How to transform only part of an array in OpenGL ES?

Question

I have one texture atlas of which I select the "non-animated textures", I put all vertex and UV coordinates in two array:

vertices = new float[] {
            // background
            0.0f, 1920f, 0.0f, 
            0.0f, 0.0f, 0.0f, 
            1080f, 0.0f, 0.0f, 
            1080f, 1920f, 0.0f, 
            // character
            240f, 760.0f, 0.0f, 
            240f, 14.0f, 0.0f, 
            770f, 14.0f, 0.0f, 
            770f, 760.0f, 0.0f, 
            // object (Will be transformed)
            360f, 800f, 0.0f, 
            360f, 600.0f, 0.0f, 
            500f, 600.0f, 0.0f, 
            500f, 800f, 0.0f,
    };

uv = new float[] {
            // background
            0.50f, 0.25f,
            0.50f, 0.75f,
            1.0f, 0.75f,
            1.0f, 0.25f,
            // character
            0.0f, 0.25f,
            0.0f, 0.75f,
            0.25f, 0.75f,
            0.25f, 0.25f,
            // object
            0.0f, 0.75f,
            0.0f, 1.0f,
            0.25f, 1.0f,
            0.25f, 0.75f
    };

My vertex shader:

uniform mat4 matrix;
attribute vec4 vPosition;
attribute vec2 a_texCoord;
varying vec2 v_texCoord;

void main() {
    gl_Position = matrix * vPosition;
    v_texCoord = a_texCoord;
}

And my fragment shader:

precision mediump float;

varying vec2 v_texCoord;
uniform sampler2D a_texture;

void main() {
    gl_FragColor = texture2D(a_texture, v_texCoord);
}

I don't know how to animate just a part of the arrangements, also I try to create one more texture but when I run the app I only see the last texture.

What do your shaders look like in this case? – user1118321 Jun 19 '18 at 03:09 — user1118321, Jun 19 '18 at 03:09

score 2 · Accepted Answer · answered Jun 19 '18 at 05:19

So the first thing I notice is that you only have 3 rectangles that you are drawing. This is a very small number and unlikely to tax the GPU. Your best bet may be to simply make 3 separate draw calls. Setting things up to do this in a more complicated way seems needlessly complex.

Second, if you want to use more than 1 texture, you need to sample from more than 1 texture. You are only ever sampling from a single texture in your fragment shader, so that's all you'll see. Again, you could put drawing of all 3 objects into a single shader, but that just makes things more complicated without any benefit. If it were me, I would use the shader you have. I would set the texture unit used to a different texture unit before each draw call. So before each draw call, make a call to glUniform1i(a_texture_location, x) where a_texture_location is the result of glGetUniformLocation("a_texture"); and x is the texture unit where the texture for that object lives (i.e. GL_TEXTURE0, GL_TEXTURE1, etc.).

Thank. Now I can see the last texture but if I transform the array (i.e. rotate) the transformation applies to everything, I suppose just for transform a part I need to have the rectangle that I want to apply the transform in another array, and if I'm not so wrong also I suppose that I need to create another vertex buffer, pass the new vertex data to the shader, and I do not know what else needs to be done. I tried but the app crashes. — Dr. Mundo, Jun 19 '18 at 07:16

How to transform only part of an array in OpenGL ES?

1 Answers1