I am rendering TRIANGLE_STRIPS in OpenGL ES 2.0. I was wondering, would it be possible to modify the vertex shader such that instead of feeding it 4 texture vertices, you give it only one vertex that represents the centre of the TRIANGLE_STRIP, with a parameter for texture width and a height?
Assuming my texture vertex is:
GLfloat textureVertices[] = {
x, y
};
Can the vertex shader be modified to work with texSize uniform, which would represent the width/height of the TRIANGLE_STRIP? :
attribute highp vec4 position;
attribute lowp vec4 inputPointCoordinate;
uniform mat4 MVP;
uniform lowp vec4 vertexColor;
uniform float texSize;
varying lowp vec2 textureCoordinate;
varying lowp vec4 color;
void main()
{
gl_Position = MVP*position;
textureCoordinate = inputPointCoordinate.xy;
color = vertexColor;
}
No, at least not in the vertex shader. You need to get the 3 different points in the vertex shader with different attribute values so you can receive the coordinate in the fragment shader which is interpolated.
What you actually can do is pass a center into the vertex shader which is the multiplied with the same matrix as the vertex coordinates. Beside that you would need some kind of radius (or the texture dimensions vector) which will probably need to be scaled if the matrix contains the scale as well. Then you can take both of these values and pass them to the fragment shader (using varying). In the fragment shader you then need to compute the texture coordinates from those 2 parameters and the fragment position.
A simular procedure is used to draw a very nice circle or sphere using only 2 triangles (a square) but I do not suggest you do this as you will only lose on performance plus it is quite a lot of work...
Related
Background
I'm looking at this example code from the WebGL2 library PicoGL.js.
It describes a single triangle (three vertices: (-0.5, -0.5), (0.5, -0.5), (0.0, 0.5)), each of which is assigned a color (red, green, blue) by the vertex shader:
#version 300 es
layout(location=0) in vec4 position;
layout(location=1) in vec3 color;
out vec3 vColor;
void main() {
vColor = color;
gl_Position = position;
}
The vColor output is passed to the fragment shader:
#version 300 es
precision highp float;
in vec3 vColor;
out vec4 fragColor;
void main() {
fragColor = vec4(vColor, 1.0);
}
and together they render the following image:
Question(s)
My understanding is that the vertex shader is called once per vertex, whereas the fragment shader is called once per pixel.
However, the fragment shader references the vColor variable, which is only assigned once per call to each vertex, but there are many more pixels than vertices!
The resulting image clearly shows a color gradient - why?
Does WebGL automatically interpolate values of vColor for pixels in between vertices? If so, how is the interpolation done?
Yes, WebGL automatically interpolates between the values supplied to the 3 vertices.
Copied from this site
A linear interpolation from one value to another would be this
formula
result = (1 - t) * a + t * b
Where t is a value from 0 to 1 representing some position between a and b. 0 at a and 1 at b.
For varyings though WebGL uses this formula
result = (1 - t) * a / aW + t * b / bW
-----------------------------
(1 - t) / aW + t / bW
Where aW is the W that was set on gl_Position.w when the varying was
as set to a and bW is the W that was set on gl_Position.w when the
varying was set to b.
The site linked above shows how that formula generates perspective correct texture mapping coordinates when interpolating varyings
It also shows an animation of the varyings changing
The khronos OpenGL wiki - Fragment Shader has the answer. Namely:
Each fragment has a Window Space position, a few other values, and it contains all of the interpolated per-vertex output values from the last Vertex Processing stage.
(Emphasis mine)
I'm trying to display textures on quads (2 triangles) using opengl 3.3
Drawing a texture on a quad works great; however when I have ONE textures (sprite atlas) but using 2 quads(objects) to display different parts of the atlas. When in draw loop, they end up switching back and fourth(one disappears than appears again, etc) at their individual translated locations.
The way I'm drawing this is not the standard DrawElements for each quad(or object) but I package all quads, uv, translations, etc send them up to the shader as one big chunk (as "in" variables): Vertex shader:
#version 330 core
// Input vertex data, different for all executions of this shader.
in vec3 vertexPosition_modelspace;
in vec3 vertexColor;
in vec2 vertexUV;
in vec3 translation;
in vec4 rotation;
in vec3 scale;
// Output data ; will be interpolated for each fragment.
out vec2 UV;
// Output data ; will be interpolated for each fragment.
out vec3 fragmentColor;
// Values that stay constant for the whole mesh.
uniform mat4 MVP;
...
void main(){
mat4 Model = mat4(1.0);
mat4 t = translationMatrix(translation);
mat4 s = scaleMatrix(scale);
mat4 r = rotationMatrix(vec3(rotation), rotation[3]);
Model *= t * r * s;
gl_Position = MVP * Model * vec4 (vertexPosition_modelspace,1); //* MVP;
// The color of each vertex will be interpolated
// to produce the color of each fragment
fragmentColor = vertexColor;
// UV of the vertex. No special space for this one.
