Categories: Technical Solution | Symbian C++ | S60 3rd Edition | Graphics

This page was last modified 09:56, 17 September 2007.

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Overview

Using CImageDecoder to decode and scale JPEG images can be a slow and memory-consuming process, depending on image size. Here are a few tips which will increase speed and decrease memory consumption.

Solution

A generic way to decode e.g. a 5 megapixel image:

1. Create a 5 megapixel CFbsBitmap image.
2. Create the decoder.
3. Convert the image.
4. Scale the 5 megapixel output to a smaller resolution size (e.g. QVGA, VGA...)

This is, however, not efficient, since it requires memory to store the 5MP intermediate bitmap image.

Instead, you can do the following:

1. Create a CFbsBitmap image of the required size (e.g. QVGA, VGA...).
2. Create the decoder.
3. Convert the image.

Using this method, bitmap scaling will be handled during decoding, which will increase speed and decrease memory usage.

Note that some JPEG decoders may not support arbitrary scaling. The information for arbitrary scaling support can be obtained from the EFullyScaleable property of TFrameInfo.

If the decoder does not support arbitrary scaling, first perform the best suitable DCT (Discrete Cosine Transform) scaling (e.g. 1, 1/2, 1/4 or 1/8) and then perform a normal bitmap scaling on the resulting CFbsBitmap.

The steps are as follows:

1. Create the decoder.
2. Check TFrameInfo flags.
   • If arbitrary scaling supported
     1. Create a CFbsBitmap image of required size (e.g.QVGA, VGA...)
     2. Convert
   • If arbitrary scaling is not supported
     1. Create a CFbsBitmap image of best suitable DCT scaling ratio (e.g. 1, 1/2, 1/4, 1/8).
     2. Convert the image.
     3. Perform CFbsBitmap scaling on the output bitmap in order to get the final image size.

Example on DCT scaling

Original size: 2048x1536

Desired size: QVGA (320x240) - same aspect ratio

Assume that arbitrary scaling is not supported.

If DCT scaling of 1/8 ratio is used, the size of the scaled bitmap is:

2048/8 = 256 and 1536/8 = 192 (256x192)

This would result in a lower than desired resolution; upscaling 256x192 to QVGA will result in poor image quality.

If DCT scaling of 1/4 ratio is used:

2048/4 = 512 and 1536/4 = 384 (512x384)

In this case, the best choice is to use 1/4 scaling and further scale the 512x384 bitmap to 320x240 with CBitmapScaler.