4 replies to this topic

[Plerion]

Hi everyone,

I found it a bit hard to set an appropriate title, but ill describe my problem a bit more in detail now.

Im currently trying to render a triangle using screen coordinates on my android device. I started as usual with a ortho matrix that projects the coordinates into device coordinates. As it didnt work i thought ill just skip the matrix for a moment and check from hardcoded values how i need to modify my code. So i started with the following set of coordinates:
[source lang="java"] float[] squareCoords = { 20.0f * 2 / 1280.0f, 30.0f * 2 / 750.0f, 0, 20.0f * 2 / 1280.0f, 0, 0, 0, 0, 0, };[/source]

And a simple shader with:
[source lang="cpp"]attribute vec4 vPosition;void main() { gl_Position = vPosition;}[/source]

So far so good, i get a little triangle around the middle of the screen vertically mirrored, everything as expected. In a next step i changed my coordinates:
[source lang="java"] float[] squareCoords = { 20.0f, 30.0f, 0, 20.0f, 0, 0, 0, 0, 0, };[/source]

But also modified my shader:
[source lang="cpp"]attribute vec4 vPosition;void main() { gl_Position = vec4( vPosition.x * 2 / 1280.0f, vPosition.y * 2 / 750.0f, vPosition.z, vPosition.w);}[/source]

In my opinion this is exactly the same thing as before, i just moved the projection to the shader instead of hardcoded values, but now the screen is completely black, nothing gets rendered at all!

Am i missing something?

Greetings
Plerion

[PAndersson]

Do you somehow set your homogenus coordinate beforehand? To me, it seems like you do not.

[Plerion]

No, but it makes no difference if i set the w-component to 1 or 0 or any other value.

[PAndersson]

What does your glVertexAttribPointer calls look like? Anyway, you should still set your w-component to 1.

[Plerion]

glVertexAttribPtr(index /* obtained from glGetAttribLocation with vPosition and is 0 */, 3, GL_FLOAT, false, 0, elem.getStream())

getStream is a FloatBuffer where i write the coordinates from above.

Just to mention: With the first version i posted the triangle is OK, in the second version there is no triangle.