CGContextDrawImage draws image upside down when passed UIImage CGImage

Question

Does anyone know why CGContextDrawImage would be drawing my image upside down  I am loading an image in from my application   UIImage  image    UIImage imageNamed   testImage png      And then simply asking core graphics to draw it to my context   CGContextDrawImage context  CGRectMake 0  0  145  15   image CGImage     It renders in the right place  and dimensions  but the image is upside down  I must be missing something really obvious here

User · Answer

UIImage contains a CGImage as its main content member as well as scaling and orientation factors  Since CGImage and its various functions are derived from OSX  it expects a coordinate system that is upside down compared to the iPhone  When you create a UIImage  it defaults to an upside-down orientation to compensate  you can change this    Use the  CGImage property to access the very powerful CGImage functions  but drawing onto the iPhone screen etc  is best done with the UIImage methods

User · Answer

It happens because QuartzCore has the coordinate system  bottom-left   while UIKit      top-left    In the case you can extend CGContext   extension CGContext     func changeToTopLeftCoordinateSystem         translateBy x  0  y  boundingBoxOfClipPath size height      scaleBy x  1  y  -1            somewhere in render  ctx saveGState   ctx changeToTopLeftCoordinateSystem   ctx draw cgImage   in  frame

User · Answer

Even after applying everything I have mentioned  I ve still had dramas with the images   In the end  i ve just used Gimp to create a  flipped vertical  version of all my images   Now I don t need to use any Transforms   Hopefully this won t cause further problems down the track      Does anyone know why   CGContextDrawImage would be drawing my   image upside down  I am loading an   image in from my application    Quartz2d uses a different co-ordinate system  where the origin is in the lower left corner   So when Quartz draws pixel x 5   y 10  of a 100   100 image  that pixel is being drawn in the lower left corner instead of the upper left   Thus causing the  flipped  image   The x co-ordinate system matches  so you will need to flip the y co-ordinates   CGContextTranslateCTM context  0  image size height     This means we have translated the image by 0 units on the x axis and by the images height on the y axis   However  this alone will mean our image is still upside down  just being drawn  image size height  below where we wish it to be drawn   The Quartz2D programming guide recommends using ScaleCTM and passing negative values to flip the image   You can use the following code to do this -  CGContextScaleCTM context  1 0  -1 0     Combine the two just before your CGContextDrawImage call and you should have the image drawn correctly   UIImage  image    UIImage imageNamed   testImage png        CGRect imageRect   CGRectMake 0  0  image size width  image size height           CGContextTranslateCTM context  0  image size height   CGContextScaleCTM context  1 0  -1 0    CGContextDrawImage context  imageRect  image CGImage     Just be careful if your imageRect co-ordinates do not match those of your image  as you can get unintended results   To convert back the coordinates   CGContextScaleCTM context  1 0  -1 0   CGContextTranslateCTM context  0  -imageRect size height

User · Answer

Even after applying everything I have mentioned  I ve still had dramas with the images   In the end  i ve just used Gimp to create a  flipped vertical  version of all my images   Now I don t need to use any Transforms   Hopefully this won t cause further problems down the track      Does anyone know why   CGContextDrawImage would be drawing my   image upside down  I am loading an   image in from my application    Quartz2d uses a different co-ordinate system  where the origin is in the lower left corner   So when Quartz draws pixel x 5   y 10  of a 100   100 image  that pixel is being drawn in the lower left corner instead of the upper left   Thus causing the  flipped  image   The x co-ordinate system matches  so you will need to flip the y co-ordinates   CGContextTranslateCTM context  0  image size height     This means we have translated the image by 0 units on the x axis and by the images height on the y axis   However  this alone will mean our image is still upside down  just being drawn  image size height  below where we wish it to be drawn   The Quartz2D programming guide recommends using ScaleCTM and passing negative values to flip the image   You can use the following code to do this -  CGContextScaleCTM context  1 0  -1 0     Combine the two just before your CGContextDrawImage call and you should have the image drawn correctly   UIImage  image    UIImage imageNamed   testImage png        CGRect imageRect   CGRectMake 0  0  image size width  image size height           CGContextTranslateCTM context  0  image size height   CGContextScaleCTM context  1 0  -1 0    CGContextDrawImage context  imageRect  image CGImage     Just be careful if your imageRect co-ordinates do not match those of your image  as you can get unintended results   To convert back the coordinates   CGContextScaleCTM context  1 0  -1 0   CGContextTranslateCTM context  0  -imageRect size height

