I have a working WebApi that delivers a PNG image to me (tested in Fiddler). However, when I want to draw the image an on HTML Canvas, nothing happens.
Web api code :
public HttpResponseMessage GetInitialMap(int height, int width)
{
var chart = RmdbHelper.GetInitialChart(height, width);
MemoryStream memoryStream = new MemoryStream();
chart.Save(memoryStream, ImageFormat.Png);
HttpResponseMessage response = new HttpResponseMessage();
response.Content = new StreamContent(memoryStream);
response.Content.Headers.ContentType = new MediaTypeHeaderValue("image/png");
response.StatusCode = HttpStatusCode.OK;
return response;
}
This is the javascript and jquery code to call the image and try drawing
var NavChart = function() {
var ctx,
chartHeight,
chartWidth,
// other stuff
renderBackgroundMap = function() {
$.get("../api/NauticalMaps/GetInitialMap?height" + chartHeight + "&width=" + chartWidth)
.done(function (bgImage) {
var background = new Image(chartWidth, chartHeight);
background.src = bgImage;
ctx.drawImage(background, 0, 0);
})
.fail(function (error) {
showError("...");
});
};
initialize = function(canvasId) {
var canvas = document.getElementById(canvasId);
chartHeight = canvas.getAttribute("height");
chartWidth = canvas.getAttribute("width");
ctx = canvas.getContext("2d");
render();
};
return {
initialize: initialize
};
};
On the HTML page I create a NavChart object and call the initialize function. No problems until then. Fiddler shows me that the call is made and it returns a image.
HTML and call :
<div class="row">
<canvas id="zeekaart" height="550" width="1150" style="border: 1px solid lightgrey"></canvas>
</div>
<script>
var nav = new NavChart();
nav.initialize("zeekaart");
</script>
It seems that
var background = new Image(chartWidth, chartHeight);
background.src = bgImage;
the .src expects an URL or destination. But plainly using
.done(function (bgImage) {
ctx.drawImage(bgImage, 0, 0);
})
doesn't work either.
Any ideas? I don't want to write all images away in a temp folder on the server and clear them after a specified time.
A workaround presented by @Kaiido does the job well enough, like this:
var back = new Image();
back.onload = function() {
ctx.drawImage(back, 0, 0);
}
back.src = "../api/NauticalMaps/GetInitialMap?height=" + chartHeight + "&width=" + chartWidth;
However ... I finally found the solution for my problem. Part of my problem was not returning a JSON object, by returning it as a plain image. When returning purely an image, the code above will do. But, by returning the image as part of a JSON object, you can directly use the image in the browser memory. Here's my solution:
A simple class for storing the data (which will be sent automatically as json) to the browser
public class NavigationMapData
{
public byte[] MapData { get; set; }
// ... other data goes in here as well
}
The api-controller method:
public NavigationMapData Get(string id, double northing, double easting, double northing2, double easting2, int height, int width)
{
NavigationMapData data = new NavigationMapData();
// next method just returns a Bitmap object
var img = RmdbHelper.GetChart(northing, easting, northing2, easting2, height, width);
// convert the object
data.MapData = img.ToByteArray(ImageFormat.Jpeg);
return data;
}
For the completeness, the image extension for converting Image/Bitmap data into a byte[]:
public static class ImageExtensions
{
public static byte[] ToByteArray(this Image image, ImageFormat format)
{
using (MemoryStream ms = new MemoryStream())
{
image.Save(ms, format);
return ms.ToArray();
}
}
}
The javascript/jquery code:
$.get(URL_TO_API_PLUS_PARAMS)
.done(function (navMapData) {
var img = document.createElement("img");
// as seen on stackoverflow somewhere
img.src = 'data:image/jpeg;base64,' + navMapData.MapData;
ctx.drawImage(img, 0, 0);
})
.fail(function (error) {
// onError actions
})
.always(function() {
// onFinished actions
});
Now I have more control over my api calls in case a bitmap or my data would not load.
您的Web API正在发送原始图像数据,但“ src”需要一个URL。
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.