Filmless digital camera with selective image compression

In a filmless camera in which a predetermined number of successive images are captured by a CCD that produces digital data representing said images and stored in a digital memory having an allocated capacity per image, a method of compressing the digital data prior to recording comprising

• dividing the data representing one of said images into a series of blocks or predetermined uniform size wherein each block represents a predetermined number of pixels,
• compressing said data representing one of said blocks using a first method of compression,
• quantizing the coefficients from said compressed data and counting the number of non-zero coefficients,
• comparing the number of non-zero coefficients with a pre-established standard and if said non-zero coefficients are greater than said pre-established standard, compressing said data, prior to compressing said data by said first method of compression by a second and different compression method prior to recording it,
• encoding each block of said data to indicate which method of compression has been employed, and
• recording the data whether compressed by said first or by said second method of compression.

In a filmless digital camera in which a predetermined number of images are captured by a CCD that produces digital data representing said images and stored in a digital memory having a predetermined allocated capacity per image, a method of compressing the digital data comprising the steps of

• dividing the digital data representing said image into a series of blocks of uniform size containing a predetermined number of pixels,
• successively compressing the data representing each of said blocks by a DCT method with a predetermined scale factor,
• determining for each block a coefficient index equal to sum of the high frequency non-zero coefficients derived by said DCT method,
• establishing a predetermined threshold number,
• determining whether said coefficient index is greater or smaller than said threshold number,
• compressing said data for each of said blocks by the differential coding method if said coefficient index exceeds said threshold number by a predetermined amount, and
• recording said data.

In a filmless digital camera for capturing and storing data representing a number of still images, a combination comprising

• a frame transfer CCD,
• a lens system for producing a focused image of a scene to be captured on said CCD,
• digital data storage means having a predetermined storage capacity allocated for each image captured by said camera,
• means for dividing the data representing one image into discrete blocks of data each having a predetermined number of pixels,
• means for computing the DCT of successive blocks of said data,
• data compression means including
• means for quantizing the DCT coefficients using a predetermined scale factor,
• means for determining the sum of selected non-zero coefficients for each of said blocks of data,
• means for establishing a relationship between a pre-established threshold and the sum of selected non-zero coefficients in the quantized data representing each of said blocks
• means for selecting particular blocks of said data on the basis of the relationship between the pre-established threshold and the sum of selected non-zero coefficients in the quantized data for each of said blocks,
• means for compressing each of said blocks of data in accordance with an alternate compression method if said sum exceeds a pre-established threshold, and
• means for compressing each of said blocks of data by the DCT method if said sum does not exceed said pre-established threshold, and
• means for storing the data in said digital storage means.

An image comparison system comprising

• means for generating a digital block of data representing part of an image to be compressed,
• primary compression means for compressing said data block in accordance with a first algorithm that compresses the data by at least 50%,
• secondary compression means for compressing said data block in accordance with an alternate algorithm that would on decompression produce less significant artifacts than said primary compression means,
• measuring means for measuring the amount of data from said primary compression means representing high spatial frequencies in the image data,
• compression control means responsive to said measuring means for deciding whether said data block is compressed by said primary compression means or said secondary compression means, and
• means for recording the compressed data block.

An image compression system comprising

• means for producing a block of digital data representing a portion of an image,
• quality factor control means,
• primary compression means under the control of said quality factor control means for compressing said data block in accordance with a first algorithm that reduces the data in spatially high frequencies but which upon decompression produces visually objectionable errors from bright points in said image,
• secondary compression means for compressing said data block in accordance with an alternate algorithm that on decompression does not produce objectionable errors from bright spots in said image,
• measuring means for measuring the amount of data from said primary compression means resulting from high spatial frequencies in the image data,
• compression control means responsive to said measuring means for controlling whether said data block is compressed by said primary compression means or said secondary compression means.