User · Answer

Swift 3 CoreGraphics Solution  If you want to use CG for whatever your reason might be  instead of UIImage  this Swift 3 construction based on previous answers did solve the issue for me   if let cgImage   uiImage cgImage       cgContext saveGState       cgContext translateBy x  0 0  y  cgRect size height      cgContext scaleBy x  1 0  y  -1 0      cgContext draw cgImage  in  cgRect      cgContext restoreGState

User · Answer

drawInRect is certainly the way to go   Here s another little thing that will come in way useful when doing this   Usually the picture and the rectangle into which it is going to go don t conform   In that case drawInRect will stretch the picture   Here s a quick and cool way to make sure that the picture s aspect ration isn t changed  by reversing the transformation  which will be to fit the whole thing in      Picture and irect don t conform  so there ll be stretching  compensate     float xf   Picture size width irect size width      float yf   Picture size height irect size height      float m   MIN xf  yf       xf    m      yf    m      CGContextScaleCTM ctx  xf  yf         Picture drawInRect  irect

User · Answer

I m not sure for UIImage  but this kind of behaviour usually occurs when coordinates are flipped  Most of OS X coordinate systems have their origin at the lower left corner  as in Postscript and PDF  But CGImage coordinate system has its origin at the upper left corner   Possible solutions may involve an isFlipped property or a scaleYBy -1 affine transform

User · Answer

Swift 5 answer based on  ZpaceZombor s excellent answer  If you have a UIImage  just use   var image  UIImage         image draw in  CGRect    If you have a CGImage use my category below  Note  Unlike some other answers  this one takes into account that the rect you want to draw in might have y    0  Those answers that don t take that into account are incorrect and won t work in the general case   extension CGContext       final func drawImage image  CGImage  inRect rect  CGRect               flip coords         let ty  CGFloat    rect origin y   rect size height          translateBy x  0  y  ty          scaleBy x  1 0  y  -1 0             draw image         let rect  y zero   CGRect x  rect origin x  y  0  width  rect width  height  rect height          draw image  in  rect  y zero             flip back         scaleBy x  1 0  y  -1 0          translateBy x  0  y  -ty              Use like this   let imageFrame  CGRect       let context  CGContext        let img  CGImage          context drawImage image  img  inRect  imageFrame

User · Answer

Instead of   CGContextDrawImage context  CGRectMake 0  0  145  15   image CGImage     Use    image drawInRect CGRectMake 0  0  145  15      In the middle of your begin end CGcontext methods   This will draw the image with the correct orientation into your current image context - I m pretty sure this has something to do with the UIImage holding onto knowledge of the orientation while the CGContextDrawImage method gets the underlying raw image data with no understanding of orientation

User · Answer

Swift 5 answer based on  ZpaceZombor s excellent answer  If you have a UIImage  just use   var image  UIImage         image draw in  CGRect    If you have a CGImage use my category below  Note  Unlike some other answers  this one takes into account that the rect you want to draw in might have y    0  Those answers that don t take that into account are incorrect and won t work in the general case   extension CGContext       final func drawImage image  CGImage  inRect rect  CGRect               flip coords         let ty  CGFloat    rect origin y   rect size height          translateBy x  0  y  ty          scaleBy x  1 0  y  -1 0             draw image         let rect  y zero   CGRect x  rect origin x  y  0  width  rect width  height  rect height          draw image  in  rect  y zero             flip back         scaleBy x  1 0  y  -1 0          translateBy x  0  y  -ty              Use like this   let imageFrame  CGRect       let context  CGContext        let img  CGImage          context drawImage image  img  inRect  imageFrame

