GDI + colors integer manipulation

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Hi !

Currently, I'm trying to compute colors interpolation with a simple linear interpolation method, but I have a "capacity overshooting" issue when I manipulate those numbers.
I'm not very good at maths, but I understand that when I make my calculations, I overshoot the max value of integers. So I need to convert my R,G & B values in a range [0.0 to 1.0].Right ?

Is there a good way to do this without rounding errors or another problem I haven't seen ?

This is what I do:

Dim R as Double = Color1.R / 255
'...the same with G,B and Alpha

'...do my calculations...
'...convert from range [0.0 - 1.0] to [0 - 255] like this:
Dim RR as Integer = R * 255

I also need working with floats for 3D calculations (according to what I read), so I need to understand the best practices, but don't find good explanations on the web.

Thanks in advance for your help and sorry for my strange english :sigh:

Posted 26-Aug-10 13:10pm

norrisMiou

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2 solutions

Solution 2

Hi Christian and thanks

Then you need to decide, do you scale all your values, so if you have a value of 2.0, any 1.0 becomes .5 so they stay in proportion

Yes, all values need to be in proportion. I think this part is easy as I know minimum and maximum values (0-255).

Does this work ? In C# you need to divide by a float ( 255.0 ) to get a float value.

Hum that was just a "pseudo code", but today I tried this in a simple project and it seems to work fine.

Dim Rdbl As Double = inptR.Value / 255.0  'NumericUpDown controls
Dim Gdbl As Double = inptG.Value / 255.0
Dim Bdbl As Double = inptB.Value / 255.0

lblR.Text = "R: " & Rdbl.ToString  'debug labels
lblG.Text = "G: " & Gdbl.ToString
lblB.Text = "B: " & Bdbl.ToString


Dim Rint As Integer = CInt(Rdbl * 255)
Dim Gint As Integer = CInt(Gdbl * 255)
Dim Bint As Integer = CInt(Bdbl * 255)

lblRint.Text = "R: " & Rint  'debug labels
lblGint.Text = "G: " & Gint
lblBint.Text = "B: " & Bint

The results seems to be always good. When I have "1" in Double ,integer conversion is 255, that's ok.
But I don't understand why, when I enter 128 in Integer, the result in Double is 0.501960784313725 :confused:
Why it's not 0.5000000..... ??

I don't know if that will be a problem for my interpolation calculations :doh:
Maybe there is a better way to do this ?

Posted 27-Aug-10 14:09pm

norrisMiou

Updated 27-Aug-10 14:11pm

v2

Comments

Christian Graus 27-Aug-10 20:44pm

You should push comment, not answer, to comment. Yes that works, b/c you mae your 255 a float with 255.0. 256/2 is 128. Your maximum is 255. In any case, computer floating point is not 100% accurate.

norrisMiou 3-Sep-10 23:07pm

Thanks Christian, that's help me. I'm so crazy, I did not think that 255/2 isn't equal to 128.

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Christian Graus · Accepted Answer · 2010-08-26T13:30:00

Solution 1

Does this work ? In C# you need to divide by a float ( 255.0 ) to get a float value. Then you need to decide, do you scale all your values, so if you have a value of 2.0, any 1.0 becomes .5 so they stay in proportion, or do you just say if R > 1.0, R = 1.0.

Your English is just fine.

Posted 26-Aug-10 13:30pm

Christian Graus