BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which:
FIG. 1 is a block diagram of a compression system according to the prior art;
FIG. 2 is a block diagram of a compression system according to the present invention; and
FIGS. 3 and 4 illustrate further embodiments of a compression system according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, in a known compression scheme, an image 10 is compressed at 12 according to a JPEG algorithm to produce a compressed image 14. The JPEG algorithm applies a degree of compression Q. The compressed image data is compared at 16 with a threshold size, for example 12 kb. If less than the threshold, the image data is output at 18 to a memory. If greater than the threshold, Q is increased to give a greater degree of compression and the comparison is repeated. As discussed above, if the image contains a large amount of fine detail, this process can lead to a result which is visually unsatisfactory.
Referring now to FIG. 2, similar items are denoted by similar reference numerals. In this case however, the original image 10 is passed through a scaler 20 before undergoing compression. The scaler 20 defaults to applying zero scaling so that its output 22 is an image which is 1:1 with the input image 10. However, if the result of the comparison at 16 is such that a Q above a predetermined value would be required, the scaler 20 is operated to scale down the input image to have a smaller number of pixels to reduce the pixel resolution of the image. As one example, an input image which contains 1024 by 736 pixels might be reduced to 676 by 486 pixels.
The overall result of such an operation is that the reproduced image has a lower pixel resolution but significantly less compression artifacts, whereby the subjective visual quality to the end user is much more acceptable. The predetermined limit value of Q can readily be determined empirically.
The use of a scaler in this manner is a particularly convenient approach, as many image sensors and cameras incorporate a scaler as a standard facility, frequently for the purpose of matching a standard image sensor to a variety of screen sizes (for example in mobile phones). Therefore, in many instances it is possible to incorporate the illustrated scaler in existing devices with no or minimal extra circuitry.
FIG. 3 illustrates a modification in which the scaler 20 is not controlled by the compression factor Q, but in accordance with light level L. Lower light levels cause the image signal to contain more noise across the image, and also noise on individual pixels which can cause undesirably high levels of compression. The effect of this can be reduced by scaling to a lower pixel resolution before compression.
FIG. 4 illustrates a further modification in which scaling is controlled by both Q and L. A measurement of incident light level can readily be derived from pixel values of the image sensor itself, or from a separate light intensity sensor.
Although described with reference to JPEG compression of still images, the illustrated scaler may be applied to other types of image compression, for example GIF or PNG. The illustrated scaler could also be applied to video compression such as MPEG, but this would require the scaling to be reversed at the viewing end.
The illustrated apparatus may conveniently be implemented within the signal processing circuitry of a solid state image sensor, or of a camera, where the appropriate means for scaling and for compression will already be present. This is particularly useful for applications with limited memory available, such as cameras in mobile phones. The requisite circuitry can be formed in a co-processor arranged for operation with the image sensing IC, for example. It is also to be appreciated that the illustrated scaler can also be applied in other situations, such as in image processing or editing on a PC.
Patent Application
20080031533
