Fill a curve with a transparent gradient brush and save it C#

Question

0.00/5 (No votes)

See more:

UPDATE :

Quote:
The WinForms implementation of Form only permits a solid colour to be used as the transparency key. It's not possible to have varying levels of transparency.

Hello,
I'm actually working with Windows Forms and GDI+.
I've got my background with a color that i MakeTransparent when I save my image as png later.
I have to draw some polygons on this background.
I also have to draw a closed curve filled with a linear gradient brush, going from white to transparent color.

The problem is, when I save the form as a png, the gradient brush is going from white to dark gray/black. It's like if i loose the transparency wanted when i fill my curve with my gradient brush.

I've got something like that :
https://image.noelshack.com/fichiers/2020/37/1/1599462293-transparent.png[^]

The red part is transparent (it's without the gradient brush).

Thanks for all the help that you can bring me to.

What I have tried:

This is my gradient brush :

C#

LinearGradientBrush linGrBrush2 = new LinearGradientBrush(closedCurveRect, Color.FromArgb(94, 94, 94), Color.Transparent, LinearGradientMode.Vertical);

And I try to fill my closed curve like that :

C#

g.FillClosedCurve(linGrBrush2, closedCurve.ToArray(), FillMode.Alternate, tension);

When I save my image, I convert my background to transparent :

C#

bmp1.MakeTransparent(Color.FromArgb(94, 94, 94));
bmp1.Save(filename + ".png", pngEncoder);

Posted 4-Sep-20 0:16am

Mineodo68

Updated 8-Sep-20 21:37pm

v6

Add a Solution

Comments

Richard Deeming 4-Sep-20 9:38am

The first argument to Color.FromArgb is the alpha value, so your brush is filling from "completely transparent grey" to "completely transparent". I don't think that's actually going to do anything useful.

Mineodo68 4-Sep-20 10:36am

Hello,
Sorry, i forgot to change this value (that was for my test). It's without an alpha value.

[no name] 4-Sep-20 14:01pm

You say you're using a gradient that goes to transparent, on a transparent background. At what point is transparent no longer transparent?

Mineodo68 7-Sep-20 3:00am

Yeah, you got that. A part of my image (drawn without gradient brush) is transparent. But all the part with the gradient brush isn't transparent...

Mineodo68 7-Sep-20 3:06am

I just uploaded a picture to illustrate.

[no name] 7-Sep-20 21:31pm

What you want is probably possible, but it depends on program / framework. Paint.Net handles transparent well; MS Paint doesn't. Bit fiddling.

https://stackoverflow.com/questions/4988869/how-do-i-set-the-background-colour-of-a-transparent-png-wpf

Mineodo68 8-Sep-20 2:57am

I already did something like that, but it only works with a color without gradient.
Thanks for your help, maybe someone will find something.

[no name] 8-Sep-20 11:43am

If you "understand the gradient", you should be able to pick what you want; even if it means using a specific color as a "mask" ... as they do in the movies when replacing the background. (Or use a "spray paint" technique and do your own gradient)

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Gerry Schmitz · Answer 1 · 2020-09-08T06:12:00

.net - C# Create Gradient Image - Stack Overflow[^]

This is a mod of the above (changed one of gradient colors and the file type). The point is you can save a transparent gradient; you just have to think more about how you apply your other artifacts.

Bitmap bitmap = new Bitmap( 100, 100 ); // 100x100 pixels
         Graphics graphics = Graphics.FromImage( bitmap );

         System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(
            new System.Drawing.Rectangle( 0, 0, 100, 100 ),
            System.Drawing.Color.Blue,
            System.Drawing.Color.Transparent,
            LinearGradientMode.Vertical );

         brush.SetSigmaBellShape( 0.5f );

         graphics.FillRectangle( brush, new System.Drawing.Rectangle( 0, 0, 100, 100 ) );
         bitmap.Save( "gradientImage.png", ImageFormat.Png );

         bitmap.Dispose();
         graphics.Dispose();
         brush.Dispose();