[openMSX] MSX2 color (hue) shift (default blue background)

By ren

Paladin (733)

ren's picture

04-04-2017, 18:33

I suddenly started noticing there's quite a noticeable color shift in the blue background color when switching over from the boot/logo screen to basic, more brightly colored, closer to purple.

I checked some real HW vids on YT, and it's hard to tell, but from some vids I do get the impression background gets somewhat more brightly colored when it goes into basic, but it seems less noticeable than what I'm noticing in openMSX.

(In blueMSX it seems it stays exactly the same color/hue btw.)

I wonder if other people have noticed this, and feel this is off, or feel it's different compared to real HW? (I do am using a rather basic TN TFT.)

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By Manuel

Ascended (13469)

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04-04-2017, 19:40

Which MSX2? Did you try some other machines too?

By wouter_

Champion (384)

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04-04-2017, 20:11

I just checked on an Philips NMS 8255 ... the blue color in the bootlogo is different from the blue background color in BASIC (RGB 007 vs 117).
It may very well be that the colors openMSX shows are different from the colors you're used to on your real MSX. It depends a lot on the type of monitor/TV you connect to your MSX. Try playing with the openMSX color settings to make them match more closely: brightness, contrast, gamma and color_matrix.

By ren

Paladin (733)

ren's picture

05-04-2017, 16:05

@Manuel NMS 8250, but seems the same for other models (inc. MSX2+).

@Wouter Thanks for the pointers. color_matrix seems like an interesting toy to play with, but adjusting the gamma a bit, using a value closer to 1.0 (I believe the default is 1.33), might suffice already!

This is what I measured, using raw screenshots, with default (1.33) and no gamma correction (1.0). I believe blueMSX makes raw screenshots by default ('Monitor emulation' was set to none in any case, gamma to 1.0 (default)).
Note that both blue- & openMSX do apply the set gamma correction when making raw screenshots.
The value between parenthesis is the gamma setting:

       openMSX (1.33)  openMSX (1.0)  blueMSX (1.0)
Boot   0,0,255         0,0,255        6,6,247
Basic  54,54,255       32,32,255      32,32,247

By ren

Paladin (733)

ren's picture

05-04-2017, 16:57

Btw I also had a look at RGB color conversion (forums).

Isn't 3-bit 1, supposed to translate to 8-bit 36 (instead of the 32 I measure)?
The step is 255/7 (max RGB-8 / max RGB-3) ≈ 36.43, thus:

0   0      0
1  36.43  36
2  72.86  73
3 109.29 109
4 145.71 146
5 182.14 182
6 218.57 219
7 255    255

By ren

Paladin (733)

ren's picture

06-04-2017, 23:43

I goofed (again), seems the default gamma is 1.1, not 1.33 (must have changed it some point in time..)

Anyhow, some other questions:
* is default 1.1 based upon 'seems to generate most faithful reproduction on our boxes'?
* adjusting gamma correction has no effect whatsoever on the boot screen, that's why I noticed the substantial difference, a flaw?

(-edit- Though perhaps I should learn some more on what gamma correction is exactly supposed to do, as I now realize it has no effect on any 'full-on' primary or secondary (& b/w).. (though my LCD monitor's gamma setting does have an effect on those..) (Though I'm not sure how serious I should take that setting.. Wink)

By Manuel

Ascended (13469)

Manuel's picture

07-04-2017, 00:04

The manual of openMSX has a link to an explanation about gamma: http://openmsx.org/manual/user.html#gamma

By ren

Paladin (733)

ren's picture

07-04-2017, 00:07

Yes thanks, was just checking it Wink

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