CGContextSetRBStrokeColor to replace, not add, for alpha?

Posted by Peter Hajas on Stack Overflow See other posts from Stack Overflow or by Peter Hajas
Published on 2010-05-06T23:21:49Z Indexed on 2010/05/06 23:28 UTC
Read the original article Hit count: 384

Filed under:

cgcontext

|

quartz-2d

|

uiimage

|

uiimageview

|

iphone-sdk-3.0

I use CGContexts to allow the user to draw in my application, here's an example:

CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound); CGContextSetLineWidth(UIGraphicsGetCurrentContext(), size); CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(),r,g,b,a);

I want the user to have an "eraser" tool that actually erases - it can't draw white (that would add white to the image, which may not have a white background) or alpha (or else the stroke won't do anything). Is there a way to do this? Perhaps replacing contents of a CGContext or UIImage?

© Stack Overflow or respective owner

Related posts about cgcontext

Quartz 2D: draw from a CGContext to another CGContext

as seen on Stack Overflow - Search for 'Stack Overflow'
I have a CGBitmapContext (bitmapContext) and I would like to draw some rectangle part (rect) of it to the current CGContext (context). Right now I do that way: CGContextRef context = UIGraphicsGetCurrentContext(); CGImageRef cgImage = CGBitmapContextCreateImage(bitmapContext); CGContextClipToRect(context… >>> More
Drawing triangle/arrow on a line with CGContext

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi, I am using the framework of route-me for working with locations. In this code the path between two markers(points) will be drawn as a line. My Question: "What code should I add if I want to add an arrow in the middle(or top) of the line, so that it points the direction" Thanks - (void)drawInContext:(CGContextRef)theContext { … >>> More
What way to use the CGContext to draw is suitable?

as seen on Stack Overflow - Search for 'Stack Overflow'
I know that the CGContext cannot call it to draw directly, and it needs to fill the drawing logic in the drawInContext, and call the CGContext to draw using "setNeedsDisplay", so, I designed a cmd to execute, but it cause some problems... like this : http://stackoverflow.com/questions/2617827/why-i-cant-draw-in-a-loop-using-uiview-in-iphone I… >>> More
CGContext - PDF margin

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi All I am showing PDF content on a view using this code using Quartz Sample // PDF page drawing expects a Lower-Left coordinate system, so we flip the coordinate system // before we start drawing. CGContextTranslateCTM(context, 0.0, self.bounds.size.height); CGContextScaleCTM(context, 1.0… >>> More
Drawing a PDF Right-Side-Up in CGContext

as seen on Stack Overflow - Search for 'Stack Overflow'
I'm overriding the drawRect: method in a custom UIView, and I'm doing some custom drawing. All was going well, until I needed to draw a PDF resource (a vector glyph, to be precise) into the context. First I retrieve the PDF from a file: NSURL *pdfURL = [NSURL fileURLWithPath:[[[NSBundle mainBundle]… >>> More

Related posts about quartz-2d

"Beveled" Shapes in Quartz 2D

as seen on Stack Overflow - Search for 'Stack Overflow'
I'm familiar with some of the basics of Quartz 2D drawing, like drawing basic shapes and gradients and so on, but I'm not sure how to draw a shape with a "beveled" look, like this: Essentially we've got a shine on one corner, and maybe some shading in the opposite corner. I think -- I didn't make… >>> More
drawing different quartz 2d

as seen on Stack Overflow - Search for 'Stack Overflow'
Hello, I'm trying to make a small application or demo thats a tab bar application. Each bar item loads in a new view. I've created a quartzView class that is called on by the controller: - (void)viewDidLoad { quartzView *drawingView = [[quartzView alloc] initWithFrame:CGRectMake(0,0,320,480)]; … >>> More
Freehand drawing with Quartz 2D

as seen on Stack Overflow - Search for 'Stack Overflow'
I am trying to develop a simple freehand-drawing app for red-marking some of my images. Below is a method that is called from within the drawRect. I want to ask if there is a way to avoid add the path every time the path is redrawn? Also, the way the draw method is set up, the drawing gets a bit… >>> More
Horizontal lines and PDF drawing on iPhone/iPad Quartz 2D

as seen on Stack Overflow - Search for 'Stack Overflow'
I'm trying to display a PDF of music using the Quartz 2D calls: CGPDFDocumentGetPage and CGPDFPageGetDrawingTransform The problem is that some horizontal lines in the PDF get drawn thicker than others. The biggest problem is that I notice this in Preview on the Mac and on iPhone/iPad. Even when… >>> More
Simple iPhone drawing app with Quartz 2D

as seen on Stack Overflow - Search for 'Stack Overflow'
I am making a simple iPhone drawing program as a personal side-project. I capture touches event in a subclassed UIView and render the actual stuff to a seperate CGLayer. After each render, I call [self setNeedsLayout] and in the drawRect: method I draw the CGLayer to the screen context. This all… >>> More