UV = vertexUV;
}
Is the vertex shader working as I think it would with a large chunk of data - that it draws each segment passed up as uniform individually because it does not seem like it? Is my train of thought correct on this?
For completeness this is my fragment shader:
#version 330 core
// Interpolated values from the vertex shaders
in vec3 fragmentColor;
// Interpolated values from the vertex shaders
in vec2 UV;
// Ouput data
out vec4 color;
// Values that stay constant for the whole mesh.
uniform sampler2D myTextureSampler;
void main()
{
// Output color = color of the texture at the specified UV
color = texture2D( myTextureSampler, UV ).rgba;
}
A request for more information was made so I will put how i bind this data up to the vertex shader. The following code is just one I use for my translations. I have more for color, rotation, scale, uv, etc:
gl.BindBuffer(gl.ARRAY_BUFFER, tvbo)
gl.BufferData(gl.ARRAY_BUFFER, len(data.Translations)*4, gl.Ptr(data.Translations), gl.DYNAMIC_DRAW)
tAttrib := uint32(gl.GetAttribLocation(program, gl.Str("translation\x00")))
gl.EnableVertexAttribArray(tAttrib)
gl.VertexAttribPointer(tAttrib, 3, gl.FLOAT, false, 0, nil)
...
gl.DrawElements(gl.TRIANGLES, int32(len(elements)), gl.UNSIGNED_INT, nil)
You have just single sampler2D
which means you have just single texture at your disposal
regardless on how many of them you bind.
If you really need to pass the data as single block
then you should add sampler per each texture you got
not sure how many objects/textures you have
but you are limited by gfx hw limit on texture units with this way of data passing
also you need to add another value to your data telling which primitive use which texture unit
and inside fragment then select the right texture sampler ...
You should add stuff like this:
// vertex
in int usedtexture;
out int txr;
void main()
{
txr=usedtexture;
}
// fragment
uniform sampler2D myTextureSampler0;
uniform sampler2D myTextureSampler1;
uniform sampler2D myTextureSampler2;
uniform sampler2D myTextureSampler3;
in vec2 UV;
in int txr;
out vec4 color;
void main
{
if (txr==0) color = texture2D( myTextureSampler0, UV ).rgba;
else if (txr==1) color = texture2D( myTextureSampler1, UV ).rgba;
else if (txr==2) color = texture2D( myTextureSampler2, UV ).rgba;
else if (txr==3) color = texture2D( myTextureSampler3, UV ).rgba;
else color=vec4(0.0,0.0,0.0,0.0);
}
This way of passing is not good for these reasons:
number of used textures is limited to HW texture units limit
if your rendering would need additional textures like normal/shininess/light maps
then you need more then 1 texture per object type and your limit is suddenly divided by 2,3,4...
You need if/switch statements inside fragment which can slow things down considerably
Yes you can do it brunch less but then you would need to access all textures all the time increasing heat stress on gfx without reason...
This kind of passing is suitable for
all textures inside single image (as you mentioned texture atlas)
which can be faster this way and reasonable for scenes with small number of object types (or materials) but large object count...
Since I needed more input on this matter, I linked this page to reddit and someone was able to help me with one response! Anyways the reddit link is here:
https://www.reddit.com/r/opengl/comments/3gyvlt/opengl_passing_all_scene_data_into_shader_each/
The issue of seeing two individual textures/quads after passing all vertices as one data structure over to vertex shader was because my element indices were off. I needed to determine the correct index of each set of vertices for my 2 triangle(quad) objects. Simply had to do something like this:
vertexInfo.Elements = append(vertexInfo.Elements, uint32(idx*4), uint32(idx*4+1), uint32(idx*4+2), uint32(idx*4), uint32(idx*4+2), uint32(idx*4+3))
I want to draw multiple polygon shapes (where each shape has it's own set of vertices).
I want to be able to position these shapes independently of each other.
Which API can i use to set the a_Position for the vertex shader?
A) gl.vertexAttrib3f
B) gl.vertexAttribPointer + gl.enableVertexAttribArray
thanks.
