I did a study a while back which compressed every 10th frame of the
entire run of Viewperf using the entire range of JPEG quality and
subsampling settings and computed an average compression ratio from
these hundreds of images for each quality level (spreadsheet available
on request.)
Based on that study, I would propose using the following quality and
subsampling levels, which correspond to meaningful average compression
ratios:
Quality level 9 = JPEG quality level 100, no subsampling (ratio ~=
10:1) [this should be lossless, except for round-off error]
Quality level 8 = JPEG quality level 92, no subsampling (ratio ~= 20:1)
[this should be perceptually lossless, based on current research]
Quality level 7 = JPEG quality level 86, no subsampling (ratio ~= 25:1)
Quality level 6 = JPEG quality level 79, no subsampling (ratio ~= 30:1)
Quality level 5 = JPEG quality level 77, 4:2:2 subsampling (ratio ~= 40:1)
Quality level 4 = JPEG quality level 62, 4:2:2 subsampling (ratio ~= 50:1)
Quality level 3 = JPEG quality level 42, 4:2:2 subsampling (ratio ~= 60:1)
Quality level 2 = JPEG quality level 41, 4:1:1 subsampling (ratio ~= 70:1)
Quality level 1 = JPEG quality level 29, 4:1:1 subsampling (ratio ~= 80:1)
Quality level 0 = JPEG quality level 15, 4:1:1 subsampling (ratio ~= 100:1)
Anything below JPEG quality level=15 is generally not going to be usable.
ossm...@users.sourceforge.net wrote:
> + { 512, 32, 6, 0, 0, 0, 4, 10, 1 }, // 0
> + { 2048, 64, 6, 1, 1, 1, 8, 25, 1 }, // 1
> + { 4096, 128, 8, 3, 3, 2, 24, 25, 0 }, // 2
> + { 8192, 256, 12, 5, 5, 2, 32, 50, 1 }, // 3
> + { 16384, 512, 12, 6, 7, 3, 32, 50, 0 }, // 4
> + { 32768, 512, 12, 7, 8, 4, 32, 75, 1 }, // 5
> + { 65536, 1024, 16, 7, 8, 5, 32, 75, 0 }, // 6
> + { 65536, 1024, 16, 8, 9, 6, 64, 83, 0 }, // 7
> + { 65536, 2048, 24, 9, 9, 7, 64, 92, 0 }, // 8
> + { 65536, 2048, 32, 9, 9, 9, 96,100, 0 } // 9
> };
> const int TightEncoder::defaultCompressLevel = 6;
>
>
> Modified: trunk/common/rfb/TightEncoder.h
> ===================================================================
> --- trunk/common/rfb/TightEncoder.h 2009-03-11 14:33:49 UTC (rev 3659)
> +++ trunk/common/rfb/TightEncoder.h 2009-03-11 16:15:29 UTC (rev 3660)
> @@ -35,7 +35,7 @@
> unsigned int monoMinRectSize;
> int idxZlibLevel, monoZlibLevel, rawZlibLevel;
> int idxMaxColorsDivisor;
> - int jpegQuality;
> + int jpegQuality, jpegSubSample;
> };
>
> //
>
> Modified: trunk/common/rfb/tightEncode.h
> ===================================================================
> --- trunk/common/rfb/tightEncode.h 2009-03-11 14:33:49 UTC (rev 3659)
> +++ trunk/common/rfb/tightEncode.h 2009-03-11 16:15:29 UTC (rev 3660)
> @@ -537,6 +537,13 @@
>
> jpeg_set_defaults(&cinfo);
> jpeg_set_quality(&cinfo, s_pjconf->jpegQuality, TRUE);
> + if (s_pjconf->jpegSubSample) {
> + cinfo.comp_info[0].h_samp_factor = 2;
> + cinfo.comp_info[0].v_samp_factor = 2;
> + } else {
> + cinfo.comp_info[0].h_samp_factor = 1;
> + cinfo.comp_info[0].v_samp_factor = 1;
> + }
>
> rdr::U8 *dstBuf = new rdr::U8[2048];
> JpegSetDstManager(&cinfo, dstBuf, 2048);
>
>
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