AN1368 datasheet(30/34 Pages) MICROCHIP

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AN1368

DS01368A-page 30

 2011 Microchip Technology Inc.

APPENDIX B:

DOUBLE-BUFFERING

Manipulating pixels on the screen requires direct writes

to the frame buffer. While these changes are being

executed, the screen is also refreshed. This means that

the changes are displayed immediately as the frame

buffer is being updated. This is not a suitable scheme

when the changes are slow, for example, decoding an

image or having a large number of widgets on a screen.

The display may appear slow in that case and can

cause user dissatisfaction.

One solution to this problem is to use a double-buffering

scheme supported by the Microchip Graphics Library. In

this scheme, the changes are not directly written to the

frame buffer, but instead, they are written to a separate

buffer, called the ‘Draw Buffer’. After all the changes are

made, the draw buffer and frame buffer are exchanged.

Now the draw buffer becomes the frame buffer, and

because of all the changes drawn there, the changes

appear spontaneous to the user. Of course, there will be

a delay, as all the changes have to be written to the draw

buffer before displaying it. This delay is generally more

tolerable than displaying the changes slowly.

After exchanging of the buffers, the latest buffer (which

is now the frame buffer) is copied to the new draw buf-

fer in the background, then the new draw buffer is in

sync with what is being displayed. New changes are

then written to the draw buffer and the cycle continues.

As the double-buffering scheme uses two buffers, the

RAM requirement will double.

In the Microchip Graphics Library, if double-buffering is

enabled, the frame buffer and draw buffer are

exchanged after the changes of all the widgets on a

screen are done (i.e., the new screen appears after the

whole screen is updated and not after updating an

individual widget).

The workflow of double-buffering is as follows:

In the initial stage, the draw buffer and frame buffer

have the same contents, as shown in Figure B-1.

FIGURE B-1:

INITIAL STAGE

In the drawing stage, only the draw buffer is

modified, and therefore, the frame buffer is not

visible to the user, as shown in Figure B-2.

FIGURE B-2:

DRAWING STAGE

In the swap stage, the draw buffer is swapped

with the frame buffer (by exchange of pointers),

as shown in Figure B-3.

FIGURE B-3:

SWAP STAGE

In the final stage, the frame buffer is copied to

the draw buffer, so the buffers have the same

contents, as shown in Figure B-4.

FIGURE B-4:

FINAL STAGE

The workflow cycle continues.

When to Use Double-Buffering?

Use double-buffering when:

• There are large numbers of widgets on a screen

• The decoding and displaying of images are

required

• The display resolution is larger than the WQVGA,

even if the above points do not hold true

When Not to Use Double-Buffering?

Do not use double-buffering when:

• The immediate display of changes is required. For

example, plotting a graph.

• The RAM available for display is limited.

• The display resolution is smaller than the

WQVGA (this is not a hard rule).

How to Use Double-Buffering?

Double-buffering can be used as follows:

• Enable double-buffering by defining,

USE_DOUBLE_BUFFERING

in the

GraphicsConfig.h

• See the section on double-buffering in the

Microchip Graphics Library Help file for code

examples.

Draw Buffer

Frame Buffer

Draw Buffer

Frame Buffer

Draw Buffer

Frame Buffer

Exchange

Frame Buffer

Draw Buffer

Copy

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