SOLVING that age-old problem of what to get for the ''person who has everything'', has never been easy. It is one of those situations which leaves the hopeful buyer in a state of self-doubt, never knowing if the swift, generous smile by the recipient is as genuine as the thought which went into getting the gift. But there is some hope - in the form of the Christmas Showcase. The event at Hong Kong's Convention and Exhibition Centre, which runs from today until Tuesday, has 500 booths with more than 300 exhibitors - all displaying gifts and appliances for every occasion. Some of the more unusual items on offer include a three-lens camera; a compact disc greeting card; a three-dimensional (3D) picture which can be printed on plain paper; a popcorn machine and device which illuminates a page of a book or magazine - withoutglare - to allow people to read in the dark. The triple-lens camera is actually a 3D photographic invention which comes in either hard case or disposable form. The ImageTech 3D is manufactured by Image Technology (HK) Ltd, in Chai Wan, and is the only camera with 3D dimensional capability available to the retail market - and is simple to operate. It is a ''point and shoot'' device which allows the user to concentrate on the subject without worrying about camera operation. It also has a focus-free glass lens which is optimised for maximum depth of picture and an internal light sensor which tells the user when to operate the internal flash. The camera uses conventional 35 millimetre colour film, and film advance, rewind and loading are motorised. Its three glass lenses assemble the multiple images of the subject's different perspectives to create a single 3-D image. Each exposure uses three half frames of a standard 35 mm film so that a roll of 36 exposures will yield 24 prints. Negatives produced by the camera can also be used to produce conventional two-dimensional prints. Another 3D innovation is the ''Magic Eye'' picture. This new visual concept comes from the B. & S. Co of Kowloon and which uses a special method of viewing a 3D picture printed on plain paper with no special coatings of any kind. It has been made possible by the power of modern computers and the use of the image-rendering Salitsky Dot. Initially, the picture is an abstract field of colour, but after staring at it - without the use of special spectacles - a 3D image will materialise. A spokesman for the company said: ''It is a bit like riding a bicycle - once you get the hang of it, the ability is there forever. ''Once you discover the Magic Eye, a whole new world of experience will open up to you.'' Another novel gift is the Glo-Page - an internally lit, acrylic panel, which is engineered to produce a glare-free illumination for reading or writing in the dark. Manufactured by Hong Kong firm US Acrylic, the Glo-Page is ideal for both children and adults and gives the user the facility of reading or writing without the harsh glare spillover which is common with other personal reading lamps. A spokesman for the company said: ''We believe this new product will become very popular since it not only allows the user to have glare-free reading but is also designed to minimise the flow of light, so others in the immediate area are not affected by the light. For the person who has everything, conveying the spirit of the season may not be easy. But the Grandata Co Ltd may have solved that. It has produced compact disc greeting cards which have a combination of traditional and not-so-traditional Christmas messages and songs. The company also has special CDs on offer for birthdays, weddings, anniversaries, invitations, apologies, get well soon and many other categories. Another idea is to buy a popcorn-making machine from Kowloon company Winmate Industrial Ltd. These products are ideal for household use and. although mainly for the export market, have been released on trial in Hong Kong.