How to stop OpenGL from applying blending to certain content? (cocos2d/iPhone/OpenGL)
- by RexOnRoids
Supporting Info:
I use cocos2d to draw a sprite (graph background) on the screen (z:-1). I then use cocos2d to draw lines/points (z:0) on top of the background -- and make some calls to OpenGL blending functions before the drawing to SMOOTH out the lines.
Problem:
The problem is that: aside from producing smooth lines/points, calling these OpenGL blending functions seems to degrade the underlying sprite (graph background).
So there is a tradeoff: I can either have (Case 1) a nice background and choppy lines/points, or I can have (Case 2) nice smooth lines/points and a degraded background. But obviously I need both.
The Code:
I have included code of the draw() method of the CCLayer for both cases explained above. As you can see, the code producing the difference between Case 1 and Case 2 seems to be 1 or 2 lines involving OpenGL Blending.
Case 1 -- MainScene.h (CCLayer):
-(void)draw{
int lastPointX = 0;
int lastPointY = 0;
GLfloat colorMAX = 255.0f;
GLfloat valR;
GLfloat valG;
GLfloat valB;
if([self.myGraphManager ready]){
valR = (255.0f/colorMAX)*1.0f;
valG = (255.0f/colorMAX)*1.0f;
valB = (255.0f/colorMAX)*1.0f;
NSEnumerator *enumerator = [[self.myGraphManager.currentCanvas graphPoints] objectEnumerator];
GraphPoint* object;
while ((object = [enumerator nextObject])) {
if(object.filled){
/*Commenting out the following two lines induces a problem of making it impossible to have smooth lines/points, but has merit in that it does not degrade the background sprite.*/
//glEnable (GL_BLEND);
//glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glHint (GL_LINE_SMOOTH_HINT, GL_DONT_CARE);
glEnable (GL_LINE_SMOOTH);
glLineWidth(1.5f);
glColor4f(valR, valG, valB, 1.0);
ccDrawLine(ccp(lastPointX, lastPointY), ccp(object.position.x, object.position.y));
lastPointX = object.position.x;
lastPointY = object.position.y;
glPointSize(3.0f);
glEnable(GL_POINT_SMOOTH);
glHint(GL_POINT_SMOOTH_HINT, GL_NICEST);
ccDrawPoint(ccp(lastPointX, lastPointY));
}
}
}
}
Case 2 -- MainScene.h (CCLayer):
-(void)draw{
int lastPointX = 0;
int lastPointY = 0;
GLfloat colorMAX = 255.0f;
GLfloat valR;
GLfloat valG;
GLfloat valB;
if([self.myGraphManager ready]){
valR = (255.0f/colorMAX)*1.0f;
valG = (255.0f/colorMAX)*1.0f;
valB = (255.0f/colorMAX)*1.0f;
NSEnumerator *enumerator = [[self.myGraphManager.currentCanvas graphPoints] objectEnumerator];
GraphPoint* object;
while ((object = [enumerator nextObject])) {
if(object.filled){
/*Enabling the following two lines gives nice smooth lines/points, but has a problem in that it degrades the background sprite.*/
glEnable (GL_BLEND);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glHint (GL_LINE_SMOOTH_HINT, GL_DONT_CARE);
glEnable (GL_LINE_SMOOTH);
glLineWidth(1.5f);
glColor4f(valR, valG, valB, 1.0);
ccDrawLine(ccp(lastPointX, lastPointY), ccp(object.position.x, object.position.y));
lastPointX = object.position.x;
lastPointY = object.position.y;
glPointSize(3.0f);
glEnable(GL_POINT_SMOOTH);
glHint(GL_POINT_SMOOTH_HINT, GL_NICEST);
ccDrawPoint(ccp(lastPointX, lastPointY));
}
}
}
}
