Hello,

My shader knowledge is bit rusty to say at least. So i could use some help.

I have an triangulated 2d shape. I can apply (angled) gradient fills on that as long as it is evenly triangulated. If not thing go look strange.
E.g. take a look at the circle in my glvg project: http://thuis.vanderhoning.net/svn_glvg/trunk/
So either i tesselate that in a better way (giving way more tringles). Still circular gradients will be hard (maybe even impossible) to apply.

Or:

Could i use shaders to apply gradient fills. Vertex shaders would not do as the same as above aplies to them i think.
So it should be fragment shaders. (most graphics card should handle these by now i hope).

1) What kind of shader should i use glsl or some low level arb fp1.0 shader?
But more importantly how does color calculation work in a fragment shader do i have x,y and are these relative to the screen? How would i relate that the 2d shape being rendered?
2) Also how do i pass the both colors to the fragment shader?

Thanks for your answers in advance.

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Since you are able to understand written german, I would at first recommend to take a look at Tutorial_glsl. There you find a (nearly) complete reference about the commands and syntax of a glsl shader.

Now... To the coordinates: In the fragment shader you can get both Viewport-relative and "3d-space" coordinates. The first ones are always available via gl_FragCoord (see wiki article).
For "3d-space" coords you have to declare a variant and fill it in the vertex shader:

And you can use this now in the fragment shader like this:

If you need the translated vertex position (i.e. with matrix operations applied to it, you just have to write a multiplication with gl_ModelViewProjectionMatrix or just gl_ModelViewMatrix (depending on what you need) in front of the gl_Vertex when assigning the value to pos in the vertex shader.

I hope this helped, if not, just ask again

greetings
Lord Horazont

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You should use GLSL. Even my old ATI Radeon 9800Pro was able to handle that.

I think the GLSL tutorial is a good start. The fragmentshader is executed for each fragment (pixel) you render. Colors work simply be assigning a color to gl_FragColor.
In fragment shader you can access window coords of the current fragment using gl_FragCoord.xy. In case you need the window size, you will need to use an uniform variable for that. If you require world coordinates for your computation, you can use a varying from the vertexshader.


You could either use vertex attributes (simply store colors in texture coords) or use uniform variables. The third way would be a texture. You can do arbitrary texture accesses in shaders, so you can use an texture as array.

Edit:
@Lord Horazont: variant => varying ??

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Yeah!

@Coolcat: Oh ja, das hab ich wohl falsch in erinnerung gehabt, ich habs mal schnell korrigiert. Danke für den Hinweis.

Gruß Lord Horazont

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If you find any deadlinks, please send me a notification – Wenn du tote Links findest, sende mir eine Benachrichtigung.
current projects: ManiacLab; aioxmpp
zombofant network • my photostream
„Writing code is like writing poetry“ - source unknown

„Give a man a fish, and you feed him for a day. Teach a man to fish and you feed him for a lifetime. “ ~ A Chinese Proverb