[Solved] Can a shader variable have different value for each vertex?

[IsawU]

The way I understand it, if I have 3-vertex polygon and I draw it, the VERTEX shader runs 3 times (once for each vertex).

We can send variable values to the shader just before we call love.graphics.polygon(...). How do I ensure that for each vertex I get a different variable value.

As an example I have this code:
straightforward define vertex position, X offsets and the shader:

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poly = {100, 200, 600, 300, 300, 500}	--polygon coordinates
xoffsets = {100, -100, 20}		--the offsets (as a variable mostly because I need to access them from multiple places)
me = love.graphics.newShader(--[[code below]])

love.draw() function showing what I get VS what I want:

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love.graphics.setColor(255, 255, 255)
--no shader
love.graphics.polygon('fill', poly)
	
love.graphics.setColor(255, 0, 0)
love.graphics.setShader(me)
	m:send("xoffsets", xoffsets[1], xoffsets[2], xoffsets[3])
	me:send("maxp", 2)
	
	--what I get
	love.graphics.polygon('line', poly)
love.graphics.setShader()
	
love.graphics.setColor(0, 255, 0)
--what I want, not a correct solution though
love.graphics.polygon('line', poly[1]+xoffsets[1], poly[2], poly[3]+xoffsets[2], poly[4], poly[5]+xoffsets[3], poly[6])	

and the shader code:

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extern float xoffsets[3];		//array for X offsets
int pointer;				//(pseudo)pointer to xoffsets array
extern int maxp;			//maximum value of pointer (we have this many vertices on this specific shape)
	  
#ifdef VERTEX
	vec4 position( mat4 transform_projection, vec4 vertex_position )
	{
		float xoffset = xoffsets[pointer];
				
		pointer++;
		if (maxp < pointer) {
			pointer = 0;
		}
				
		vertex_position = vec4(vertex_position[0]+xoffset, vertex_position[1], vertex_position[2], vertex_position[3]);
				
		return transform_projection * vertex_position;
	}
#endif
		 
#ifdef PIXEL
	vec4 effect( vec4 color, Image texture, vec2 texture_coords, vec2 screen_coords )
	{
			//always TRUE, need to have this, otherwise maxp doesn't exist (defined but not used) HELP??
		if (maxp < 1000) {
			vec4 texcolor = Texel(texture, texture_coords);
			return texcolor * color;
		}
	}
#endif

Visual interpretation of RAW(white) VS what I want(green) VS what I get(red) You will get this sceen if you run the code.love.

screenshot-2017-07-28-18-13-40.png (13.26 KiB) Viewed 8404 times

I do understand that this cannot work, it's just the closest I got.

Can this be done in shaders, and if so, how can I deliver theese additional attributes?

[raidho36]

What you're looking for is vertex attributes. However I'm pretty sure what you really need is a transformation matrix, it's just you don't see how to use it so you instead resort to inventing some square wheels.

[IsawU]

You're spot on, yet wouldn't a transformation matrix be a mathematical description of how to move all the vertices, resulting in a triangle that has a shape of white polygon (such as red triangle)?

Square wheelely talking, how can I implement vertex attributes in LÖVE, because from reading the GLSLangSpec I got the idea that I need them too.

[raidho36]

I think matrix is what you need because it appears that you simply want the shape distorted, not directly manipulate each individual vertex.

You create a mesh and attach additional attributes using standard functionality, then you draw that mesh.

[MasterLee]

What you're looking for is vertex attributes. However I'm pretty sure what you really need is a transformation matrix, it's just you don't see how to use it so you instead resort to inventing some square wheels.

What is wrong with square wheels?

[zorg]

What is wrong with square wheels?

Tehnically those are the curved triangles present in wankel engines... and are in fact not squares at all.

[MasterLee]

What is wrong with square wheels?

Tehnically those are the curved triangles present in wankel engines... and are in fact not squares at all.

Well ok they are triangles but triangles are the opposite of a circle at least by the number of coners
real squares are possible too:

or when driven on special floor
https://www.youtube.com/watch?v=LgbWu8zJubo

[zorg]

Technically, the opposite of an infinity-gon should be one with zero points... not that it could exist.

2 sides is the lowest i believe.

[IsawU]

You create a mesh and attach additional attributes using standard functionality, then you draw that mesh.

Oh, right. I was wondering what those difficult ways to create a new Mesh meant. Thanks very much. Can this be done with lines and points too (and love.graphics.polygon as such)?

[IsawU]

2 sides is the lowest i believe.

Would work if your daily commute to work was about half a meter