HTML5 Canvas


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Consider a circle that has its origin at x , y , and that has radius radius. The points at startAngle and endAngle this circle's circumference, measured in radians clockwise from the positive x-axis, are the start and end points respectively. Otherwise, the arc is the path along the circumference of this circle from the start point to the end point, going anti-clockwise if the counterclockwise argument is true, and clockwise otherwise. If the two points are the same, or if the radius is zero, then the arc is defined as being of zero length in both directions. It must then create a new subpath with the point x , y as the only point in the subpath.

Each CanvasRenderingContext2D object has a current transformation matrix , as well as methods described in this section to manipulate it. When a CanvasRenderingContext2D object is created, its transformation matrix must be initialized to the identity transform. The transformation matrix is applied to coordinates when creating the current path, and when painting text, shapes, and paths, on CanvasRenderingContext2D objects. This API remains mostly for historical reasons. For instance, if a scale transformation that doubles the width is applied to the canvas, followed by a rotation transformation that rotates drawing operations by a quarter turn, and a rectangle twice as wide as it is tall is then drawn on the canvas, the actual result will be a square.

Changes the transformation matrix to apply a scaling transformation with the given characteristics. Changes the transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians. Changes the transformation matrix to apply a translation transformation with the given characteristics. Changes the transformation matrix to the matrix given by the arguments as described below. The scale x , y method must add the scaling transformation described by the arguments to the transformation matrix.

The x argument represents the scale factor in the horizontal direction and the y argument represents the scale factor in the vertical direction. The factors are multiples. The rotate angle method must add the rotation transformation described by the argument to the transformation matrix. The angle argument represents a clockwise rotation angle expressed in radians. The translate x , y method must add the translation transformation described by the arguments to the transformation matrix. The x argument represents the translation distance in the horizontal direction and the y argument represents the translation distance in the vertical direction.

The arguments are in coordinate space units. The transform a , b , c , d , e , f method must replace the current transformation matrix with the result of multiplying the current transformation matrix with the matrix described by:. The setTransform a , b , c , d , e , f method must reset the current transform to the identity matrix, and then invoke the transform a , b , c , d , e , f method with the same arguments.

This union type allows objects implementing any of the following interfaces to be used as image sources:. When a user agent is required to check the usability of the image argument , where image is a CanvasImageSource object, the user agent must run these steps, which return either good , bad , or aborted :. If the image argument is an HTMLImageElement object with an intrinsic width or intrinsic height or both equal to zero, then return bad and abort these steps. If the image argument is an HTMLCanvasElement object with either a horizontal dimension or a vertical dimension equal to zero, then return bad and abort these steps.

Specifically, when a CanvasImageSource object represents an animated image in an HTMLImageElement , the user agent must use the default image of the animation the one that the format defines is to be used when animation is not supported or is disabled , or, if there is no such image, the first frame of the animation, when rendering the image for CanvasRenderingContext2D APIs.

When a CanvasImageSource object represents an HTMLVideoElement , then the frame at the current playback position when the method with the argument is invoked must be used as the source image when rendering the image for CanvasRenderingContext2D APIs, and the source image's dimensions must be the intrinsic width and intrinsic height of the media resource i.

Invalid values are ignored. The fillStyle attribute represents the color or style to use inside shapes, and the strokeStyle attribute represents the color or style to use for the lines around the shapes. Both attributes can be either strings, CanvasGradient s, or CanvasPattern s. If the new value is a CanvasPattern object that is marked as not origin-clean , then the bitmap's origin-clean flag must be set to false. When set to a CanvasPattern or CanvasGradient object, the assignment is live , meaning that changes made to the object after the assignment do affect subsequent stroking or filling of shapes.

On getting, if the value is a color, then the serialization of the color must be returned. Otherwise, if it is not a color but a CanvasGradient or CanvasPattern , then the respective object must be returned. Such objects are opaque and therefore only useful for assigning to other attributes or for comparison to other gradients or patterns. The serialization of a color for a color value is a string, computed as follows: if it has alpha equal to 1. Otherwise, the color value has alpha less than 1.

Snake Game using HTML5 Canvas tag

User agents must express the fractional part of the alpha value, if any, with the level of precision necessary for the alpha value, when reparsed, to be interpreted as the same alpha value. When the context is created, the fillStyle and strokeStyle attributes must initially have the string value When the value is a color, it must not be affected by the transformation matrix when used to draw on the canvas. There are two types of gradients, linear gradients and radial gradients, both represented by objects implementing the opaque CanvasGradient interface.

