I'm stuck on something that should be simple but it is evading me. I have
searched this mailing list and regular documentation but can't figure out an
answer.
I'm using the iOS platform. Previously, my drawing code needed to draw images
at different locations in the view erasing everything prior to that (i.e. there
was only one image visible at any one time). To do this I simply invalidated
the entire view with [theView setNeedsDisplay] and in drawRect:aRect I drew the
new image. This all works fine. However, now, I want multiple images visible at
the same time and selectively replace images as necessary (by 'overwriting'
them). My approach has been to invalidate just the region within the view that
I want to draw into using [theView setNeedsDisplayInRect:aRect] and then draw
the new image at the desired location (i.e. inside aRect). This doesn't work
hence this email. The problem is that the old image is not erased prior to
drawing the new image hence all the images 'accumulate'. Here is the relevant
code:
View creation:
[self setTouchSequenceViewSys0:[[RTTouchSequenceView alloc]
initWithFrame:baseRect]];
[[self touchSequenceViewSys0] setAlpha:1.0];
[[self touchSequenceViewSys0] setOpaque:NO];
[[self touchSequenceViewSys0] setClearsContextBeforeDrawing:YES];
[[self touchSequenceViewSys0] setTouchClearImage:[[self imageArrayLookup]
objectForKey:@"touchClear"]];
[[self touchSequenceViewSys0] setTouchSequence:nil]; // to prevent any
drawing when view is added as a subview
[[self view] addSubview:[self touchSequenceViewSys0]];
View invalidation:
CGRect aRect = CGRectMake([[[[self touchSequence]
objectAtIndex:[self touchDisplayStartIndex]]
beginDrawLocationX] intValue],
[[[[self touchSequence]
objectAtIndex:[self touchDisplayStartIndex]]
beginDrawLocationY] intValue],
[[[[self touchSequence]
objectAtIndex:tmpIndex]
beginDrawLocationX] intValue]
- [[[[self touchSequence]
objectAtIndex:touchDisplayStartIndex]
beginDrawLocationX] intValue]
+ [[[[self touchSequence]
objectAtIndex:tmpIndex]
displayImage] size].width,
[[[[self touchSequence]
objectAtIndex:tmpIndex]
beginDrawLocationY] intValue]
- [[[[self touchSequence]
objectAtIndex:touchDisplayStartIndex]
beginDrawLocationY] intValue]
+ [[[[self touchSequence]
objectAtIndex:tmpIndex]
displayImage] size].height);
[[self touchSequenceViewSys0] setTouchSequence:[self
touchSequence]];
[[self touchSequenceViewSys0] setTouchDisplayStartIndex:[self
touchDisplayStartIndex]];
[[self touchSequenceViewSys0] setTouchDisplayEndIndex:tmpIndex];
[[self touchSequenceViewSys0] setNeedsDisplayInRect:aRect];
View drawing:
- (void)drawRect:(CGRect)rect {
[super drawRect:rect];
if (touchSequence) {
BOOL isDisplayDone = NO;
while (!isDisplayDone) {
RTTouchSequenceItem *item = [[self touchSequence]
objectAtIndex:[self touchDisplayStartIndex]];
CGPoint aPoint = CGPointMake([[item beginDrawLocationX]
doubleValue],
[[item beginDrawLocationY]
doubleValue]);
//[[self touchClearImage] drawAtPoint:aPoint
blendMode:kCGBlendModeNormal alpha:0.0]; // <-- see note below
[[item displayImage] drawAtPoint:aPoint
blendMode:kCGBlendModeNormal alpha:[[item
displayImageAlpha] doubleValue]]; // <-- alpha is in fact 1.0
if ([self touchDisplayStartIndex] == [self touchDisplayEndIndex]) {
isDisplayDone = YES;
}
else {
[self setTouchDisplayStartIndex:[self touchDisplayStartIndex] +
1];
}
}
}
[self setTouchSequence:nil]; // can do this since touchSequence is not
retained
}
The commented out line:
[[self touchClearImage] drawAtPoint:aPoint blendMode:kCGBlendModeNormal
alpha:0.0];
was an attempt to draw a transparent black rectangle first followed by the
desired image. That had no effect. I've also tried variations on kCGBlendMode,
alpha and opaque settings but, under all combinations, fundamentally, the new
image is additive with respect to the old image(s).
How do I clear a region on the view prior to drawing a new image? Apologies is
this is a hopelessly 'newbie' question.....
Andrew Coad
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