Image data compression device, encoder, electronic apparatus, and method of compressing image data

An encoder for executing image data compression process, comprising:

• an image input interface for executing an interfacing process for inputting image data;
• a quantizing section for quantizing the image data with a quantization step changing in accordance with a quantization parameter;
• a FIFO buffer section for buffering the quantized data corresponding to a plurality of frames and quantized by the quantizing section;
• and
• a host interface for executing an interfacing process with a host which reads out the quantized data stored in the FIFO buffer section asynchronously with writing to the FIFO buffer section, wherein
• the host finds a predicted data size of the encoded data for the precedent frame from the data size of the quantized data for the precedent frame to the present frame,
• sets or releases an upper limit threshold of the quantization parameter based on the number of assignable bits assignable to a compressing process in order for maintaining a predetermined bit rate, or the number of remaining frames, or both the number of the assignable bits and the number of the remaining frames,
• finds the quantization parameter so as to be equal to or less than the upper limit threshold of the quantization parameter using the predicted data size when setting the upper limit threshold of the quantization parameter, and
• finds the quantization parameter irrespective of the upper limit threshold of the quantization parameter using the predicted data size when releasing the upper limit threshold of the quantization parameter, and
• the quantizing section changes the quantization step based on the quantization parameter found by the host.