User · Answer

Best of both worlds  use UIImage s drawAtPoint  or drawInRect  while still specifying your custom context   UIGraphicsPushContext context    image drawAtPoint CGPointZero      UIImage will handle all especial cases  UIGraphicsPopContext      Also you avoid modifying your context with CGContextTranslateCTM or CGContextScaleCTM which the second answer does

User · Answer

It happens because QuartzCore has the coordinate system  bottom-left   while UIKit      top-left    In the case you can extend CGContext   extension CGContext     func changeToTopLeftCoordinateSystem         translateBy x  0  y  boundingBoxOfClipPath size height      scaleBy x  1  y  -1            somewhere in render  ctx saveGState   ctx changeToTopLeftCoordinateSystem   ctx draw cgImage   in  frame

User · Answer

Swift 3 0  amp  4 0  yourImage draw in  CGRect  blendMode  CGBlendMode  alpha  ImageOpacity    No Alteration needed

User · Answer

I m not sure for UIImage  but this kind of behaviour usually occurs when coordinates are flipped  Most of OS X coordinate systems have their origin at the lower left corner  as in Postscript and PDF  But CGImage coordinate system has its origin at the upper left corner   Possible solutions may involve an isFlipped property or a scaleYBy -1 affine transform

User · Answer

We can solve this problem using the same function   UIGraphicsBeginImageContext image size    UIGraphicsPushContext context     image drawInRect CGRectMake gestureEndPoint x gestureEndPoint y 350 92     UIGraphicsPopContext     UIGraphicsEndImageContext

User · Answer

Best of both worlds  use UIImage s drawAtPoint  or drawInRect  while still specifying your custom context   UIGraphicsPushContext context    image drawAtPoint CGPointZero      UIImage will handle all especial cases  UIGraphicsPopContext      Also you avoid modifying your context with CGContextTranslateCTM or CGContextScaleCTM which the second answer does

User · Answer

I m not sure for UIImage  but this kind of behaviour usually occurs when coordinates are flipped  Most of OS X coordinate systems have their origin at the lower left corner  as in Postscript and PDF  But CGImage coordinate system has its origin at the upper left corner   Possible solutions may involve an isFlipped property or a scaleYBy -1 affine transform

User · Answer

You can also solve this problem by doing this     Using an Image as a mask by directly inserting UIImageObject CGImage causes   the same inverted display problem  This is solved by saving it to a CGImageRef first     CGImageRef image    UImageObject CGImage      CGContextDrawImage context  boundsRect  image    Nevermind    Stupid caching

User · Answer

Even after applying everything I have mentioned  I ve still had dramas with the images   In the end  i ve just used Gimp to create a  flipped vertical  version of all my images   Now I don t need to use any Transforms   Hopefully this won t cause further problems down the track      Does anyone know why   CGContextDrawImage would be drawing my   image upside down  I am loading an   image in from my application    Quartz2d uses a different co-ordinate system  where the origin is in the lower left corner   So when Quartz draws pixel x 5   y 10  of a 100   100 image  that pixel is being drawn in the lower left corner instead of the upper left   Thus causing the  flipped  image   The x co-ordinate system matches  so you will need to flip the y co-ordinates   CGContextTranslateCTM context  0  image size height     This means we have translated the image by 0 units on the x axis and by the images height on the y axis   However  this alone will mean our image is still upside down  just being drawn  image size height  below where we wish it to be drawn   The Quartz2D programming guide recommends using ScaleCTM and passing negative values to flip the image   You can use the following code to do this -  CGContextScaleCTM context  1 0  -1 0     Combine the two just before your CGContextDrawImage call and you should have the image drawn correctly   UIImage  image    UIImage imageNamed   testImage png        CGRect imageRect   CGRectMake 0  0  image size width  image size height           CGContextTranslateCTM context  0  image size height   CGContextScaleCTM context  1 0  -1 0    CGContextDrawImage context  imageRect  image CGImage     Just be careful if your imageRect co-ordinates do not match those of your image  as you can get unintended results   To convert back the coordinates   CGContextScaleCTM context  1 0  -1 0   CGContextTranslateCTM context  0  -imageRect size height

