CPCTelera 1.5 Decrunching Issue

[Typhon]

Allo all,
So I've got a small CPCTelera 1.5 (development branch) uncompression issue. 
I have successfully generated a compressed and packed png (160x200 for Mode 0 screen) ->bin (via image_conversion.mk):

PALETTE=0 2 3 4 6 9 11 13 14 15 17 18 20 24 25 26
$(eval $(call IMG2SP, SET_MODE        , 0));
$(eval $(call IMG2SP, SET_PALETTE_FW  , $(PALETTE)))
$(eval $(call IMG2SP, SET_OUTPUT      , bin))
$(eval $(call IMG2SP, SET_IMG_FORMAT  , screen))
$(eval $(call IMG2SP, SET_FOLDER      , tmp/))
$(eval $(call IMG2SP, CONVERT         , img/transition.png, 160, 200, transition, palette, tileset))
$(eval $(call IMG2SP, CONVERT_PALETTE , $(PALETTE), t_palette))

Which is then converted again successfully via compression.mk into three source files:

$(eval $(call ADD2PACK,transition,tmp/transition.bin))
$(eval $(call PACKZX7B,transition,src/))

With amongst other things, 3 output files, transition.h, transition.s, and transition.h.s. I include these in my code successfully:

// File 'src/transition.h' generated using cpct_pack
// Compresor used:  zx7b
// Files compressed: [ 'tmp/transition.bin' ]
// Uncompressed:    16400 bytes
// Compressed:      630 bytes
// Space saved:      15770 bytes
//
#ifndef transition_630_H
#define transition_630_H
// Declaration of the compressed array
extern const unsigned char transition[630];
// Address of the latest byte of the compressed array (for unpacking purposes)
#define transition_end      (transition + 630 - 1)
// Compressed and uncompressed sizes
#define transition_size_z  630
#define transition_size    16400
#endif

However, when I try to decompress and display onto the screen:

cpct_zx7b_decrunch_s(VIDEO_MEM_END, transition_end);

It *does* display (see attached screenshot), however, the decrunching routine doesn't seem to stop and overwrites the rest of my program with unpredictable results (crashes mainly).

You cannot view this attachment.

Note that (as mentioned on a previous thread on the forum.) #define VIDEO_MEM_END ((void*)0xffff)


if I don't decrunch, all is good, but the image doesn't display.

What gives?

[ervin]

This is an interesting problem.
I notice that the uncompressed file size is 16400 bytes instead of 16384. I don't know if that is a problem, but it does seem odd.
Decompressing 16400 bytes from 0xFFFF down would cause 16 bytes of the stack to be overwritten (the 16 bytes below 0xC000).

The documentation for cpct_zx7b_decrunch_s says the following:

;;    * *source_end* should be a 16-bit pointer to the latest byte of the array where compressed
;; data is held. ZX7B algorithm will read the array from this byte backwards till its start.
;; No runtime checks are performed: if this value is incorrect, undefined behaviour will follow.
;; Typically, garbage data of undefined size will be produced, potentially overwriting undesired
;; memory parts.

A few years ago @Arnaud had a slightly different problem, and @ronaldo gave a very helpful response.
Not sure if that might help you.
https://www.cpcwiki.eu/forum/programming/cpctelera-1-5-0-file-compression-problem/

[Arnaud]

Hi,
i think @ervin is right, the decompression is overwriting the stack.

To solve your problem try to set the size of your image to 0, 0 (image size by default) and no args after image:
$(eval $(call IMG2SP, CONVERT        , img/transition.png, 0, 0, transition, , ))

I have done this some weeks ago and the size file was OK.

[ervin]

This discussion may also help.
CPCtelera - png to screen (cpcwiki.eu)

[Typhon]

Hi,
i think @ervin is right, the decompression is overwriting the stack.

To solve your problem try to set the size of your image to 0, 0 (image size by default) and no args after image:
$(eval $(call IMG2SP, CONVERT        , img/transition.png, 0, 0, transition, , ))

I have done this some weeks ago and the size file was OK.

Thank you, I'll try that later. I do notice that the video tuturial I was using from Prof.Retroman also had the same issue, though he wasn't doing anything afterwards in his code, so I don't think he realised he had the same issue.

[Typhon]

Hi,
i think @ervin is right, the decompression is overwriting the stack.

To solve your problem try to set the size of your image to 0, 0 (image size by default) and no args after image:
$(eval $(call IMG2SP, CONVERT        , img/transition.png, 0, 0, transition, , ))

I have done this some weeks ago and the size file was OK.

I can confirm that works! Thank you!

// File 'src/transition.h' generated using cpct_pack
// Compresor used:  zx7b
// Files compressed: [ 'tmp/transition.bin' ]
// Uncompressed:    16384 bytes
// Compressed:      612 bytes
// Space saved:      15772 bytes
//
#ifndef transition_612_H
#define transition_612_H
// Declaration of the compressed array
extern const unsigned char transition[612];
// Address of the latest byte of the compressed array (for unpacking purposes)
#define transition_end (transition + 612 - 1)
// Compressed and uncompressed sizes
#define transition_size_z 612
#define transition_size 16384
#endif