5.7 Example scripts 139
PRESENTATION OF NEGATIVE VALUES IN UBASIC
When reading properties with the uBasic command get_prop, negative val-
ues are represented as so-called 2-complements. Depending on the storage
size of the property (1 byte or 2 byte), either the value 2
16
+x or 2
8
+x is dis-
played.
Example: A value of 65500 is displayed. The real value is –36:
2
16
+(–36) = 65536–36 = 65500.
In the table we have printed the property cases that can be modified by a
script in bold letters. All other properties are read-only, or modifying them
has not yet been tested. In general, however, it is better to change the
camera settings by simulating key presses (section 5.1). Changing camera
settings by setting property values can be sometimes problematic because
only part of the camera may be affected by a specific property case. For in-
stance, a property case may influence the camera optics while leaving the
user interface untouched.
This example is a property-based version of the script flashoff.lua
from section 5.1:
-- flashof2.lua by Berthold Daum
--[[
@title Flash off
]]
props = require "propcase"
set_prop(props.SHOOTING,2)
exit_alt()
Here, the flash is switched off by assigning a value of 2 to property 143
(property 16 on DIGIC II). The script works all right—the flash is indeed
switched off. But the display does not reflect the new state. It still indicates
that the flash is in AUTO mode. When using the camera in unattended
mode (e.g., in an RC-controlled vehicle), nobody cares. But personally, I pre-
fer a display that truly reflects the state of the camera, so I would choose
the original version of the script as shown in section 5.1.
5.7 Example scripts
Scripting is a popular topic on the CHDK web pages. Most of the scripts
deal with timers, bracketing, or motion detection. Timers are typically used
for operating the camera autonomously (e.g., from a kite or a balloon) or
To Next Page
Loading...