Once a gradient has been created see below , stops are placed along it to define how the colors are distributed along the gradient. The color of the gradient at each stop is the color specified for that stop.

What is HTML Canvas?

Between each such stop, the colors and the alpha component must be linearly interpolated over the RGBA space without premultiplying the alpha value to find the color to use at that offset. Before the first stop, the color must be the color of the first stop. After the last stop, the color must be the color of the last stop. When there are no stops, the gradient is transparent black. Adds a color stop with the given color to the gradient at the given offset.

Throws an IndexSizeError exception if the offset is out of range. Throws a SyntaxError exception if the color cannot be parsed. Returns a CanvasGradient object that represents a linear gradient that paints along the line given by the coordinates represented by the arguments. Returns a CanvasGradient object that represents a radial gradient that paints along the cone given by the circles represented by the arguments.

If either of the radii are negative, throws an IndexSizeError exception. The addColorStop offset , color method on the CanvasGradient interface adds a new stop to a gradient. If the offset is less than 0 or greater than 1 then an IndexSizeError exception must be thrown. If multiple stops are added at the same offset on a gradient, they must be placed in the order added, with the first one closest to the start of the gradient, and each subsequent one infinitesimally further along towards the end point in effect causing all but the first and last stop added at each point to be ignored.

The createLinearGradient x0 , y0 , x1 , y1 method takes four arguments that represent the start point x0 , y0 and end point x1 , y1 of the gradient. The method must return a linear CanvasGradient initialized with the specified line. Linear gradients must be rendered such that all points on a line perpendicular to the line that crosses the start and end points have the color at the point where those two lines cross with the colors coming from the interpolation and extrapolation described above.

The points in the linear gradient must be transformed as described by the current transformation matrix when rendering. The createRadialGradient x0 , y0 , r0 , x1 , y1 , r1 method takes six arguments, the first three representing the start circle with origin x0 , y0 and radius r0 , and the last three representing the end circle with origin x1 , y1 and radius r1. The values are in coordinate space units. If either of r0 or r1 are negative, an IndexSizeError exception must be thrown.

Otherwise, the method must return a radial CanvasGradient initialized with the two specified circles. Abort these steps.


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This effectively creates a cone, touched by the two circles defined in the creation of the gradient, with the part of the cone before the start circle 0. The resulting radial gradient must then be transformed as described by the current transformation matrix when rendering. Gradients must be painted only where the relevant stroking or filling effects requires that they be drawn. Patterns are represented by objects implementing the opaque CanvasPattern interface. Returns a CanvasPattern object that uses the given image and repeats in the direction s given by the repetition argument.

The allowed values for repetition are " repeat " both directions , " repeat-x " horizontal only , " repeat-y " vertical only , and " no-repeat " neither. If the repetition argument is empty, the value repeat is used. If the image has no image data, throws an InvalidStateError exception. If the second argument isn't one of the allowed values, throws a SyntaxError exception. If the image isn't yet fully decoded, then the method returns null.

To create objects of this type, the createPattern image , repetition method is used. When the method is invoked, the user agent must run the following steps:. Modifying this image after calling the createPattern method must not affect the pattern. Patterns must be painted so that the top left of the first image is anchored at the origin of the coordinate space, and images are then repeated horizontally to the left and right, if the repeat-x string was specified, or vertically up and down, if the repeat-y string was specified, or in all four directions all over the canvas, if the repeat string was specified, to create the repeated pattern that is used for rendering.

The images are not scaled by this process; one CSS pixel of the image must be painted on one coordinate space unit in generating the repeated pattern. When rendered, however, patterns must actually be painted only where the stroking or filling effect requires that they be drawn, and the repeated pattern must be affected by the current transformation matrix. Pixels not covered by the repeating pattern if the repeat string was not specified must be transparent black.

If the original image data is a bitmap image, the value painted at a point in the area of the repetitions is computed by filtering the original image data. The user agent may use any filtering algorithm for example bilinear interpolation or nearest-neighbor. When the filtering algorithm requires a pixel value from outside the original image data, it must instead use the value from wrapping the pixel's coordinates to the original image's dimensions.

That is, the filter uses 'repeat' behavior, regardless of the value of repetition. If a radial gradient or repeated pattern is used when the transformation matrix is singular, the resulting style must be transparent black otherwise the gradient or pattern would be collapsed to a point or line, leaving the other pixels undefined. Linear gradients and solid colors always define all points even with singular transformation matrices. There are three methods that immediately draw rectangles to the bitmap. They each take four arguments; the first two give the x and y coordinates of the top left of the rectangle, and the second two give the width w and height h of the rectangle, respectively.