User · Answer

drawInRect is certainly the way to go   Here s another little thing that will come in way useful when doing this   Usually the picture and the rectangle into which it is going to go don t conform   In that case drawInRect will stretch the picture   Here s a quick and cool way to make sure that the picture s aspect ration isn t changed  by reversing the transformation  which will be to fit the whole thing in      Picture and irect don t conform  so there ll be stretching  compensate     float xf   Picture size width irect size width      float yf   Picture size height irect size height      float m   MIN xf  yf       xf    m      yf    m      CGContextScaleCTM ctx  xf  yf         Picture drawInRect  irect

User · Answer

You can also solve this problem by doing this     Using an Image as a mask by directly inserting UIImageObject CGImage causes   the same inverted display problem  This is solved by saving it to a CGImageRef first     CGImageRef image    UImageObject CGImage      CGContextDrawImage context  boundsRect  image    Nevermind    Stupid caching

User · Answer

Even after applying everything I have mentioned  I ve still had dramas with the images   In the end  i ve just used Gimp to create a  flipped vertical  version of all my images   Now I don t need to use any Transforms   Hopefully this won t cause further problems down the track      Does anyone know why   CGContextDrawImage would be drawing my   image upside down  I am loading an   image in from my application    Quartz2d uses a different co-ordinate system  where the origin is in the lower left corner   So when Quartz draws pixel x 5   y 10  of a 100   100 image  that pixel is being drawn in the lower left corner instead of the upper left   Thus causing the  flipped  image   The x co-ordinate system matches  so you will need to flip the y co-ordinates   CGContextTranslateCTM context  0  image size height     This means we have translated the image by 0 units on the x axis and by the images height on the y axis   However  this alone will mean our image is still upside down  just being drawn  image size height  below where we wish it to be drawn   The Quartz2D programming guide recommends using ScaleCTM and passing negative values to flip the image   You can use the following code to do this -  CGContextScaleCTM context  1 0  -1 0     Combine the two just before your CGContextDrawImage call and you should have the image drawn correctly   UIImage  image    UIImage imageNamed   testImage png        CGRect imageRect   CGRectMake 0  0  image size width  image size height           CGContextTranslateCTM context  0  image size height   CGContextScaleCTM context  1 0  -1 0    CGContextDrawImage context  imageRect  image CGImage     Just be careful if your imageRect co-ordinates do not match those of your image  as you can get unintended results   To convert back the coordinates   CGContextScaleCTM context  1 0  -1 0   CGContextTranslateCTM context  0  -imageRect size height

User · Answer

During the course of my project I jumped from Kendall s answer to Cliff s answer to solve this problem for images that are loaded from the phone itself   In the end I ended up using CGImageCreateWithPNGDataProvider instead   NSString  imageFileName      NSBundle mainBundle  resourcePath  stringByAppendingPathComponent   clockdial png     return CGImageCreateWithPNGDataProvider CGDataProviderCreateWithFilename  imageFileName UTF8String    NULL  YES  kCGRenderingIntentDefault     This doesn t suffer from the orientation issues that you would get from getting the CGImage from a UIImage and it can be used as the contents of a CALayer without a hitch

User · Answer

func renderImage size  CGSize  - gt  UIImage       return UIGraphicsImageRenderer size  size  image   rendererContext in            flip y axis         rendererContext cgContext translateBy x  0  y  size height          rendererContext cgContext scaleBy x  1  y  -1              draw image rotated offsetted         rendererContext cgContext saveGState           rendererContext cgContext translateBy x  translate x  y  translate y          rendererContext cgContext rotate by  rotateRadians          rendererContext cgContext draw cgImage  in  drawRect          rendererContext cgContext restoreGState

