| | <<<Back 1 day (to 2019/04/01) | 20190402 |
| i3v | As a follow up for the https://bugs.ghostscript.com/show_bug.cgi?id=700931#c2 : | 01:55.38 |
| | @kens, could you please help me with one more thing: | 01:55.46 |
| | When you do DeviceRGB to DeviceCMYK conversion for an image: | 01:55.52 |
| | 1) RGB lives in the DefaultRGB color space (this part is documented) | 01:56.03 |
| | 2) CMYK would be in DefaultCMYK color space (right?) | 01:56.11 |
| | But what is the rendering intent? Could it be explicitly specified? | 01:56.19 |
| | PS: maybe "intent" is not the best word here, I'm not sure. The most interesting part for me is "clipping". E.g. I've just tested that both rgb(255,0,0) and rgb(250,0,0) are converted to cmyk(0,1,1,0). | 02:56.12 |
| | I've tried to add a preliminary step that sets DefaultRenderingIntent to Perceptual, but this does not seem to change anything. | 03:00.43 |
| ray_laptop | i3v: your question is probably best handled by mvrhel (Dr. Michael Vrhel, our color expert, and the architect of the GS color handling). Note there is a "-dUseFastColor=true" option that converts RGB to CMYK using the PostScript model with blkackgeneration and undercolorremoval functions | 04:09.10 |
| | s/blkackgeneration/blackgeneration/ | 04:09.34 |
| | also without -dUseFastColor, RenderingIntent only selects from those rendering intents available in the CMYK ICC profile (by default the one in iccprofiles/default_cmyk.icc) that provides the information to convert back and forth between RGB and CMYK | 04:12.37 |
| | mvrhel is in the Pacific TZ so will be online in about 11 hours | 04:13.55 |
| | as far as clipping, generally, the CMYK ICC profile has a more limited "gamut" than RGB (being subtractive) so I am not surprised that there are colors that are gamut-clipped from RGB->CMYK | 04:15.48 |
| | but, Michael Vrhel will (I'm sure) correct any inaccuracies in what I've said (and probably be able to say it more clearly, and certainly more accurately) | 04:17.12 |
| kens | i3v whn we get RGB and CMYK inputs from an input source these are uncharacterised sources. They are not 'DefaultRGB' or 'DefaultCMYK' which have a soecific meaning in PDF terms. | 07:02.45 |
| | The mapping from Device space into device-independent colour space is controlled by the edfault profiles though. | 07:03.17 |
| | In ghostpdl/iccprofiles are default_cmyk.icc, default_gray.icc and default_rgb.icc | 07:03.51 |
| | These are used to map the components of the rekevant device space into the device-independent colour space. There is no rendering intent at this point, the colours aren't being rendered. | 07:04.45 |
| | A link is generated from the input ICC profile to the output ICC profile, and the final device space colours are generated, At this point there is a rendering intent. | 07:05.18 |
| | You can, of course, use a different set of profiles to map from Device space into device-independent colour space. | 07:05.57 |
| | ZeroWalker, the PCL5 technical reference manual is available online I belive. However you need to be aware that 'PCL5' isn't a simple, single specification. DFifferent devices created by HP interpret PCL5 in different ways. So 'decoding' PCL5 is not as straght-forward as it may sound. | 07:09.32 |
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