Shapes are painted without affecting the current default path , and are subject to the clipping region , and, with the exception of clearRect , also shadow effects , global alpha , and global composition operators. Paints the box that outlines the given rectangle onto the canvas, using the current stroke style. The clearRect x , y , w , h method must run the following steps:. Let pixels be the set of pixels in the specified rectangle that also intersect the current clipping region.

Clear regions that cover the pixels in pixels in the canvas element. If either height or width are zero, this method has no effect, since the set of pixels would be empty. The fillRect x , y , w , h method must paint the specified rectangular area using the fillStyle. If either height or width are zero, this method has no effect. The strokeRect x , y , w , h method must take the result of tracing the path described below, using the CanvasRenderingContext2D object's line styles, and fill it with the strokeStyle.

If both w and h are zero, the path has a single subpath with just one point x , y , and no lines, and this method thus has no effect the trace a path algorithm returns an empty path in that case. Fills or strokes respectively the given text at the given position. If a maximum width is provided, the text will be scaled to fit that width if necessary. Returns a TextMetrics object with the metrics of the given text in the current font.

Returns the advance width of the text that was passed to the measureText method. The CanvasRenderingContext2D interface provides the following methods for rendering text directly to the canvas. The fillText and strokeText methods take three or four arguments, text , x , y , and optionally maxWidth , and render the given text at the given x , y coordinates ensuring that the text isn't wider than maxWidth if specified, using the current font , textAlign , and textBaseline values.

HTML5 Canvas Tutorial for Beginners - An Intro to Becoming a Pro - Ep. 1

Specifically, when the methods are called, the user agent must run the following steps:. Run the text preparation algorithm , passing it text , the CanvasRenderingContext2D object, and, if the maxWidth argument was provided, that argument. Let glyphs be the result. Paint the shapes given in glyphs , as transformed by the current transformation matrix , with each CSS pixel in the coordinate space of glyphs mapped to one coordinate space unit.

For fillText , fillStyle must be applied to the shapes and strokeStyle must be ignored. For strokeText , the reverse holds: strokeStyle must be applied to the result of tracing the shapes using the CanvasRenderingContext2D object for the line styles, and fillStyle must be ignored. These shapes are painted without affecting the current path, and are subject to shadow effects , global alpha , the clipping region , and global composition operators.

W3C Recommendation 19 November 2015

If the text preparation algorithm used a font that has an origin that is not the same as the origin specified by the entry settings object even if "using a font" means just checking if that font has a particular glyph in it before falling back to another font , then set the bitmap's origin-clean flag to false. The measureText method takes one argument, text.

When the method is invoked, the user agent must run the text preparation algorithm, pass a new TextMetrics object with its attributes set as described in the following list. If doing these measurements requires using a font that has an origin that is not the same as that of the Document object that owns the canvas element even if "using a font" means just checking if that font has a particular glyph in it before falling back to another font , then the method must throw a SecurityError exception.

Otherwise, it must return the new TextMetrics object. The TextMetrics interface is used for the objects returned from measureText. It has one attribute, width , which is set by the measureText method. Glyphs rendered using fillText and strokeText can spill out of the box given by the font size the em square size and the width returned by measureText the text width. This version of the specification does not provide a way to obtain the bounding box dimensions of the text. If the text is to be rendered and removed, care needs to be taken to replace the entire area of the canvas that the clipping region covers, not just the box given by the em square height and measured text width.

This would be provided in preference to a dedicated way of doing multiline layout. The context always has a current default path. There is only one current path, it is not part of the drawing state. The current path is a path , as described above. Informs the user of the canvas location for the fallback element, based on the current path. If the given element has focus, draws a focus outline around the current path following the platform or user agent conventions for focus outlines as defined by the user agent. The beginPath method must empty the list of subpaths in the context's current path so that the it once again has zero subpaths.

The fill method must fill all the subpaths of the current path, using fillStyle , and using the non-zero winding number rule. Open subpaths must be implicitly closed when being filled without affecting the actual subpaths. Thus, if two overlapping but otherwise independent subpaths have opposite windings, they cancel out and result in no fill. If they have the same winding, that area just gets painted once. The stroke method must trace the path, using the CanvasRenderingContext2D object for the line styles, and then fill the combined stroke area using the strokeStyle attribute.

