Update the calculation of bitmap image based on more complicated rules for most use cases

README.md

@@ -124,6 +124,19 @@ In most cases, vector SVG is preferred. But however, sometimes you may want the
 
 By default it use the SVG viewBox size. You can also specify a desired size during image loading using `.imageThumbnailPixelSize` context option. And you can specify whether or not to keep aspect ratio during scale using `.imagePreserveAspectRatio` context option.
 
+Note: When using `imageThumbnailPixelSize`, pass 0 for width or height will remove the limit of this length. For example, given a SVG image which original size is `(40,50)`:
+
+| preserve aspect ratio | width | height | effect |
+| --- | --- | --- | --- |
+| true | 50 | 50 | (40,50) |
+| true | 0 | 100 | (80,100) |
+| true | 20 | 0 | (20,25) |
+| true | 0 | 0 | (40,50) |
+| false | 50 | 50 | (50,50) |
+| false | 0 | 100 | (40,100) |
+| false | 20 | 0 | (20,50) |
+| false | 0 | 0 | (40,50) |
+
 Note: Once you pass the `imageThumbnailPixelSize`, we will always generate the bitmap representation. If you do want the vector format, do not pass them, let `UIImageView` to dynamically stretch the SVG.
 
 + Objective-C

SDWebImageSVGCoder/Classes/SDImageSVGCoder.m

@@ -192,28 +192,55 @@ - (UIImage *)createBitmapSVGWithData:(nonnull NSData *)data targetSize:(CGSize)t
     }
 
     CGSize size = SDCGSVGDocumentGetCanvasSize(document);
-    CGRect rect = CGRectMake(0, 0, size.width, size.height);
-    CGRect targetRect = rect;
-    if (!CGSizeEqualToSize(targetSize, CGSizeZero)) {
-        targetRect = CGRectMake(0, 0, targetSize.width, targetSize.height);
+    // Invalid size
+    if (size.width == 0 || size.height == 0) {
+        return nil;
     }
 
-    CGFloat xRatio = targetRect.size.width / rect.size.width;
-    CGFloat yRatio = targetRect.size.height / rect.size.height;
-
-    // If we specify only one length of the size (width or height) we want to keep the ratio for that length
-    if (preserveAspectRatio) {
-        if (xRatio <= 0) {
-            xRatio = yRatio;
-            targetRect.size = CGSizeMake(rect.size.width * xRatio, targetSize.height);
-        } else if (yRatio <= 0) {
-            yRatio = xRatio;
-            targetRect.size = CGSizeMake(targetSize.width, rect.size.height * yRatio);
+    CGFloat xScale;
+    CGFloat yScale;
+    // Calculation for actual target size, see rules in documentation
+    if (targetSize.width <= 0 && targetSize.height <= 0) {
+        // Both width and height is 0, use original size
+        targetSize.width = size.width;
+        targetSize.height = size.height;
+        xScale = 1;
+        yScale = 1;
+    } else {
+        CGFloat xRatio = targetSize.width / size.width;
+        CGFloat yRatio = targetSize.height / size.height;
+        if (preserveAspectRatio) {
+            // If we specify only one length of the size (width or height) we want to keep the ratio for that length
+            if (targetSize.width <= 0) {
+                yScale = yRatio;
+                xScale = yRatio;
+                targetSize.width = size.width * yRatio;
+            } else if (targetSize.height <= 0) {
+                xScale = xRatio;
+                yScale = xRatio;
+                targetSize.height = size.height * xRatio;
+            } else {
+                xScale = MIN(xRatio, yRatio);
+                yScale = MIN(xRatio, yRatio);
+            }
+        } else {
+            // If we specify only one length of the size but don't keep the ratio, use original size
+            if (targetSize.width <= 0) {
+                targetSize.width = size.width;
+                yScale = yRatio;
+                xScale = 1;
+            } else if (targetSize.height <= 0) {
+                xScale = xRatio;
+                yScale = 1;
+                targetSize.height = size.height;
+            } else {
+                xScale = xRatio;
+                yScale = yRatio;
+            }
         }
     }
-
-    CGFloat xScale = preserveAspectRatio ? MIN(xRatio, yRatio) : xRatio;
-    CGFloat yScale = preserveAspectRatio ? MIN(xRatio, yRatio) : yRatio;
+    CGRect rect = CGRectMake(0, 0, size.width, size.height);
+    CGRect targetRect = CGRectMake(0, 0, targetSize.width, targetSize.height);
 
     CGAffineTransform scaleTransform = CGAffineTransformMakeScale(xScale, yScale);
     CGAffineTransform transform = CGAffineTransformIdentity;