Your question makes it sound like you're really new to WebGL? Maybe you should read some tutorials? But in answer to your question:
gl.vertexAttrib3f only lets you supply a single constant value to a GLSL attribute so you'll need to use gl.vertexAttribPointer and gl.enableVertexAttribArray. You'll also need to set up buffers with your vertex data.
gl.vertexAttrib3f only point is arguably to let you pass in a constant in the case that you have a shader that uses multiple attributes but you don't have data for all of them. For example lets say you have a shader that uses both textures and so needs texture coordinates and it also has vertex colors. Something like this
vertex shader
attribute vec4 a_position;
attribute vec2 a_texcoord;
attribute vec4 a_color;
varying vec2 v_texcoord;
varying vec4 v_color;
uniform mat4 u_matrix;
void main() {
gl_Position = u_matrix * a_position;
// pass texcoord and vertex colors to fragment shader
v_texcoord = a_texcoord;
v_color = v_color;
}
fragment shader
precision mediump float;
varying vec2 v_texcoord;
varying vec4 v_color;
uniform sampler2D u_texture;
void main() {
vec4 textureColor = texture2D(u_texture, v_texcoord);
// multiply the texture color by the vertex color
gl_FragColor = textureColor * v_color;
}
This shader requires vertex colors. If your geometry doesn't have vertex colors then you have 2 options (1) use a different shader (2) turn off the attribute for vertex colors and set it to a constant color, probably white.
gl.disableVertexAttribArray(aColorLocation);
gl.vertexAttrib4f(aColorLocation, 1, 1, 1, 1);
Now you can use the same shader even though you have no vertex color data.
Similarly if you have no texture coordinates you could pass in a white 1 pixel shader and set the texture coordinates to some constant.
gl.displayVertexAttribArray(aTexcoordLocation);
gl.vertexAttrib2f(aTexcoordLocation, 0, 0);
gl.bindTexture(gl.TEXTURE_2D, some1x1PixelWhiteTexture);
In that case you could also decide what color to draw with by setting the vertex color attribute.
gl.vertexAttrib4f(aColorLocation, 1, 0, 1, 1); // draw in magenta
I want to use one shader for both, coloured rectangles and/or rectangles which render textures. This is my approach:
precision mediump float;
uniform vec4 u_color;
uniform sampler2D u_image;
varying vec2 v_texCoord;
void main(void) {
vec4 texCol = texture2D(u_image, v_texCoord);
gl_FragColor = texCol * u_color;
}
But I get nothing. Where is my mistake?
If you do not have a valid texture specified for the sampler, the texture2D call will return transparent black — vec4(0.0, 0.0, 0.0, 0.0). Multiplying by that value will always produce black again.
Instead of trying to use “no texture”, use a texture which has exactly one pure white texel. This will cause the multiplication to yield the u_color.
Or, you can simply switch between two shader programs, one for untextured geometry and one for textured geometry. Switching programs (gl.useProgram) is not very expensive.
I am experimenting with GLSL for OpenGL ES 2.0. I have a quad and a texture I am rendering. I can successfully do it this way:
//VERTEX SHADER
attribute highp vec4 vertex;
attribute mediump vec2 coord0;
uniform mediump mat4 worldViewProjection;
varying mediump vec2 tc0;
void main()
{
// Transforming The Vertex
gl_Position = worldViewProjection * vertex;
// Passing The Texture Coordinate Of Texture Unit 0 To The Fragment Shader
tc0 = vec2(coord0);
}
//FRAGMENT SHADER
varying mediump vec2 tc0;
uniform sampler2D my_color_texture;
void main()
{
gl_FragColor = texture2D(my_color_texture, tc0);
}
So far so good. However, I'd like to do some pixel-based filtering, e.g. Median. So, I'd like to work in pixel coordinates rather than in normalized (tc0) and then convert the result back to normalized coords. Therefore, I'd like to use gl_FragCoord instead of a uv attribute (tc0). But I don't know how to go back to normalized coords because I don't know the range of gl_FragCoords. Any idea how I could get it? I have got that far, using a fixed value for 'normalization', though it's not working perfectly as it is causing stretching and tiling (but at least is showing something):
//FRAGMENT SHADER
varying mediump vec2 tc0;
uniform sampler2D my_color_texture;
void main()
{
gl_FragColor = texture2D(my_color_texture, vec2(gl_FragCoord) / vec2(256, 256));
}
So, the simple question is, what should I use in the place of vec2(256, 256) so that I could get the same result as if I were using the uv coords.
Thanks!
gl_FragCoord is in screen coordinates, so to get normalized coords you need to divide by the viewport width and height. You can use a uniform variable to pass that information to the shader, since there is no built in variable for it.
You can also sample the texture by un-normalized coordinates if:
sampling by texture() from GL_TEXTURE_RECTANGLE
sampling by texelFetch() from a regular texture or texture buffer