User · Answer

Relevant Quartz2D docs  https   developer apple com library ios documentation 2DDrawing Conceptual DrawingPrintingiOS GraphicsDrawingOverview GraphicsDrawingOverview html   apple ref doc uid TP40010156-CH14-SW4     Flipping the Default Coordinate System      Flipping in UIKit drawing modifies the backing CALayer to align a drawing environment having a LLO coordinate system with the default coordinate system of UIKit  If you only use UIKit methods and function for drawing  you shouldn   t need to flip the CTM  However  if you mix Core Graphics or Image I O function calls with UIKit calls  flipping the CTM might be necessary       Specifically  if you draw an image or PDF document by calling Core Graphics functions directly  the object is rendered upside-down in the view   s context  You must flip the CTM to display the image and pages correctly       To flip a object drawn to a Core Graphics context so that it appears correctly when displayed in a UIKit view  you must modify the CTM in two steps  You translate the origin to the upper-left corner of the drawing area  and then you apply a scale translation  modifying the y-coordinate by -1  The code for doing this looks similar to the following    CGContextSaveGState graphicsContext   CGContextTranslateCTM graphicsContext  0 0  imageHeight   CGContextScaleCTM graphicsContext  1 0  -1 0   CGContextDrawImage graphicsContext  image  CGRectMake 0  0  imageWidth  imageHeight    CGContextRestoreGState graphicsContext

User · Answer

I m not sure for UIImage  but this kind of behaviour usually occurs when coordinates are flipped  Most of OS X coordinate systems have their origin at the lower left corner  as in Postscript and PDF  But CGImage coordinate system has its origin at the upper left corner   Possible solutions may involve an isFlipped property or a scaleYBy -1 affine transform

User · Answer

During the course of my project I jumped from Kendall s answer to Cliff s answer to solve this problem for images that are loaded from the phone itself   In the end I ended up using CGImageCreateWithPNGDataProvider instead   NSString  imageFileName      NSBundle mainBundle  resourcePath  stringByAppendingPathComponent   clockdial png     return CGImageCreateWithPNGDataProvider CGDataProviderCreateWithFilename  imageFileName UTF8String    NULL  YES  kCGRenderingIntentDefault     This doesn t suffer from the orientation issues that you would get from getting the CGImage from a UIImage and it can be used as the contents of a CALayer without a hitch

User · Answer

Supplemental answer with Swift code  Quartz 2D graphics use a coordinate system with the origin in the bottom left while UIKit in iOS uses a coordinate system with the origin at the top left  Everything usually works fine but when doing some graphics operations  you have to modify the coordinate system yourself  The documentation states      Some technologies set up their graphics contexts using a different   default coordinate system than the one used by Quartz  Relative to   Quartz  such a coordinate system is a modified coordinate system and   must be compensated for when performing some Quartz drawing   operations  The most common modified coordinate system places the   origin in the upper-left corner of the context and changes the y-axis   to point towards the bottom of the page    This phenomenon can be seen in the following two instances of custom views that draw an image in their drawRect methods     On the left side  the image is upside-down and on the right side the coordinate system has been translated and scaled so that the origin is in the top left   Upside-down image  override func drawRect rect  CGRect            image     let image   UIImage named   rocket        let imageRect   CGRect x  0  y  0  width  image size width  height  image size height          context     let context   UIGraphicsGetCurrentContext           draw image in context     CGContextDrawImage context  imageRect  image CGImage       Modified coordinate system  override func drawRect rect  CGRect            image     let image   UIImage named   rocket        let imageRect   CGRect x  0  y  0  width  image size width  height  image size height          context     let context   UIGraphicsGetCurrentContext           save the context so that it can be undone later     CGContextSaveGState context          put the origin of the coordinate system at the top left     CGContextTranslateCTM context  0  image size height      CGContextScaleCTM context  1 0  -1 0          draw the image in the context     CGContextDrawImage context  imageRect  image CGImage          undo changes to the context     CGContextRestoreGState context