As a result of how the algorithm to trace a path is defined, overlapping parts of the paths in one stroke operation are treated as if their union was what was painted. The stroke style is affected by the transformation during painting, even if the path is the current default path.

Paths, when filled or stroked, must be painted without affecting the current path, and must be subject to shadow effects , global alpha , the clipping region , and global composition operators. Zero-length line segments must be pruned before stroking a path. Empty subpaths must be ignored. The drawFocusIfNeeded element method, when invoked, must run the following steps:. If element is not focused or is not a descendant of the element with whose context the method is associated, then abort these steps.

If the user has requested the use of particular focus outlines e. Some platforms only draw focus outlines around elements that have been focused from the keyboard, and not those focused from the mouse. Other platforms simply don't draw focus outlines around some elements at all unless relevant accessibility features are enabled.

This API is intended to follow these conventions. User agents that implement distinctions based on the manner in which the element was focused are encouraged to classify focus driven by the focus method based on the kind of user interaction event from which the call was triggered if any. The focus outline should not be subject to the shadow effects , the global alpha , or the global composition operators , but should be subject to the clipping region.

When the focus area is clipped by the canvas element, only the visual representation of the focus outline is clipped to the clipping region. If the focus area is not on the screen, then scroll the focus outline into view when it receives focus. Inform the user of the location given by the path. The full location of the corresponding fallback element is passed to the accessibility API, if supported.

User agents may wait until the next time the event loop reaches its "update the rendering" step to inform the user. To properly drive magnification based on a focus change, a system accessibility API driving a screen magnifier needs the bounds for the newly focused object. The methods above are intended to enable this by allowing the user agent to report the bounding box of the path used to render the focus outline as the bounds of the element element passed as an argument, if that element is focused, and the bounding box of the area to which the user agent is scrolling as the bounding box of the current selection.

The clip method must create a new clipping region by calculating the intersection of the current clipping region and the area described by the path, using the non-zero winding number rule. Open subpaths must be implicitly closed when computing the clipping region, without affecting the actual subpaths.

The new clipping region replaces the current clipping region. When the context is initialized, the clipping region must be set to the rectangle with the top left corner at 0,0 and the width and height of the coordinate space. The isPointInPath method must return true if the point given by the x and y coordinates passed to the method, when treated as coordinates in the canvas coordinate space unaffected by the current transformation, is inside the intended path as determined by the non-zero winding number rule; and must return false otherwise. Points on the path itself must be considered to be inside the path.

If either of the arguments is infinite or NaN, then the method must return false. To draw images onto the canvas, the drawImage method can be used. If the first argument isn't an img , canvas , or video element, throws a TypeMismatchError exception.

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If the one of the source rectangle dimensions is zero, throws an IndexSizeError exception. If the image isn't yet fully decoded, then nothing is drawn. When the drawImage method is invoked, the user agent must run the following steps:. Check the usability of the image argument. If this returns aborted , then an exception has been thrown and the method doesn't return anything; abort these steps.

Create HTML5 Canvas documents in Animate

If it returns bad , then abort these steps without drawing anything. Otherwise it returns good ; continue with these steps. If not specified, the dw and dh arguments must default to the values of sw and sh , interpreted such that one CSS pixel in the image is treated as one unit in the bitmap's coordinate space.


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If the sx , sy , sw , and sh arguments are not specified, they must default to 0, 0, the image's intrinsic width in image pixels, and the image's intrinsic height in image pixels, respectively. If the image has no intrinsic dimensions, the concrete object size must be used instead, as determined using the CSS " Concrete Object Size Resolution " algorithm, with the specified size having neither a definite width nor height, nor any additional constraints, the object's intrinsic properties being those of the image argument, and the default object size being the size of the bitmap. When the source rectangle is outside the source image, the source rectangle must be clipped to the source image and the destination rectangle must be clipped in the same proportion.

When the destination rectangle is outside the destination image the bitmap , the pixels that land outside the bitmap are discarded, as if the destination was an infinite canvas whose rendering was clipped to the dimensions of the bitmap. If one of the sw or sh arguments is zero, abort these steps. Nothing is painted. Paint the region of the image argument specified by the source rectangle on the region of the rendering context's bitmap specified by the destination rectangle, after applying the current transformation matrix to the destination rectangle.

The image data must be processed in the original direction, even if the dimensions given are negative. This specification does not define the algorithm to use when scaling the image, if necessary. When a canvas is drawn onto itself, the drawing model requires the source to be copied before the image is drawn back onto the canvas, so it is possible to copy parts of a canvas onto overlapping parts of itself.

