VDP2 render timing problem

[rtoumazet]

While displaying the "set time" screen, moving the cursor (displayed by the VDP1) doesn't move the yellow arrows (displayed by the VDP2). The date numbers aren't updated also, you need to click the "Reload cache" button to get the correct display.

It's actually a synchronization problem between the UI thread and the emu thread.

[rtoumazet]

Actually the problem is elsewhere : rendering is done through OpenGL for both the Saturn and Imgui. Imgui UI displays one frame every 1/60s (16.67ms). During that time, Saturn image calculation and rendering must be done too. If rendering is too long, the Saturn frame data is dropped and a new frame is started. In that case rendering is always cut before being complete, leading to a partly rendered screen, or even to a black screen. What needs to be done is to keep Saturn data when it takes longer to display than a Imgui frame, and to display it when complete during the next Imgui frame.

[rtoumazet]

Current implementation :	UI thread	Rendering thread
opengl()->isThereSomethingToRender()	opengl()->displayFramebuffer() called from vdp2->run()
opengl()->generateTextures()
opengl()->render() (uses parts_listmutex)

Mutexes declared in opengl.h

• texture_key_id_linkmutex
  • used in opengl->addOrUpdateTexture()
  • used in opengl()->generateTextures()
• parts_listmutex
  • used in opengl->render()

[rtoumazet]

Tasks from the UI thread :

• check if there's something to render from the emulation thread
• delete unused OpenGL textures
• generate to be used OpenGL textures
• render parts

Tasks from the emulation thread

• generate parts from Saturn memory (VDP1 & VDP2)
• stores parts textures data
• marks textures for deletion if needed
• displays Saturn framebuffer
• signals when something has to be rendered

[rtoumazet]

Concurrent access listing

Location	Variable	Function	UI thread	Emu thread	Mutex
Texture	texturestorage	storeTexture()	none	W	X
Texture	texturestorage	getTexture()	R	R	X
Texture	texturestorage	isTextureLoadingNeeded()	none	R/W	X
Texture	texturestorage	discardCache()	R/W	R/W	X
Texture	texturestorage	setCache()	none	R/W	X
Texture	texturestorage	cleanCache()	none	R/W	X
Opengl	partslist	displayFramebuffer()	none	W	X
Opengl	partslist	render()	W	none	X
Opengl	texture_key_idlink	isThereSomethingToRender()	W	none
Opengl	texture_key_idlink	addOrUpdateTexture()	none	R/W
Opengl	texture_key_idlink	generateTextures()	R/W	none
Opengl	textures_todelete	isThereSomethingToRender()	W	none
Opengl	textures_todelete	addOrUpdateTexture()	none	R/W
Opengl	textures_todelete	generateTextures()	R	none

[rtoumazet]

Saturn textures

Stored into a texture pool in Texture.h. Each texture contains raw data, and is called from OpenGL to generate OpenGL textures to be used while rendering.

OpenGL textures

Created in generateTextures(), where unused OpenGL textures are first deleted, then generated if needed.

[rtoumazet]

UI thread (ie data processing) :

• checks if there's data available ( !partslist.empty() )
• if there is any : get the data, delete / create the OpenGL textures, render to the texture
• notify the emu thread when done

Emu thread (ie data preparation) :

• runs until displayFramebuffer() end, where condition var is set when data is sent to the UI thread.
• thread waits until notification
• when notified, execution resumes for the next Saturn frame.

[rtoumazet]

Synchronization issue is fixed, but rendering is now slower. Time to investigate what can be done to improve this.