# Drawing IZOLINES of temperature

Hello everybody!

I stuck with my problem.

PROBLEM:

• very slow algorithm
• no idea how to improve it
• any existing algorithm in Java/ JOGL?

HOW I DO:
I use shader to calculate izolines of temperature and i obtain for example this:

http://imageshack.us/photo/my-images/26/izoline.png

now i must to draw in JOGL black lines on color edges. I read all pixels from GL buffer.

``````public BufferedImage toImage(GL gl, int w, int h)
{

ByteBuffer glBB = ByteBuffer.allocate(3 * w * h);
gl.glReadPixels(0, 0, w, h, GL.GL_BGR, GL.GL_BYTE, glBB);

BufferedImage bi = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
int[] bd = ((DataBufferInt) bi.getRaster().getDataBuffer()).getData();

for (int y = 0; y < h; y++) {
for (int x = 0; x < w; x++) {
int b = 2 * glBB.get();
int g = 2 * glBB.get();
int r = 2 * glBB.get();
bd[(h - y - 1) * w + x] = (r << 16) | (g << 8) | b | 0xFF000000;
}
}

return bi;
}
``````

But trying find edge pixel by pixel is soooo long in time. Like this:

``````BufferedImage bi = toImage(m_gl, Resources.dCanvasWidth, Resources.dCanvasHeight);
for(int i=0; i< 20; ++i)
{
System.out.println( i );
for(int i1=0; i1< bi.getData().getHeight()-1; ++i1)
{
int[] buff1;
int[] buff2;

buff1 = bi.getData().getPixel(i, i1, (int[]) null );
buff2 = bi.getData().getPixel(i, i1, (int[]) null );

if( !Arrays.equals(buff1, buff2) )
{
m_gl.glDrawPixels(i, i1,GL.GL_RGB, GL.GL_INT, blackPixel);
m_gl.glFlush();

}

}
``````

Can somebody help me solve this problem?

Hi

Maybe you can compute iso-contours in the fragment shader too.

I tried but i havent neighbour pixels.

When I calculate it without neigbours but by function calculating my color. I tried obtain izo by 10 degrees it doted or all black, or line is to width.

Do it in two passes and you’ll have access to neighboring pixels in the second pass. You can’t have each pixel depend on what’s being put into neighboring pixels or you’ll get a paradox. =S

Hmm i agree with you but how i can write somthing like that?

``````
gl.glVertexAttrib1f(vs_fTempAtribute, (float) t1.getValueInNode( Resources.mesh.getFace( Resources.mesh.getArea(dAreaCounter).getFaceNr(dFaceCounter) ).m_dwVertex[0]));
gl.glVertex2d(  Resources.mesh.getVertex( Resources.mesh.getFace( Resources.mesh.getArea(dAreaCounter).getFaceNr(dFaceCounter) ).m_dwVertex[0] ).m_position.x,
Resources.mesh.getVertex( Resources.mesh.getFace( Resources.mesh.getArea(dAreaCounter).getFaceNr(dFaceCounter) ).m_dwVertex[0] ).m_position.y);
gl.glVertexAttrib1f(vs_fTempAtribute, (float) t1.getValueInNode( Resources.mesh.getFace( Resources.mesh.getArea(dAreaCounter).getFaceNr(dFaceCounter) ).m_dwVertex[1]));
gl.glVertex2d(  Resources.mesh.getVertex( Resources.mesh.getFace( Resources.mesh.getArea(dAreaCounter).getFaceNr(dFaceCounter) ).m_dwVertex[1] ).m_position.x,
Resources.mesh.getVertex( Resources.mesh.getFace( Resources.mesh.getArea(dAreaCounter).getFaceNr(dFaceCounter) ).m_dwVertex[1] ).m_position.y);
gl.glVertexAttrib1f(vs_fTempAtribute, (float) t1.getValueInNode( Resources.mesh.getFace( Resources.mesh.getArea(dAreaCounter).getFaceNr(dFaceCounter) ).m_dwVertex[2]));
gl.glVertex2d(  Resources.mesh.getVertex( Resources.mesh.getFace( Resources.mesh.getArea(dAreaCounter).getFaceNr(dFaceCounter) ).m_dwVertex[2] ).m_position.x,
Resources.mesh.getVertex( Resources.mesh.getFace( Resources.mesh.getArea(dAreaCounter).getFaceNr(dFaceCounter) ).m_dwVertex[2] ).m_position.y);
``````

First i put in shader a temperature and it calculate color. Where put second step? On this same vertices? I do not imagine that

Render what you’re currently doing directly to a texture using a framebuffer object (FBO). That way you’ll have the data easily accessible later. Then render (using just a single fullscreen quad) this texture to the screen (back buffer) using a different shader that finds your edges or whatever and adds them to the screen. This is called post-processing a rendered image and is pretty much what you’re doing on the CPU right now, am I right?

In the code is not as easy as in theory ;( I can’t imagine how … I need to sleep mayby tomorrow will be better.

You can use FBObject to implement theagentd’s suggestion.