If the original image data is a bitmap image, the value painted at a point in the destination rectangle is computed by filtering the original image data. When the filtering algorithm requires a pixel value from outside the original image data, it must instead use the value from the nearest edge pixel. That is, the filter uses 'clamp-to-edge' behaviour. When the filtering algorithm requires a pixel value from outside the source rectangle but inside the original image data, then the value from the original image data must be used.

Thus, scaling an image in parts or in whole will have the same effect. This does mean that when sprites coming from a single sprite sheet are to be scaled, adjacent images in the sprite sheet can interfere. This can be avoided by ensuring each sprite in the sheet is surrounded by a border of transparent black, or by copying sprites to be scaled into temporary canvas elements and drawing the scaled sprites from there.

Images are painted without affecting the current path, and are subject to shadow effects , global alpha , the clipping region , and global composition operators. If the image argument is not origin-clean , set the bitmap's origin-clean flag to false. Each canvas element whose primary context is a CanvasRenderingContext2D object must have a hit region list associated with its bitmap.

The hit region list is a list of hit regions. A path on the canvas element's bitmap for which this region is responsible. A bounding circumference on the canvas element's bitmap that surrounds the hit region's path as it stood when it was created. Optionally, a non-empty string representing an ID for distinguishing the region from others. A control is a reference to an Element node, to which, in certain conditions, the user agent will route events, and from which the user agent will determine the state of the hit region for the purposes of accessibility tools.

The control is ignored when it is not a descendant of the canvas element. Adds a hit region to the canvas bitmap based on the current default path. The argument is an object with the following members:. While both ID and control are optional, when calling addHitRegion, at least one of the two needs to be present to create a hit region. Removes a hit region from the canvas bitmap. The argument is the ID of a region added using addHitRegion.

Canvas tutorial. Last updated by: uxitten , Apr 19, , AM. Learn the best of web development Get the latest and greatest from MDN delivered straight to your inbox. I'm okay with Mozilla handling my info as explained in this Privacy Policy. Sign up now. Please check your inbox to confirm your subscription. This is good so far, but what we want to happen is for the snake to automatically move to either side once it hits an edge and cannot go further. We do this by using the following code:. This code takes an axis type, and then works out which way to go based on the current position.

So, for example if the snake has hit the right edge of the canvas, then the function is passed a y parameter, telling the function it needs to move either up or down. This function will then check if the current position is above or below the middle of the canvas. The sample principles apply for the horizontal case. If you now reload the page, you will see that the Snake keeps moving when it hits an edge, and cleverly opts to head in the direction furthest from an edge.

The idea is to have an array of xy coordinates that will represent all the square points which the Snake body occupies:. What happens here is that every time there is a call to drawSnake, the function adds the new position to the snakeBody array, keeping it in memory. What you should see now is a little snake moving around, with a tiny body rather than an infinitely-long line. Now we need to add 1 element to the game which is currently not implemented in any way; we need the food.

To add this element to the game, we need to do the following:. We already know how to draw squares, but how do we place them in a random location, one which is not occupied by the Snake? The former problem is solved by using the Math library, and the latter by detecting whether the snakeBody array contains an array of the random point that is suggested for locating the food square. Here is the code:. Unfortunately comparing an array of arrays is not a straightforward procedure in javascript, so I had to create a function called hasPoint, which using Array.

A special note, make sure to move the red fillcolor line to the drawSnake function, as demonstrated below:. Put makeFoodItem ; right in the IF block for the canvas-supported initial startup code, and what you should see is a little green square on the map, awesome. For an added bonus, move your Snake so he eats the food…. Intentionally, no, unintentionally, yes. The little code which helps to give the impression of the Snake moving by removing the trailing squares also clears the area where the Snake has been, including green squares.

How nice is that? First, start by putting this code in the canvas-supported IF statement block, precisely in this order:. The makeFoodItem function must come before the drawSnake function, otherwise the code will break because of the suggestedPoint variable being missing. You can guess where this is heading, and here is the answer:. Firstly, it now refers to the snakeLength variable instead of just 3, and importantly, the IF statement comparison of the currentPosition variable with the suggestedPoint variable allows us to capture when the Snake eats the food, so then we generate the next food item on the canvas, and we increment the snakeLength variable, which makes the Snake grow longer.

Refresh the browser and see it in action. Right now, if the Snake moves over itself, it simply continues its path.

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