User · Answer

Instead of   CGContextDrawImage context  CGRectMake 0  0  145  15   image CGImage     Use    image drawInRect CGRectMake 0  0  145  15      In the middle of your begin end CGcontext methods   This will draw the image with the correct orientation into your current image context - I m pretty sure this has something to do with the UIImage holding onto knowledge of the orientation while the CGContextDrawImage method gets the underlying raw image data with no understanding of orientation

User · Answer

I use this Swift 5  pure Core Graphics extension that correctly handles non-zero origins in image rects   extension CGContext            Draw  image  flipped vertically  positioned and scaled inside  rect       public func drawFlipped   image  CGImage  in rect  CGRect            self saveGState           self translateBy x  0  y  rect origin y   rect height          self scaleBy x  1 0  y  -1 0          self draw image  in  CGRect origin  CGPoint x  rect origin x  y  0   size  rect size           self restoreGState             You can use it exactly like CGContext s regular draw   in   method   ctx drawFlipped myImage  in  myRect

User · Answer

We can solve this problem using the same function   UIGraphicsBeginImageContext image size    UIGraphicsPushContext context     image drawInRect CGRectMake gestureEndPoint x gestureEndPoint y 350 92     UIGraphicsPopContext     UIGraphicsEndImageContext

User · Answer

If anyone is interested in a no-brainer solution for drawing an image in a custom rect in a context    func drawImage image  UIImage  inRect rect  CGRect  context  CGContext            flip coords     let ty  CGFloat    rect origin y   rect size height      CGContextTranslateCTM context  0  ty      CGContextScaleCTM context  1 0  -1 0         draw image     let rect  y zero   CGRect origin  CGPoint x  rect origin x  y 0   size  rect size      CGContextDrawImage context  rect  y zero  image CGImage         flip back     CGContextScaleCTM context  1 0  -1 0      CGContextTranslateCTM context  0  -ty       The image will be scaled to fill the rect

User · Answer

Swift 3 0  amp  4 0  yourImage draw in  CGRect  blendMode  CGBlendMode  alpha  ImageOpacity    No Alteration needed

User · Answer

If anyone is interested in a no-brainer solution for drawing an image in a custom rect in a context    func drawImage image  UIImage  inRect rect  CGRect  context  CGContext            flip coords     let ty  CGFloat    rect origin y   rect size height      CGContextTranslateCTM context  0  ty      CGContextScaleCTM context  1 0  -1 0         draw image     let rect  y zero   CGRect origin  CGPoint x  rect origin x  y 0   size  rect size      CGContextDrawImage context  rect  y zero  image CGImage         flip back     CGContextScaleCTM context  1 0  -1 0      CGContextTranslateCTM context  0  -ty       The image will be scaled to fill the rect

User · Answer

Swift 3 CoreGraphics Solution  If you want to use CG for whatever your reason might be  instead of UIImage  this Swift 3 construction based on previous answers did solve the issue for me   if let cgImage   uiImage cgImage       cgContext saveGState       cgContext translateBy x  0 0  y  cgRect size height      cgContext scaleBy x  1 0  y  -1 0      cgContext draw cgImage  in  cgRect      cgContext restoreGState

User · Answer

I use this Swift 5  pure Core Graphics extension that correctly handles non-zero origins in image rects   extension CGContext            Draw  image  flipped vertically  positioned and scaled inside  rect       public func drawFlipped   image  CGImage  in rect  CGRect            self saveGState           self translateBy x  0  y  rect origin y   rect height          self scaleBy x  1 0  y  -1 0          self draw image  in  CGRect origin  CGPoint x  rect origin x  y  0   size  rect size           self restoreGState             You can use it exactly like CGContext s regular draw   in   method   ctx drawFlipped myImage  in  myRect

