opengl color matrix

Graphics and GPU Programming Programming OpenGL

Started by dasan11 October 23, 2006 09:36 PM

7 comments, last by dasan11 17 years, 6 months ago

122

Author

October 23, 2006 09:36 PM

I want to use the color matrix to copy the red channel to green channel. Before the operation,the color of the pixel is (1,0,0), after the operation,the color changes to (1,1,0). how to do this?

zimerman

176

October 24, 2006 04:03 AM

try (should copy red to green & keep the other three):

1 0 0 0
1 0 0 0
0 0 1 0
0 0 0 1

cheers
zimerman

dasan11

122

Author

October 24, 2006 07:35 AM

Quote:Original post by zimerman
try (should copy red to green & keep the other three):

1 0 0 0
1 0 0 0
0 0 1 0
0 0 0 1

cheers
zimerman

GLfloat color[16]={1,0,0,0,
1,0,0,0,
0,0,1,0,
0,0,0,1};
glClear(GL_COLOR_BUFFER_BIT);
glColor4f(1.0, 0.0, 0.0, 0.5);
glBegin (GL_TRIANGLES);
glVertex2f(0.9, 0.9);
glVertex2f(0.3, 0.5);
glVertex2f(0.9, 0.1);
glEnd();
glMatrixMode(GL_COLOR);
glMultMatrixf(color);
glMatrixMode(GL_MODELVIEW);

glBegin (GL_TRIANGLES);
glVertex2f(0.4, 0.9);
glVertex2f(0.3, 0.5);
glVertex2f(0.4, 0.1);
glEnd();
why the two triangles are still red?
How to write?

zimerman

176

October 24, 2006 07:47 AM

OpenGL uses column major storage for matrices, so

GLfloat color[16]={1,0,0,0,
1,0,0,0,
0,0,1,0,
0,0,0,1};

equals

1 1 0 0
0 0 0 0
0 0 1 0
0 0 0 1

try

GLfloat color[16]={1,1,0,0,
0,0,0,0,
0,0,1,0,
0,0,0,1};

cheers
zimerman

dasan11

122

Author

October 24, 2006 10:51 AM

I tried it .

GLfloat color[16]={1,1,0,0,
0,0,0,0,
0,0,1,0,
0,0,0,1};

But it's still red.

void display(void)
{
GLfloat color[16]={1,1,0,0,
0,0,0,0,
0,0,1,0,
0,0,0,1};
glClear(GL_COLOR_BUFFER_BIT);
glColor4f(1.0, 0.0, 0.0, 0.5);
glBegin (GL_TRIANGLES);
glVertex2f(0.9, 0.9);
glVertex2f(0.3, 0.5);
glVertex2f(0.9, 0.1);
glEnd();
glMatrixMode(GL_COLOR);
glLoadIdentity();
glMultMatrixf(color);
glMatrixMode(GL_MODELVIEW);

glBegin (GL_TRIANGLES);
glVertex2f(0.4, 0.9);
glVertex2f(0.3, 0.5);
glVertex2f(0.4, 0.1);
glEnd();

glFlush();
}

Kalidor

1,087

October 24, 2006 11:16 AM

The color matrix is only used during pixel transfer operations such as glDrawPixels or glTex[Sub]Image2D.

zimerman

176

October 24, 2006 11:17 AM

did a quick google look-up:

http://oss.sgi.com/projects/performer/mail/info-performer/perf-03-12/0020.html

...Kalidor is right...sry for raising your hopes (never used w/ color matrices myself)...

zimerman (wearing sackcloth & ashes)

[Edited by - zimerman on October 24, 2006 11:17:33 AM]

ff8

134

October 24, 2006 02:02 PM

you can make copy of the buffer after drawing everything then enable it as texture on quad that fill the whole screen and then u can use color_matrix for coping the channs this will be bit slower tho.
-EDIT :
also if ur graphic card support shader u can do this
<Vertex_Shader>:

void main(){  gl_FrontColor = vec4(gl_Color.x,gl_Color.x,0,1.0);gl_Position = ftransform();}

<Pixel_Shader>:

void main(){gl_FragColor = gl_Color;}

take care.

[Edited by - ff8 on October 24, 2006 2:02:38 PM]

dasan11

122

Author

October 24, 2006 07:56 PM

Thank you .I use glReadPixels to do this transform.

GLfloat *pixels=(GLfloat *)malloc(200*200*4);
GLfloat color[16]={1,0,1,0,
0,1,0,0,
0,0,0,0,
0,0,0,1};
glClear(GL_COLOR_BUFFER_BIT);
glColor4f(1.0, 0.0, 0.0, 0.5);
glBegin (GL_TRIANGLES);
glVertex2f(0.9, 0.9);
glVertex2f(0.3, 0.5);
glVertex2f(0.9, 0.1);
glEnd();
glReadPixels(0,0,200,200,GL_RGBA,GL_UNSIGNED_BYTE,pixels);
glMatrixMode(GL_COLOR);
glLoadIdentity();
glMultMatrixf(color);
glDrawPixels(200,200,GL_RGBA,GL_UNSIGNED_BYTE,pixels);

opengl color matrix

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