Playback quality in ActionScript 3 format

Question

I am developing an image downloader for Flash 10. Is there a way to read the jpeg quality of the images being viewed.

Answer 1

Unfortunately, this cannot be done directly:

The quality factor is not saved directly in the JPEG file, so you cannot read the quality factor from the file. (from: support for Microsoft pages ...)

More details:

The quantization table used to compress the image is saved in the JFIF header, but the JPEG quality. The factor that was used to create the quantization table is not saved along with the image and, therefore, the original JPEG quality factor is lost. (from: JPEG compression rates as image quality conversion , Surendar Chandra and Carla Schlatter Ellis)

The above quote is from an article that discusses ways to estimate the level of compression (by examining the quantization tables used in the image), but it doesn’t look simple: An example is here , which is part of Image Magick codebase, but it is written in C.

The Magick image has been ported to Haxe , which can be executed in Flash code, so you might be able to get something working, but I'm afraid it is not my ability to explain how to do this.

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EDIT: Just found a similar question on SuperUser, which also mentions Image Magick.

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EDIT: You may also be interested in answers to this question , which asks how to get the image size without downloading the whole file (useful for solving with images larger than Flash can handle).

Answer 2

I used libjpeg to complete this work, maybe you can refer to my code

#include <stdio.h> #include <math.h> #include "jpeglib.h" #include <setjmp.h> static const unsigned int std_luminance_quant_tbl[DCTSIZE2] = { 16, 11, 10, 16, 24, 40, 51, 61, 12, 12, 14, 19, 26, 58, 60, 55, 14, 13, 16, 24, 40, 57, 69, 56, 14, 17, 22, 29, 51, 87, 80, 62, 18, 22, 37, 56, 68, 109, 103, 77, 24, 35, 55, 64, 81, 104, 113, 92, 49, 64, 78, 87, 103, 121, 120, 101, 72, 92, 95, 98, 112, 100, 103, 99 }; int ReadJpegQuality(const char *filename) { FILE * infile = fopen(filename, "rb"); fseek(infile,0,SEEK_END); size_t sz = ftell(infile); fseek(infile,0,SEEK_SET); unsigned char* buffer = new unsigned char[sz]; fread(buffer,1,sz,infile); fclose(infile); struct jpeg_decompress_struct cinfo; struct jpeg_error_mgr jerr; cinfo.err = jpeg_std_error(&jerr); jpeg_create_decompress(&cinfo); jpeg_mem_src(&cinfo,(unsigned char*)buffer,sz); jpeg_read_header(&cinfo, TRUE); int tmp_quality = 0; int linear_quality = 0; const int aver_times = 3; int times = 0; int aver_quality = 0; for(int i=0;i<DCTSIZE2;i++) { long temp = cinfo.quant_tbl_ptrs[0]->quantval[i]; if(temp<32767L&&temp>0) { linear_quality = ceil((float)(temp*100L - 50L)/std_luminance_quant_tbl[i]); if(linear_quality==1) tmp_quality = 1; else if(linear_quality==100) tmp_quality = 100; else if(linear_quality>100) { tmp_quality = ceil((float)5000/linear_quality); } else { tmp_quality = 100 - ceil((float)linear_quality/2); } aver_quality += tmp_quality; if(aver_times==++times) { aver_quality /= aver_times; break; } } } jpeg_destroy_decompress(&cinfo); return aver_quality; } int main(int argc,char** argv) { printf("quality: %d\n",ReadJpegQuality("test1.jpg")); return 0; } 

this method uses libjpeg to read the quantization table of the jpg file, and then uses the quantization table to calculate quality arguments.