I haven't tried the code yet, but based on the commit message, this is a 
great contribution. Perhaps it would be worth to create a complete tool 
showing the actual results.
I think this code belongs to Morphic/MorphicExtras rather than Graphics. 
Graphics doesn't depend on Morphic, and it shouldn't, but these changes 
would introduce that dependency.

Levente

On Thu, 21 Mar 2019, David T. Lewis wrote:

> Hi Maximilian,
>
> This looks like a good addition. It is small and useful, and it aids
> understanding.
>
> I am not very knowledgeable about the BitBlt rules, so maybe someone
> else can look at it also? To me, this looks like a good contribution
> for trunk.
>
> Dave
>
>
> On Wed, Mar 20, 2019 at 12:51:46PM +0000, commits at source.squeak.org wrote:
>> A new version of Graphics was added to project The Inbox:
>> http://source.squeak.org/inbox/Graphics-ms.407.mcz
>> 
>> ==================== Summary ====================
>> 
>> Name: Graphics-ms.407
>> Author: ms
>> Time: 20 March 2019, 1:51:42.768596 pm
>> UUID: 40a66bd7-815f-43f7-bbd0-31dc8da3a483
>> Ancestors: Graphics-mt.406
>> 
>> Adds a preview morph for results of all BitBlt rules.
>> 
>> When first using BitBlt, it wasn't easy to understand the effects of the possible rules on pixel combinating for me.
>> I implemented two example methods which preview the effects of rule 0 to 41 (these are all, I believe) on quadratic existing forms with colors and 1 bit alpha channel (previewAllBitBltRules uses ToolIcons flag/collection) plus Forms with more alpha values (previewAllBitBltRulesWithAlpha uses ToolIcons flag and a Color red + 0.4 alpha preview). They generate a combined morph for all rules applied on these forms (scaled, because icons are small) and opens it in your hand.
>> Internally, this morph can be generated for any rule, forms and scale. Rules which throw exceptions are replaced with a fallback form.
>> 
>> =============== Diff against Graphics-mt.406 ===============
>> 
>> Item was added:
>> + ----- Method: BitBlt class>>previewAllBitBltRules (in category 'examples') -----
>> + previewAllBitBltRules
>> + 
>> + 	(self previewBitBltRules: (0 to: 41)
>> + 		on: ToolIcons flag
>> + 		and: ToolIcons collection
>> + 		fallback: ToolIcons exception
>> + 		scaledTo: 32) openInHand.!
>> 
>> Item was added:
>> + ----- Method: BitBlt class>>previewAllBitBltRulesWithAlpha (in category 'examples') -----
>> + previewAllBitBltRulesWithAlpha
>> + 
>> + 	(self previewBitBltRules: (0 to: 41)
>> + 		on: ToolIcons flag
>> + 		and: ((Color red alpha: 0.4) iconOrThumbnailOfSize: 12)
>> + 		fallback: ToolIcons exception
>> + 		scaledTo: 32) openInHand.!
>> 
>> Item was added:
>> + ----- Method: BitBlt class>>previewBitBltRules:on:and:fallback:scaledTo: (in category 'examples') -----
>> + previewBitBltRules: rules on: aForm1 and: aForm2 fallback: fallbackForm scaledTo: aNumberOrPoint
>> + 	"Returns a combined morph of the result of each rule applied on aForm1 combined with 
>> + 	aForm2 scaled to aNumberOrPoint. If the combination faild with a rule, fallbackForm is 
>> + 	shown instead. The number of each rule is appended at the bottom of each result."
>> + 
>> + 	| resultMorph tileExtent |
>> + 	tileExtent := aNumberOrPoint asPoint.
>> + 	resultMorph := Morph new
>> + 		color: Color transparent;
>> + 		extent: (rules size * tileExtent x)@tileExtent y;
>> + 		yourself.
>> + 
>> + 	rules withIndexDo: [ :rule :index | | form formMorph numberLabel |
>> + 		form := aForm1 copy.
>> + 		[aForm2 copy displayOn: form at: 0 at 0 rule: rule]
>> + 			on: Exception 
>> + 			do: [form := fallbackForm].
>> + 		formMorph := (form scaledToSize: tileExtent) asMorph
>> + 			position: (index*tileExtent x)@0;
>> + 			yourself.
>> + 		resultMorph addMorph: formMorph.
>> + 
>> + 		numberLabel := rule asString asMorph
>> + 	      	center: ((index+0.5)*tileExtent x)@tileExtent y;
>> + 			yourself.
>> + 		resultMorph addMorph: numberLabel].
>> + 
>> + 	^ resultMorph!
>> 
>>