely inappropriate
(although technically possible). It is planned to fix the problem by
checking the palette difference for every PutImage invocation. NB -
the effect is seen on dynamic color displays only.
Dmitry Karasik, <email@example.com>.