User · Answer

UIImage contains a CGImage as its main content member as well as scaling and orientation factors  Since CGImage and its various functions are derived from OSX  it expects a coordinate system that is upside down compared to the iPhone  When you create a UIImage  it defaults to an upside-down orientation to compensate  you can change this    Use the  CGImage property to access the very powerful CGImage functions  but drawing onto the iPhone screen etc  is best done with the UIImage methods

User · Answer

Relevant Quartz2D docs  https   developer apple com library ios documentation 2DDrawing Conceptual DrawingPrintingiOS GraphicsDrawingOverview GraphicsDrawingOverview html   apple ref doc uid TP40010156-CH14-SW4     Flipping the Default Coordinate System      Flipping in UIKit drawing modifies the backing CALayer to align a drawing environment having a LLO coordinate system with the default coordinate system of UIKit  If you only use UIKit methods and function for drawing  you shouldn   t need to flip the CTM  However  if you mix Core Graphics or Image I O function calls with UIKit calls  flipping the CTM might be necessary       Specifically  if you draw an image or PDF document by calling Core Graphics functions directly  the object is rendered upside-down in the view   s context  You must flip the CTM to display the image and pages correctly       To flip a object drawn to a Core Graphics context so that it appears correctly when displayed in a UIKit view  you must modify the CTM in two steps  You translate the origin to the upper-left corner of the drawing area  and then you apply a scale translation  modifying the y-coordinate by -1  The code for doing this looks similar to the following    CGContextSaveGState graphicsContext   CGContextTranslateCTM graphicsContext  0 0  imageHeight   CGContextScaleCTM graphicsContext  1 0  -1 0   CGContextDrawImage graphicsContext  image  CGRectMake 0  0  imageWidth  imageHeight    CGContextRestoreGState graphicsContext

User · Answer

func renderImage size  CGSize  - gt  UIImage       return UIGraphicsImageRenderer size  size  image   rendererContext in            flip y axis         rendererContext cgContext translateBy x  0  y  size height          rendererContext cgContext scaleBy x  1  y  -1              draw image rotated offsetted         rendererContext cgContext saveGState           rendererContext cgContext translateBy x  translate x  y  translate y          rendererContext cgContext rotate by  rotateRadians          rendererContext cgContext draw cgImage  in  drawRect          rendererContext cgContext restoreGState

User · Answer

Supplemental answer with Swift code  Quartz 2D graphics use a coordinate system with the origin in the bottom left while UIKit in iOS uses a coordinate system with the origin at the top left  Everything usually works fine but when doing some graphics operations  you have to modify the coordinate system yourself  The documentation states      Some technologies set up their graphics contexts using a different   default coordinate system than the one used by Quartz  Relative to   Quartz  such a coordinate system is a modified coordinate system and   must be compensated for when performing some Quartz drawing   operations  The most common modified coordinate system places the   origin in the upper-left corner of the context and changes the y-axis   to point towards the bottom of the page    This phenomenon can be seen in the following two instances of custom views that draw an image in their drawRect methods     On the left side  the image is upside-down and on the right side the coordinate system has been translated and scaled so that the origin is in the top left   Upside-down image  override func drawRect rect  CGRect            image     let image   UIImage named   rocket        let imageRect   CGRect x  0  y  0  width  image size width  height  image size height          context     let context   UIGraphicsGetCurrentContext           draw image in context     CGContextDrawImage context  imageRect  image CGImage       Modified coordinate system  override func drawRect rect  CGRect            image     let image   UIImage named   rocket        let imageRect   CGRect x  0  y  0  width  image size width  height  image size height          context     let context   UIGraphicsGetCurrentContext           save the context so that it can be undone later     CGContextSaveGState context          put the origin of the coordinate system at the top left     CGContextTranslateCTM context  0  image size height      CGContextScaleCTM context  1 0  -1 0          draw the image in the context     CGContextDrawImage context  imageRect  image CGImage          undo changes to the context     CGContextRestoreGState context

[ios] CGContextDrawImage draws image upside down when passed UIImage.CGImage

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