Color patterns

As also evident through the analysis in Section III, spatial changes within a screen also impact energy consumption. We call how colors are distributed on the screen its color pattern. The color pattern determines the spatial changes within the screen. Even if the percentage of pixels of each color remains constant (then the LCD consumes constant energy), different pixel arrangements can introduce different switching activities in the hardware including the LCDC, system bus, and external bus, because data for the screen have to be constantly transferred from the framebuffer sitting in the off-chip memory to the LCDC, and then to the LCD for refreshing the storage capacitors. We measure the energy consumption of the system when the system is idle with full-screen windows of checkerboard patterns on iPAQ1. A checkerboard pattern consists of alternating white and black blocks. For each pattern, the white and black blocks each take up half of the screen pixels so that the LCD energy consumption does not vary. However, as expected, the system energy consumption increases when block size decreases, which leads to more spatial changes within the screen. Table X shows the system energy consumption for one second and also gives the percentage energy increase compared with that of presenting a fully white screen. It also shows the system energy for presenting the starting home screen for Pocket PC 2002. The energy difference in Table X is due to the showing of the screen only. The screen with smaller blocks takes more CPU time and energy to generate the screen data, which is a separate issue from what we are concerned with here. The above results imply that a plain GUI is more energy-efficient than fancy ones.

Table X: Different color patterns on iPAQ1

Pattern	Energy (Joule)	Over white (%)
Full white	0.575	0
Full black	0.581	1.0
120$\times$160 block	0.577	0.3
30$\times$40 block	0.584	1.6
12$\times$16 block	0.588	2.3
3$\times$4 block	0.598	4.0
MS home	0.590	2.6